SIPCORE                                                        E. Burger
Internet-Draft                                             NeuStar, Inc.
Obsoletes: RFC 2976                                            H. Kaplan
(if approved)                                                Acme Packet
Intended status: Standards Track                             C. Holmberg
Expires: January 6, 2010                                        Ericsson
                                                            July 5, 2009

  Session Initiation Protocol (SIP) INFO Method and Package Framework

Status of this Memo

   This Internet-Draft is submitted to IETF in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on January 6, 2010.

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   Copyright (c) 2009 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   Provisions Relating to IETF Documents in effect on the date of
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   Please review these documents carefully, as they describe your rights
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   This document provides new semantics for the SIP INFO method of RFC
   2976.  These new semantics defined here are fully backwards
   compatible with the old semantics.  Core to the new semantics is 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 session, 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",
   document are to be interpreted as described in RFC 2119 [RFC2119].
   The terminology in this document conforms to the Internet Security
   Glossary [RFC4949].

   Be aware this document strictly follows RFC 3261 [RFC3261] for the
   definition of the terms User Agent Server (UAS) and User Agent Client
   (UAC).  Specifically, 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.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  5
   2.  Applicability  . . . . . . . . . . . . . . . . . . . . . . . .  6
   3.  Info Package Behavior  . . . . . . . . . . . . . . . . . . . .  7
     3.1.  UAS Behavior . . . . . . . . . . . . . . . . . . . . . . .  7
     3.2.  UAC Behavior . . . . . . . . . . . . . . . . . . . . . . .  8
     3.3.  Package Versions . . . . . . . . . . . . . . . . . . . . .  9
     3.4.  Advertisement Example  . . . . . . . . . . . . . . . . . . 10
   4.  The INFO Method Request  . . . . . . . . . . . . . . . . . . . 11
     4.1.  INFO Requests  . . . . . . . . . . . . . . . . . . . . . . 11
     4.2.  INFO Request Body  . . . . . . . . . . . . . . . . . . . . 12
     4.3.  Responses to the INFO Request Method . . . . . . . . . . . 13
     4.4.  Routing Behavior . . . . . . . . . . . . . . . . . . . . . 14
     4.5.  Behavior of Registrars . . . . . . . . . . . . . . . . . . 14
     4.6.  OPTIONS Processing . . . . . . . . . . . . . . . . . . . . 14
     4.7.  Order of Delivery  . . . . . . . . . . . . . . . . . . . . 15
   5.  Formal INFO Method Definition  . . . . . . . . . . . . . . . . 15
     5.1.  INFO Method  . . . . . . . . . . . . . . . . . . . . . . . 15
     5.2.  INFO Headers . . . . . . . . . . . . . . . . . . . . . . . 17
       5.2.1.  Info-Package header  . . . . . . . . . . . . . . . . . 17
       5.2.2.  Recv-Info header . . . . . . . . . . . . . . . . . . . 18
   6.  Legacy Uses of INFO  . . . . . . . . . . . . . . . . . . . . . 18
   7.  Info Package Requirements  . . . . . . . . . . . . . . . . . . 19
     7.1.  Applicability  . . . . . . . . . . . . . . . . . . . . . . 19
     7.2.  Info Package Name  . . . . . . . . . . . . . . . . . . . . 19
     7.3.  Info Package Parameters  . . . . . . . . . . . . . . . . . 19
     7.4.  Info Package Tags  . . . . . . . . . . . . . . . . . . . . 19
     7.5.  INFO Bodies  . . . . . . . . . . . . . . . . . . . . . . . 20
     7.6.  UAC generation of INFO requests  . . . . . . . . . . . . . 20
     7.7.  UAS processing of INFO requests  . . . . . . . . . . . . . 20
     7.8.  Rate of INFO Requests  . . . . . . . . . . . . . . . . . . 21
     7.9.  IANA Registrations . . . . . . . . . . . . . . . . . . . . 21
     7.10. Security Considerations  . . . . . . . . . . . . . . . . . 21
     7.11. Examples . . . . . . . . . . . . . . . . . . . . . . . . . 21
   8.  Syntax . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
   9.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 22
     9.1.  Update to Registration of SIP INFO Method  . . . . . . . . 22
     9.2.  Registration of the Info-Package Header Field  . . . . . . 22
     9.3.  Registration of the Recv-Info Header Field . . . . . . . . 23
     9.4.  Creation of the Info Packages Registry . . . . . . . . . . 23
     9.5.  Registration of the Info-Package Content-Disposition . . . 24
     9.6.  SIP Response Code 469 Registration . . . . . . . . . . . . 24
   10. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
     10.1. Simple Info Package  . . . . . . . . . . . . . . . . . . . 24
     10.2. Multipart INFO Example . . . . . . . . . . . . . . . . . . 24
   11. Modifications to SIP Change Process  . . . . . . . . . . . . . 25
   12. Security Considerations  . . . . . . . . . . . . . . . . . . . 25

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   13. References . . . . . . . . . . . . . . . . . . . . . . . . . . 26
     13.1. Normative References . . . . . . . . . . . . . . . . . . . 26
     13.2. Informative References . . . . . . . . . . . . . . . . . . 27
   Appendix A.  Info Package Considerations . . . . . . . . . . . . . 29
     A.1.  Appropriateness of Usage . . . . . . . . . . . . . . . . . 29
     A.2.  Dialog Fate-Sharing  . . . . . . . . . . . . . . . . . . . 30
     A.3.  Messaging Rates and Volume . . . . . . . . . . . . . . . . 30
     A.4.  Is there a better alternative? . . . . . . . . . . . . . . 30
     A.5.  Alternatives for Common INFO Use . . . . . . . . . . . . . 32
       A.5.1.  State Updates  . . . . . . . . . . . . . . . . . . . . 32
       A.5.2.  User Stimulus: Touch Tones and Others  . . . . . . . . 32
       A.5.3.  Direct Signaling Channel . . . . . . . . . . . . . . . 33
       A.5.4.  Proxy-Aware Signaling  . . . . . . . . . . . . . . . . 33
       A.5.5.  Dialog Probe . . . . . . . . . . . . . . . . . . . . . 33
       A.5.6.  Malicious Indicator  . . . . . . . . . . . . . . . . . 34
   Appendix B.  Legacy INFO Usages  . . . . . . . . . . . . . . . . . 34
     B.1.  ISUP . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
     B.2.  QSIG . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
     B.3.  MSCML  . . . . . . . . . . . . . . . . . . . . . . . . . . 35
     B.4.  MSML . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
     B.5.  Video Fast Update  . . . . . . . . . . . . . . . . . . . . 35
   Appendix C.  Acknowledgements  . . . . . . . . . . . . . . . . . . 35
   Appendix D.  Change Log  . . . . . . . . . . . . . . . . . . . . . 36
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 38

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1.  Introduction

   The SIP protocol [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 [RFC4028].  The purpose of
   the INFO message [RFC2976] is to carry application level information
   along the SIP signaling path.  Note the INFO method does not change
   the SIP session state.  It may, however, change application state for
   applications using the SIP session.

   While INFO has been widely adopted for specific application use
   cases, such as ISUP and DTMF exchange, RFC 2976 [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 are many useful ways a UAS
   can render an image.  Said differently, there are different contexts
   for an image in SIP.  The image could be a caller-id picture, a
   contact icon, a photo for sharing, and so on.  The sender does not
   know which image to send to the receiver if the receiver supports an
   image content type.  Likewise, the receiver does not know the context
   of an image the client is sending if the receiver supports receiving
   more than one image 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
   [RFC3458].  RFC 3458 goes into this and other issues in detail.

   Event Packages [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.  However, while the mechanism described here is
   similar to subscription-based events, there is no formal relationship
   between this mechanism and the subscription mechanism.  In
   particular, when a UAC issues a SUBSCRIBE, it creates a dialog usage.

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   The mechanism defined here creates neither a separate subscription
   dialog nor a subscription usage within an existing session.  Instead,
   it uses the INVITE method and its responses to indicate supported
   Info Packages and the INFO method to convey the Info Packages.

   Each UA enumerates which Info Packages it can receive.  If a first UA
   indicates it can receive a package and a second UA can send the
   package, the second UA 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 to this document to detect legacy UAs.  A
   legacy UA will not include a Recv-Info header in their SIP session
   establishment or modification requests.  Conversely, a UA that
   supports Info Packages will have a Recv-Info header.  Section 3
   describes Info Package advertisement 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 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-session 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 session.  SIP proxy servers will
   receive, and potentially act on, mid-session 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
   also 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 [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

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   header field in the INFO request.  As described in Section 4.1, the
   mechanism described here is backwards compatible with legacy, RFC
   2976 INFO mechanisms.

3.  Info Package Behavior

   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.  UAS Behavior

   A UAS supporting this document MUST advertise the set of Info
   Packages it is willing to receive in Recv-Info header(s) in dialog
   usage requests and responses for session establishment or target
   refresh.  This includes INVITE, UPDATE, PRACK, ACK, and their non-
   failure responses (101-199 and 2xx only).

   Once a UAS indicates support for an Info Package by sending a Recv-
   Info header with one or more package names, the UAS MUST be prepared
   to receive an INFO containing that package.  Note this may occur
   before dialog negotiation completes.

   Recall the UAC of an INVITE may choose to receive (be a UAS for) INFO
   methods.  This UA may chose not to offer any packages in the initial
   INVITE and subsequently advertise packages from the target UA's
   subsequent responses, in order to support third-party call control

   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 may also list multiple
   packages by including multiple Recv-Info headers.  The UAS may also
   combine multiple Recv-Info headers with one or more packages in each
   header value.  If the UAS prefers to receive 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.  The UAS MUST NOT list a package more than
   once.  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

   There is an important issue to consider when the UAS advertises
   support for multiple packages that one might interpret to be similar

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   or equivalent.  The UAC has no method of knowing whether the UAS
   would like the UAC to send a single INFO request with the preferred
   package or for the UAC to send multiple INFO requests with the same
   or similar information.  The behavior is entirely up to the UAC and
   the rules specified by the package definitions.

   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
   a UAS that understands Info Packages but does not wish to receive any
   from a legacy UAS that does not understand Info Packages.  A UAC
   conforming to this document can always send or receive legacy INFO
   usages without packages.

   Info Package capability advertisement occurs within the context of a
   session negotiation exchange.  The Info Package capability set
   received by the UAC within the last exchange is the one the UAC will
   use to chose Info Packages from.  Also note that due to glare, an
   INFO request may be in flight prior to the UAC receiving an updated
   capability set removing a given Info Package.  Thus, the UAS MUST be
   prepared to handle an INFO request with an Info Package payload with
   a newly delisted Info Package.  Proper handling does include
   rejecting the request with a 469.  See Section 4.3 for more on this

3.2.  UAC Behavior

   A UAC MUST NOT send INFO requests for a given INFO package until the
   UAC receives an "INVITE dialog usage" request or response (for
   session establishment or target refresh) with a Recv-Info header
   listing the given Info Package.

   At any time during an "INVITE dialog usage" request or response, if a
   UAS sends one or more Recv-Info headers, the UAC MUST replace the old
   set of supported Info Packages with the collection of Info Packages
   enumerated by the current message.

   If the UAS does not send any Recv-Info headers in a message, then the
   list of supported Info Packages does not change.

   A UAC MUST cease sending INFO requests for a given INFO package when
   the UAC receives an "INVITE dialog usage" request or response (for
   session establishment or target refresh) that does not contain a
   Recv-Info header listing the given Info Package.  Note the UAC MUST
   be prepared to receive a 469 response (Section 4.3) at any time, even
   if the UAS advertised it could receive the Info Package.  This
   situation can occur if the UAC sends the INFO request at the same
   time the UAS advertises it no longer supports the Info Package in

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   If the UAC receives a Recv-Info header with the value 'nil', the UAC
   MUST NOT send any INFO methods that contain Info Packages.

   The UAS may advertise support for multiple Info Packages.  If some of
   these packages have similar or equivalent functionality and the UAC
   supports multiple such packages, the UAC SHOULD chose to send Info
   Package payload(s) from the Info Package listed lexically earlier in
   the last Recv-Info advertisement the UAC received from the UAS.  This
   document cannot make this protocol action a must strength, as the
   concept of "similar or equivalent" is highly Info Package specific.

   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 [RFC3261].  One could
   envision a particular Info Package implementation that relied on
   either of these headers.  See Section 7 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 [RFC3841] is not sufficient to determine if the endpoints
   support Info Packages, as the INFO method supported might be the
   obsolete RFC 2976 [RFC2976] version.

   For Info Packages, this draft does not provide a means of requiring
   support for a specific Info Package.  If the UAS does not indicate
   support for an Info Package that the UAC requires, and the UAC
   requires the use of that package, the UAC can use any supported
   RFC3261 [RFC3261] method to terminate the session.

   A UAC MAY send a legacy INFO [RFC2976] method at any time.

3.3.  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 not
   all payloads have sufficient capability to carry payload versions.

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   In this situation, it is highly unlikely payloads will 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 advertise package support but cannot advertise package version
   support.  Even if we did allow for package versioning, as a parameter
   to the Recv-Info header value, for example, it is lexically
   equivalent to having a new Info Package.

   UACs MUST NOT depend on any lexical parsing of the Info Package name
   for versioning, such as "fooV1" and "fooV2" or "foo.1" and "foo.2".

3.4.  Advertisement Example

   Here is an INVITE.  The initiating UA advertises the following.

   Via: SIP/2.0/TCP;branch=z9hG4bK776
   Max-Forwards: 70
   To: Bob <>
   From: Alice <>;tag=1928301774
   CSeq: 314159 INVITE
   Recv-Info: P, R
   Contact: <>
   Content-Type: application/sdp
   Content-Length: ...


   This means the initiating UA is willing to receive from Info Packages
   P and R.

   In this next message, the target UA responds with a 200 OK:
  SIP/2.0 200 OK
  Via: SIP/2.0/TCP;branch=z9hG4bK776;received=
  To: Bob <>;tag=a6c85cf
  From: Alice <>;tag=1928301774
  CSeq: 314159 INVITE
  Contact: <>
  Recv-Info: R, T
  Content-Type: application/sdp
  Content-Length: ...


   This indicates the target UA is willing to receive from Info Packages
   R and T.

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   The initiating UA then confirms in an ACK, as shown.

   ACK SIP/2.0
   Via: SIP/2.0/TCP;branch=z9hG4bK776
   To: Bob <>;tag=a6c85cf
   From: Alice <>;tag=1928301774
   CSeq: 314163 ACK
   Recv-Info: R
   Content-Length: 0

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

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 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.

   The UAC MAY send an INFO in a legacy usage context.  See Appendix B
   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
   session 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 session identifiers defined in RFC 3261 [RFC3261] must match
   those of the provisional or final responses to the INVITE.  As a

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   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 session
   negotiation has not completed, and we allow early INFO requests,
   multiple target UAs may respond.  This initial session establishment
   phase is the only time the UAS need be prepared to receive multiple
   INFO requests, as one would expect there may be messages from non-
   authoritative forked dialogs prior to their termination.

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

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

   The signaling path for the INFO method is the signaling path
   established as a result of the session 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

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 package specification
   indicates it is appropriate to include a payload with the request,
   the UAC MUST include the payload with the MIME type specified by the
   Info Package.

   If the Info Package definition directs the UAC to send a request
   without a payload, the UAC MUST send the INFO request without a body.

   Some SIP extensions, which are orthogonal to INFO, may insert body
   parts unrelated to the INFO payload.  User Agents MUST conform to RFC

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   3261 as updated by body-handling [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

   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 session.  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 session 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

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   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 469 Bad INFO
   Package.  In the terminology of Multiple Dialog Usages [RFC5057],
   this represents a Transaction Only failure.

   If a server receives an INFO request with a body it understands, but
   the request has no Info-Package header, the UAS MAY use the body as
   it sees fit.  If the UAS accepts the INFO request, the UAS MUST
   respond to the INFO request with a 200 OK.  This enables legacy use
   of INFO.  If the UAS needs to enforce strict compliance with the
   current INFO framework described here, the UAS MUST reject the
   request with a 469.

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

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

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 [RFC3261] as defined for the BYE request.

   The INFO message MUST NOT change the state of the SIP session.  Of
   course, outside the INFO machinery specific failure responses as
   documented in the SIP dialog usages document [RFC5057], may cause the
   INVITE session 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

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

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   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.

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.  All a UAS
   can determine is the UAC sent one INFO message after another.  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

   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.  Overlapping
   requests can occur even if the particular instance of an application
   (Info Package) does not allow it, as the mechanism described here is
   package-agnostic.  Thus, 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].

   This table expands on Tables 2 and 3 in [RFC3261].

     Header                    Where    INFO
     ------                    -----    ----
     Accept                      R       o

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     Accept-Encoding             R       o
     Accept-Encoding            2xx      o
     Accept-Encoding            415      c
     Accept-Language             R       o
     Accept-Language            2xx      o
     Accept-Language            415      c
     Alert-Info                          -
     Allow                       R       o
     Allow                      200      -
     Allow                      405      o
     Authentication-Info        2xx      o
     Authorization               R       o
     Call-ID                     c       m
     Call-Info                           o
     Contact                             -
     Content-Disposition                 o
     Content-Encoding                    o
     Content-Language                    o
     Content-Length                      o
     Content-Type                        *
     CSeq                        c       m
     Date                                o
     Error-Info               3xx-6xx    o
     Expires                             -
     From                        c       m
     Geolocation                 R       o
     Max-Breadth                 R       -
     Max-Forwards                R       o
     MIME-Version                        o
     Min-Expires                         -
     Organization                        o
     Priority                    R       -
     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,18x    o
     Require                             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

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     Server                      r       o
     Subject                     R       o
     Supported                   R       o
     Supported                  2xx      o
     Timestamp                           o
     To                          c       m  (w/ Tag)
     Unsupported                420      o
     User-Agent                          o
     Via                                 m
     Warning                     r       o
     WWW-Authenticate           401      m
     WWW-Authenticate           407      o

                Figure 1: Table 1: Summary of Header Fields

5.2.  INFO Headers

   This table expands on tables 2 and 3 in [RFC3261].

Info-Package   R      -   -   -   -   -   -   -   o*  -   -   -   -   -
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   -   -   -

   * Info-Package is MANDATORY for INFO messages sent using Info
   Packages as described in this document.  Info-Package is OPTIONAL for
   legacy (RFC2976) INFO messages.

                    Table 2: INFO-related Header Fields

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

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   Required [RFC5226].

5.2.2.  Recv-Info header

   This document adds Recv-Info to the definition of the element
   "general-header" in the SIP [RFC3261] message grammar.  Section 3
   describes the Recv-Info header usage.

6.  Legacy Uses of INFO

   Several RFC-defined and other standards-defined uses of RFC 2976 INFO
   [RFC2976] exist and are in use, as well as numerous proprietary uses.
   Appendix B 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
   [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.

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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

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 ABNF
   production described in Section 8.  It MUST include the information
   that appears in the IANA registration of the token.  For information
   on registering such types, see Section 9.

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

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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 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

   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 as a result
   of the overlapping INFO request.  Of course, if there are other
   problems with the request that results in a failure, the UAC issues
   the appropriate response code.  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 session.

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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 session.

   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.
   Intermediaries will have access to the payload and past the first
   hop, there is no way to assure subsequent hops will not transmit the
   payload in clear text.  The only way to ensure secure transport at
   the application level is to have the security at the application
   level.  The most common method of achieving this is to use end-to-end
   security techniques such as S/MIME [RFC3851].  If the application
   demands this level of security, the Info Package definition MUST
   indicate such.

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.

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8.  Syntax

   This section describes the syntax extensions required for the INFO
   method.  The previous sections describe the semantics.  Note the
   formal syntax definitions described in this document use the ABNF
   format used in SIP [RFC3261] and contain references to elements
   defined therein.

   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 Info-package-list
   Info-package-list   =  "nil"
                       / Info-package-type *( COMMA Info-package-type )
   Info-package-type   =  Info-package-name *( ";" Info-package-param)
   Info-package-name   =  token
   Info-package-param  =  token

   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

   Method:      INFO
   Reference:   [RFC2976]


   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.

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   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
   [RFC5226] policy.

   The following data elements populate the Info Package Registry.
   o  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.
   o  Info Package Payload MIME Types: A list of zero or more registered
      MIME types from the MIME Type Registry.
   o  Standards Status: Values are "Standards Track" or empty.  See
      below for a discussion and rules on this field.
   o  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 [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:

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   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

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

9.6.  SIP Response Code 469 Registration

   Please register the 469 response code in the Session Initiation
   Protocol Parameters - Response Codes registry as follows.
   Response Code: 469
   Default Reason Phrase: Bad INFO Package
   Reference: RFCXXXX

10.  Examples

10.1.  Simple Info Package

   Here Alice sends Bob a simple Info Package payload.

   INFO sip:alice@ SIP/2.0
   Via: SIP/2.0/UDP;branch=z9hG4bKnabcdef
   To: Alice <>;tag=1234567
   From: Bob <>;tag=abcdefg
   Call-Id: 123456mcmxcix
   CSeq: 2 INFO
   Contact: <sip:bob@>
   Info-Package: foo
   Content-type: application/foo
   Content-length: 24

   I am a foo message type

10.2.  Multipart INFO Example

   Other SIP extensions can put payloads into an INFO method,
   independent of the Info Package.  In this case, the Info Package
   payload gets put into a Multipart MIME body, with the content
   disposition indicating which body belongs to the Info Package.  Since
   there is one and only one Info Package payload in the message, we

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   only need to tag which body part goes with the Info Package.

   INFO sip:alice@ SIP/2.0
   Via: SIP/2.0/UDP;branch=z9hG4bKnabcdef
   To: Alice <>;tag=1234567
   From: Bob <>;tag=abcdefg
   Call-Id: 123456mcmxcix
   CSeq: 7 INFO
   Contact: <sip:bob@>
   Info-Package: foo
   mumble-extension: <cid:abcd9999qq>
   Content-Type: multipart/mixed;boundary="theboundary"
   Content-Length: ...

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

   <mumble stuff>

   Content-Type: application/foo
   Content-Disposition: Info-Package
   Content-length: 24

   I am a foo message type

11.  Modifications to SIP Change Process

   This document updates RFC 3427 [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 DISPATCH chairs to assign an
   individual to perform expert review of the proposal if the work is
   not a RAI 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

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   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

13.  References

13.1.  Normative References

              Camarillo, G., "Message Body Handling in the Session
              Initiation Protocol (SIP)",
              draft-ietf-sip-body-handling-06 (work in progress),
              March 2009.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate

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              Requirement Levels", RFC 2119, BCP 14, March 1997.

   [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.

   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 5226,
              May 2008.

13.2.  Informative References

              Shanmugham, S. and D. Burnett, "Media Resource Control
              Protocol Version 2 (MRCPv2)",
              draft-ietf-speechsc-mrcpv2-19 (work in progress),
              June 2009.

              Sharratt, G. and A. Saleem, "Media Server Markup Language
              (MSML)", draft-saleem-msml-08 (work in progress),
              February 2009.

   [RFC0768]  Postel, J., "User Datagram Protocol", STD 6, RFC 768,
              August 1980.

   [RFC0793]  Postel, J., "Transmission Control Protocol", STD 7,
              RFC 793, September 1981.

   [RFC2616]  Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
              Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
              Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.

   [RFC2976]  Donovan, S., "The SIP INFO Method", RFC 2976,
              October 2000.

   [RFC3080]  Rose, M., "The Blocks Extensible Exchange Protocol Core",
              RFC 3080, March 2001.

   [RFC3265]  Roach, A., "Session Initiation Protocol (SIP)-Specific
              Event Notification", RFC 3265, June 2002.

   [RFC3372]  Vemuri, A. and J. Peterson, "Session Initiation Protocol
              for Telephones (SIP-T): Context and Architectures",
              BCP 63, RFC 3372, September 2002.

   [RFC3427]  Mankin, A., Bradner, S., Mahy, R., Willis, D., Ott, J.,

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              and B. Rosen, "Change Process for the Session Initiation
              Protocol (SIP)", BCP 67, RFC 3427, December 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.

   [RFC3458]  Burger, E., Candell, E., Eliot, C., and G. Klyne, "Message
              Context for Internet Mail", RFC 3458, January 2003.

   [RFC3725]  Rosenberg, J., Peterson, J., Schulzrinne, H., and G.
              Camarillo, "Best Current Practices for Third Party Call
              Control (3pcc) in the Session Initiation Protocol (SIP)",
              BCP 85, RFC 3725, April 2004.

   [RFC3841]  Rosenberg, J., Schulzrinne, H., and P. Kyzivat, "Caller
              Preferences for the Session Initiation Protocol (SIP)",
              RFC 3841, August 2004.

   [RFC3851]  Ramsdell, B., "Secure/Multipurpose Internet Mail
              Extensions (S/MIME) Version 3.1 Message Specification",
              RFC 3851, July 2004.

   [RFC4028]  Donovan, S. and J. Rosenberg, "Session Timers in the
              Session Initiation Protocol (SIP)", RFC 4028, April 2005.

   [RFC4145]  Yon, D. and G. Camarillo, "TCP-Based Media Transport in
              the Session Description Protocol (SDP)", RFC 4145,
              September 2005.

   [RFC4240]  Burger, E., Van Dyke, J., and A. Spitzer, "Basic Network
              Media Services with SIP", RFC 4240, December 2005.

   [RFC4497]  Elwell, J., Derks, F., Mourot, P., and O. Rousseau,
              "Interworking between the Session Initiation Protocol
              (SIP) and QSIG", BCP 117, RFC 4497, May 2006.

   [RFC4730]  Burger, E. and M. Dolly, "A Session Initiation Protocol
              (SIP) Event Package for Key Press Stimulus (KPML)",
              RFC 4730, November 2006.

   [RFC4949]  Shirey, R., "Internet Security Glossary, Version 2",
              RFC 4949, August 2007.

   [RFC4960]  Stewart, R., "Stream Control Transmission Protocol",
              RFC 4960, September 2007.

   [RFC4975]  Campbell, B., Mahy, R., and C. Jennings, "The Message

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              Session Relay Protocol (MSRP)", RFC 4975, September 2007.

   [RFC5022]  Van Dyke, J., Burger, E., and A. Spitzer, "Media Server
              Control Markup Language (MSCML) and Protocol", RFC 5022,
              September 2007.

   [RFC5057]  Sparks, R., "Multiple Dialog Usages in the Session
              Initiation Protocol", RFC 5057, November 2007.

   [RFC5168]  Levin, O., Even, R., and P. Hagendorf, "XML Schema for
              Media Control", RFC 5168, March 2008.

              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", World Wide Web Consortium
              Recommendation REC-voicexml21-20070619, June 2007,

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
   session, 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.

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A.2.  Dialog Fate-Sharing

   INFO, by design, is a method within an INVITE dialog usage.  RFC 5057
   [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 session 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 session.  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 [RFC0768].  Appropriate mechanisms for
   such traffic include MSRP [RFC4975], COMEDIA [RFC4145], or HTTP

A.4.  Is there a better alternative?

   The first alternative for application level interaction is SIP
   Events, also known as SUBSCRIBE/NOTIFY [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 session that does
   not share the fate of the related INVITE-initiated session.
   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 session's proxy
   path.  In fact, SIUBSCRIBE/NOTIFY is your only option if you need to

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   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 session establishment.  If the
   SUBSCRIBE application is co-resident with the INVITE application, the
   application will have to manage two SIP sessions instead of one.
   Tracking the application level state dominates memory and processing
   for some applications, and as such, the doubling of SIP sessions is
   not an issue.  However, for other applications, this may be an issue.

   The MESSAGE method [RFC3428] defines one-time instant message
   exchange, typically for sending MIME contents for rendering to the

   Another model for application level information exchange is to
   establish a communication channel in the media plane.  One model for
   this is MRCPv2 [I-D.ietf-speechsc-mrcpv2].  Here, the INVITE-
   initiated session establishes a separate reliable, connection-
   oriented channel, such as a TCP [RFC0793] or SCTP [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 [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.  In addition, 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

   Another model is to use a totally externally signaled channel, such
   as HTTP [RFC2616].  In this model, the user agent knows about a
   rendezvous point to direct HTTP requests to for the transfer of

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   information.  Examples include encoding of a prompt to retrieve in
   the SIP Request URI in RFC 4240 [RFC4240] or the encoding of a SUBMIT
   target in a VoiceXML [W3C.REC-voicexml21-20070619] script.

   MSRP [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 sessions.
   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

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.
   o  State updates
   o  User stimulus
   o  Direct signaling channel
   o  Proxy-aware signaling
   o  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 [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

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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 [RFC4975] or MRCPv2 [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 [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

   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 [RFC3261].

A.5.5.  Dialog Probe

   Some implementations in the wild use INFO to probe if an INVITE-
   initiated session is alive.  While this works, it is NOT RECOMMENDED.
   In particular, RFC 4028 [RFC4028] describes how to ensure an INVITE-
   initiated session is alive.

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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 session, 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 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 session is
   torn down, as presumably the SPIT event will refer not to, "this
   session," which does not exist, but to "that session 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

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B.1.  ISUP

   SIP-T uses Content-Type to identify ISUP protocol elements in an INFO
   message.  See RFC3372 [RFC3372].

B.2.  QSIG

   QSIG uses Content-Type to identify QSIG protocol elements in an INFO
   message.  See RFC4497 [RFC4497].


   MSCML uses a Require to ensure the UAS understands that INFO messages
   of the MSCML type are in fact MSCML messages.  See RFC5022 [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
   [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 [RFC5168] for more information.

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,

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      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 draft-ietf-sip-info-events-03
   o  Clarified Abstract language
   o  All SIP dialogs are now refered to as sessions
   o  Clarified the image example in the Introduction
   o  Clarified the relationship (none) between SIP Event Packages and
      SIP Info Packages
   o  Really, really clarified the protocol is NOT a negotiation but an
   o  Split Section 3 into UAS and UAC behavior
   o  Moved the example in section 3 into its own sub-section, and used
      full SIP headers
   o  Clarified forking behavior
   o  Clarified language around when to send a body
   o  Added 469 error response, instead of reusing 489
   o  Clarified overlapping INFO method handling

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   o  Fixed table 1 to follow 3261, not 2543
   o  Added REFER to the INFO Headers table
   o  replaced token-nodot with token for Info-Package values
   o  Clarified end-to-end security considerations
   o  Info Package parameters are semi-colon delimited, not dot

   Changes from -02
   o  Applicability statement explicitly says we're backwards compatible
   o  Explicitly state we work like UPDATE (both early and confirmed
   o  Agreed text for IANA Considerations package registry

   Changes from -01
   o  One and only one Info Package per INFO
   o  Removed Send-Info header, greatly simplifying negotiation
   o  Multiple body part identification through Content-Disposition:
   o  Note that forking INVITEs may result in multiple INFO's coming
      back to INVITE originator
   o  Describe how a UAS can enforce strict adherence to this document
   o  Remove CANCEL INFO faux pas
   o  Better explained overlapping INFO issues and resolutions
   o  Token names are now really case sensitive
   o  Moved Info Package Considerations to an Appendix
   o  Introduced stronger, yet more open, IANA registration process
   o  Took a few more paragraphs from INFO Litmus to cover all bases.
   o  Added RFC 5168 to legacy usages

   Changes from -00
   o  Corrected ABNF.
   o  Enabled sending of legacy INFO messages.  Receiving legacy INFO
      messages was already here.
   o  Negotiation is not Offer/Answer, it is Offer/Offer.
   o  Created the explicit "nil" Info Package to indicate no info
   o  Fixed CANCEL impacting future transactions.
   o  Added Registrar behavior.
   o  Added OPTIONS processing.
   o  Clarified overlapping INFO method processing.
   o  Described multiple INFO bodies in a single INFO method.
   o  Took out Info-Package as a header for responses to the INFO
   o  Expanded on risks of using INFO and filled-in more on the
   o  Moved definitions of INFO into the body of the text and cleaned up
      IANA Considerations section

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   o  Added legacy usages descriptions

Authors' Addresses

   Eric W. Burger
   NeuStar, Inc.
   46000 Center Oak Plaza
   Sterling, VA  20166-6579


   Hadriel Kaplan
   Acme Packet
   71 Third Ave.
   Burlington, MA  01803


   Christer Holmberg
   Hirsalantie 11
   Jorvas,   02420


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