Network Working Group                                     J. Van Dyke
Internet Draft                                              E. Burger
Document: draft-burger-sipping-msuri-01.txt  SnowShore Networks, Inc.
Category: Standards Track                           November 21, 2001
Expires: May 2002


                 SIP URI Conventions for Media Servers


Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026 [1].

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups. Note that
   other groups may also distribute working documents as Internet-
   Drafts. Internet-Drafts are draft documents valid for a maximum of
   six months and may be updated, replaced, or obsoleted by other
   documents at any time. It is inappropriate to use Internet-Drafts as
   reference material or to cite them other than as "work in progress."

   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/ietf/1id-abstracts.txt

   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html.

   Discussion of this document is on the SIPPING discussion list.  The
   SIPPING list is at <mailto:sipping-request@ietf.org>.


1. Abstract

   Application Servers, SIP Proxies, and SoftSwitches (a.k.a. Media
   Gateway Controllers) act as SIP [2] User Agents to control the media
   processing capabilities of media servers.  The SIP Request URI
   identifies the desired service and provides a context for the media
   server to interpret the SIP message.  This document describes a
   standard SIP addressing mechanism to address specific services.
   Because of SIP's flexibility, the existing protocol accommodates
   these services.  This document proposes a standard URI scheme for
   important media services such as announcements and conferencing.



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2. 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 [3].

   FORMATTING NOTE: Notes, such at this one, provide additional,
   nonessential information that the reader may skip without missing
   anything essential.  The primary purpose of these non-essential
   notes is to convey information about the rationale of this document,
   or to place this document in the proper historical or evolutionary
   context.  Readers whose sole purpose is to construct a conformant
   implementation may skip such information.  However, it may be of use
   to those who wish to understand why we made certain design choices.



Table of Contents

1. Abstract...........................................................1
2. Conventions used in this document..................................2
3. Overview...........................................................2
4. Service Definition.................................................3
5. Service Indicators and URI Signatures..............................3
6. Operation..........................................................4
7. Formal Syntax......................................................5
8. Security Considerations............................................5
9. IANA Considerations................................................5
10. Examples..........................................................6
10.1. Announcement....................................................6
10.2. Conference......................................................7
11. References........................................................7
12. Changes Made in Version 01........................................8
13. Acknowledgments...................................................8
14. Author's Addresses................................................8


3. Overview

   Media servers are devices that perform media processing on real-time
   packet media.  Examples of such processing are tone detection and
   generation, packet recording (usually with transcoding), packet
   playing (usually with transcoding - also known as prompting), and
   mixing (also known as conferencing).

   These services are of general utility to a wide array of SIP UA's
   including application servers, softswitches and proxy servers.  In
   addition, the behaviors and semantics of these services are well
   understood.  For these reasons, it is both desirable and possible to
   create standard SIP interfaces for these services.

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4. Service Definition

   A service is a set of related media processing features with a well-
   defined set of properties.  The defining properties of a service are
   its SIP URI signature, the MIME types it accepts and any
   SUBSCRIBE/NOTIFY [4] event packages it supports.  The SIP URI
   signature consists of the service indicator, instance ID and any
   associated URI parameters.

   Services MUST have a unique SIP URI signature.  Services MAY offer
   support for MIME types other than "application/sdp" and
   SUBSCRIBE/NOTIFY event packages if required to implement service
   features.

   In the context of SIP control of media servers, we take advantage of
   the fact that the standard SIP URI has a user part [2].  Media
   processing services may be thought of as user automatons that
   participate in SIP sessions. It naturally follows that the user
   address, or the left-hand-side of the URI, should be utilized as a
   service indicator.

   Media servers commonly offer multiple services at a single host
   address.  Use of the user part as a service indicator enables
   service consumers to direct their requests without ambiguity.  It
   has the added benefit of enabling media services to register their
   availability with SIP Registrars just as any "real" SIP user would.
   This maintains consistency and provides enhanced flexibility in the
   deployment of media services in the network.

   For per-service security, the media server MAY use any of the
   security protocols described in [2].

   Following [2], the media server MAY issue 401 challenges for
   authentication.

   The media server, upon receiving the INVITE, notes the service
   indicator.  Depending on the service indicator, the media server
   will either honor the request or return a failure response code.


5. Service Indicators and URI Signatures

   The service indicator is the concatenation of the service name and
   an optional service instance identifier, separated by an equal sign.
   The service name MAY be modified by an optional service namespace.

   There has been much discussion about the potential for confusion if
   media services URIs are not readily distinguishable from other types
   of SIP UA's.  The use of a service namespace provides a mechanism to
   unambiguously identify standard interfaces while not constraining
   the development of private or experimental services.

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   It is proposed that standard services, such as the announcement and
   conferencing services described here, be registered with IANA using
   the "org.iana" service namespace.

   Service developers MAY use a service namespace other than "org.iana"
   for private or experimental services.

   Per [2], the service indicator is case insensitive.  The service
   name MUST be from the set alphanumeric characters plus dash (US-
   ASCII %2C).  The service name MUST NOT include an equal sign (US-
   ASCII %3C).

   The service name MAY have long- and short-forms, as SIP does for
   headers.

   A given service indicator MAY have an associated set of parameters.
   Such parameters MUST follow the convention set out in [2] for SIP
   URI parameters.  That is, a semi-colon separated list of
   keyword=values.

   Certain services may have an association with a unique service
   instance on the media server.  For example, a given media server can
   host multiple, separate conference sessions.  To identify unique
   service instances, a unique identifier modifies the service name.
   The unique identifier MUST meet the rules for a legal user part of a
   SIP URI as set out in [2].  An equal sign, US-ASCII %3D, MUST
   separate the service indicator from the unique identifier.

   Note that since the service indicator is case insensitive per [2],
   the service instance identifier is also case insensitive.


6. Operation

   The requesting client issues a SIP INVITE to the media server,
   specifying the requested service and any appropriate parameters.

   If the media server can perform the requested service, it does so,
   following the processing steps described in the service definition
   document (see IANA Considerations, below).

   If the media server cannot perform the requested service or does not
   recognize the service indicator, it MUST respond with the response
   code 488 NOT ACCEPTABLE HERE.  This is appropriate, as 488 refers to
   a problem with the user part of the URI.  Moreover, 606 is not
   appropriate, as some other media server may be able to satisfy the
   request.  [2] describes the 488 and 606 response codes.

   Some services require a unique identifier.  Most services
   automatically create a service instance upon the first INVITE with
   the given identifier.  However, if a service requires an existing
   service instance, and no such service instance exists on the media
   server, the media server MUST respond with the response code 404 NOT

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   FOUND.  This is appropriate as the service itself exists on the
   media server, but the particular service instance does not.  It is
   as if the user was not home.


7. Formal Syntax

   The following syntax specification uses the augmented Backus-Naur
   Form (BNF) as described in RFC-2234 [6].

   SERVICE-URL     = "sip:" [srvc-namespace] srvc-ind "@" hostport
                          url-parameters [headers]

   srvc-namespace  = ("org.iana" "." | 1*nmspc-part)

   nmspc-part      = 1*(ALPHA | DIGIT) "."

   srvc-ind        = srvc-name [ "=" instanceId ]

   srvc-name       = "annc" | "conf" | token

   instanceId      = token

   Section 2 of [2] defines the elements hostport and token.  See the
   IANA Considerations section for procedures for adding new service
   indicators.


8. Security Considerations

   The security issues are the same as for SIP [2], as the media server
   is simply a SIP User Agent.


9. IANA Considerations

   This document describes an extensible set of SIP Media Server
   Service Indicator types.  To promote interoperability and coherent
   interpretations of different types, we need a central repository for
   well-known service indicator types.

   IANA will create a repository for service indicator types called
   "SIP Media Server Service Indicator Types".  Following the policies
   outlined in [7], this repository is "Specification Required by RFC."
   The documents [8] and [10] describe the initial values for the
   repository.  For reference, the values are as follows.

   NOTE: Drafts describing service indicators for conferencing,
   transcoding and IVR are currently being developed.



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   SIP Media Server Service Indicator Types
   ========================================

   Value   Meaning                Reference
   -----   --------------         ---------
                   Parameter     Values
                   -----------   ------
   annc    Announcement Service   draft-burger-sipping-netann-01.txt
                   play=         URI or provisioned sequence identifier
                   early=        ( "yes" | "no" )
                   repeat=        Integer, number of repetitions
                   delay=         Integer, delay between repetitions
                   duration=      Integer, max. prompt duration
                   locale=       Language and country codes
                   param[n]=      Variable values to be substituted in
                   a sequence

   conf    Conference Service
                   <none>


10. Examples

   These examples are informative.  For the normative definitions of
   the given services, see the referenced documents.

   NOTE: The line wrapping (backslash, CRLF, and spacing before
   continued lines) in the examples is for readability purposes only.


10.1. Announcement

   The document [8] fully specifies the announcement service.  In
   brief, the announcement service can play a prompt as early media or
   after the establishment of a connection.

   The announcement service indicator is "annc".  The service has
   several associated parameters that control the content and delivery
   of the announcement.

   In the following example, the media server at ms2.carrier.net
   retrieves an audio file using HTTP [9] from the server
   prompts.carrier.net and plays it as early media.

        sip:annc@ms2.carrier.net; \
          play="http://prompts.carrier.net/audio/allcircuitsbusy.g711";
   \
          early=yes



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10.2. Conference

   The conference service creates a conference upon the first instance
   of a unique service instance identifier.  The media server places
   subsequent requests with the same service instance identifier into a
   conference.

   The conference service indicator is "conf".  There are no parameters
   for the conference service.

   In the following example, the media server at ms2.carrier.net
   creates (or places into conference) the stream associated with the
   SDP in the INVITE to the conference identified by the identifier
   "q4unfcqdscQS".

        sip:conf=q4unfcqdscQS@ms2.carrier.net

   NOTE: A draft describing the conference service in detail is in
   progress.



11. References

   1  Bradner, S., "The Internet Standards Process -- Revision 3", BCP
      9, RFC 2026, October 1996.

   2  Handley, M., Schulzrinne, H., Schooler, E., Rosenberg, J., "SIP:
      Session Initiation Protocol", draft-ietf-sip-rfc2543bis-03.txt,
      May 2001, work in progress.

   3  Bradner, S., "Key words for use in RFCs to Indicate Requirement
      Levels", BCP 14, RFC 2119, March 1997.

   4  Roach, A., "SIP-Specific Event Notification, "draft-ietf-sip-
      events-01.txt, November 2001, work in progress.

   6  Crocker, D. and Overell, P.(Editors), "Augmented BNF for Syntax
      Specifications: ABNF", RFC 2234, November 1997.

   7  Alvestrand, H. and Narten, T., "Guidelines for Writing an IANA
      Considerations Section in RFCs", BCP 26, RFC 2434, October 1998.

   8  O'Connor, W., Burger, E., "Network Announcements with SIP",
      draft-burger-sipping-netann-01.txt, November 2001, work in
      progress.

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



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12. Changes Made in Version 01
     o Added additional explanatory text to explain motivations for
        use of service indicators and benefits of the proposed format.
     o Updated description of the announcement service to be
        consistent with draft-burger-sipping-netann-01.txt.
     o Proposed an option for implementing a namespace for service
        indicators.


13. Acknowledgments

   We would like to offer our thanks to Jonathan Rosenberg of
   dynamicsoft for his constructive comments.


14. Author's Addresses

   Jeff Van Dyke
   SnowShore Networks, Inc.
   285 Billerica Rd.
   Chelmsford, MA  01824-4120
   USA

   Phone: +1 978/367-8405
   Email: jvandyke@snowshore.com


   Eric Burger
   SnowShore Networks, Inc.
   285 Billerica Rd.
   Chelmsford, MA  01824-4120
   USA

   Phone: +1 978/367-8403
   Email: eburger@snowshore.com



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