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Versions: 00 01 02 draft-ietf-mmusic-sdp-cs

MMUSIC WG                                               M. Garcia-Martin
Internet-Draft                                           S. Veikkolainen
Intended status: Standards Track                  Nokia Siemens Networks
Expires: October 17, 2008                                 April 15, 2008


 Conventions for the Use of the Session Description Protocol (SDP) for
  Circuit-Switched Bearer Connections in the Public Switched Telephone
                             Network (PSTN)
                     draft-garcia-mmusic-sdp-cs-01

Status of this Memo

   By submitting this Internet-Draft, each author represents that any
   applicable patent or other IPR claims of which he or she is aware
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   This Internet-Draft will expire on October 17, 2008.

Abstract

   This memo describes use cases, requirements, and protocol conventions
   for using the Session Description Protocol (SDP) Offer/Answer model
   for establishing circuit-switched bearer connections in the Public
   Switched Telephone Network (PSTN).  Additionally, this memo proposes
   conventions for using SDP and the SDP capability negotiation
   framework for expressing those circuit-switched bearer connections as
   alternatives to packet-switched streams.






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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Conventions Used in This Document  . . . . . . . . . . . . . .  4
   3.  Requirements . . . . . . . . . . . . . . . . . . . . . . . . .  4
   4.  Overview of Operation  . . . . . . . . . . . . . . . . . . . .  5
     4.1.  Example Call Flow  . . . . . . . . . . . . . . . . . . . .  5
   5.  Protocol Description . . . . . . . . . . . . . . . . . . . . .  7
     5.1.  Level of Compliance  . . . . . . . . . . . . . . . . . . .  7
     5.2.  Extensions to SDP  . . . . . . . . . . . . . . . . . . . .  7
       5.2.1.  Connection Data  . . . . . . . . . . . . . . . . . . .  7
       5.2.2.  Media Descriptions . . . . . . . . . . . . . . . . . .  8
       5.2.3.  Correlating the PSTN Circuit-Switched Bearer with
               SDP  . . . . . . . . . . . . . . . . . . . . . . . . .  9
         5.2.3.1.  Caller-ID Correlation Mechanism  . . . . . . . . .  9
         5.2.3.2.  User-User Information Element Correlation
                   Mechanism  . . . . . . . . . . . . . . . . . . . . 10
     5.3.  Considerations for Usage of Existing SDP . . . . . . . . . 11
       5.3.1.  Originator of the Session  . . . . . . . . . . . . . . 11
       5.3.2.  Determining the Direction of the Circuit-Switched
               Connection Setup . . . . . . . . . . . . . . . . . . . 12
     5.4.  Extensions to and Usage of the SDP Capability
           Negotiation Framework  . . . . . . . . . . . . . . . . . . 13
       5.4.1.  Offering alternative media streams . . . . . . . . . . 13
       5.4.2.  Using Additional Attributes with the SDP
               Capability Negotiation Framework . . . . . . . . . . . 16
     5.5.  Formal Syntax  . . . . . . . . . . . . . . . . . . . . . . 17
   6.  SDP Examples . . . . . . . . . . . . . . . . . . . . . . . . . 18
     6.1.  Basic SDP example: Single Circuit-Switched Audio Stream  . 19
     6.2.  PSTN Circuit-Switched Audio Bearer as an Alternative
           to RTP . . . . . . . . . . . . . . . . . . . . . . . . . . 19
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 21
     7.1.  Registration of new corr-id SDP attribute  . . . . . . . . 22
     7.2.  Registration of a new "nettype" value  . . . . . . . . . . 22
     7.3.  Registration of new "addrtype" values  . . . . . . . . . . 22
     7.4.  Registration of a new "proto" value  . . . . . . . . . . . 22
     7.5.  Registration of a new Capability Negotiation Option Tag  . 23
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 23
   9.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 23
   10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 23
     10.1. Normative References . . . . . . . . . . . . . . . . . . . 23
     10.2. Informative References . . . . . . . . . . . . . . . . . . 24
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 25
   Intellectual Property and Copyright Statements . . . . . . . . . . 26







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1.  Introduction

   The Session Description Protocol (SDP) [RFC4566] is intended for
   describing multimedia sessions for the purposes of session
   announcement, session invitation, and other forms of multimedia
   session initiation.  SDP is most commonly used for describing media
   streams that are transported over the Real-Time Transport Protocol
   (RTP) [RFC3550], using the profiles for audio and video media defined
   in RTP Profile for Audio and Video Conferences with Minimal Control
   [RFC3551].

   However, SDP can be used to describe other transport protocols than
   RTP.  Previous work includes SDP conventions for describing ATM
   bearer connections [RFC3108] and the Message Session Relay Protocol
   [RFC4975].

   SDP is commonly carried in Session Initiation Protocol (SIP)
   [RFC3261] messages in order to agree on a common media description
   among the endpoints.  An Offer/Answer Model with Session Description
   Protocol (SDP) [RFC3264] defines a framework by which two endpoints
   can exchange SDP media descriptions and come to an agreement as to
   which media streams should be used, along with the media related
   parameters.

   In some scenarios it might be desirable to establish the media stream
   over a circuit-switched bearer connection even if the signaling for
   the session is carried over an IP bearer.  An example of such a
   scenario is two mobile devices capable of both circuit-switched and
   packet-switched communication over a low-bandwidth radio bearer.  The
   radio bearer may not be suitable for carrying real-time audio media,
   and using a circuit-switched bearer would offer a better perceived
   quality of service.  So, according to this scenario, SDP and its
   higher layer session control protocol (e.g., the Session Initiation
   Protocol (SIP) [RFC3261]) are used over regular IP connectivity,
   while the audio is received through the classical circuit-switched
   bearer.

   Setting up a signaling relationship in the IP domain instead of just
   setting up a circuit-switched call offers also the possibility of
   negotiating in the same session other IP based media that is not
   constrained by jitter or delay, for example, text messaging or
   presence information.

   At a later point in time the mobile device might move to an area
   where a high-bandwidth packet-switched bearer, for example a Wireless
   Local Area Network (WLAN) connection, is available.  At this point
   the mobile device may perform a handover and move the audio media
   streams over to the high-speed bearer.  This implies a new exchange



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   of SDP offer/answer that lead to a re-negotiation of the media
   streams.

   The rest of the document is structured as follows: Section 2 provides
   the document conventions, Section 3 introduces the requirements,
   Section 4 presents an overview of the proposed solutions, and
   Section 5 contains the protocol description.  Section 6 provides a
   few examples of descriptions of circuit-switched audio streams in
   SDP.  Section 7 and Section 8 contain the IANA and Security
   considerations, respectively.


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


3.  Requirements

   This section presents the general requirements that are specific for
   the circuit-switched audio media stream.

   REQ-1:  A mechanism for endpoints to negotiate and agree on a
           circuit-switched bearer for audio media MUST be available.

   REQ-2:  The mechanism MUST allow the endpoints to combine circuit-
           switched audio media streams with other complementary media
           streams, for example, text messaging.

   REQ-3:  It MUST be possible for an endpoint to offer an audio stream
           where the circuit-switched bearer is an alternative to the IP
           bearer, and vice versa.

   REQ-4:  The mechanism MUST allow the endpoint to negotiate the
           direction of the circuit-switched connection, i.e., which
           endpoint is active when initiating the circuit-switched
           connection.

   REQ-5:  The mechanism MUST be independent of the type of the circuit-
           switched access (e.g., Integrated Services Digital Network
           (ISDN), Global System for Mobile Communication (GSM), etc.)






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   REQ-6:  The mechanism MUST be backwards compatible with SDP [RFC4566]
           and the SDP Offer/Answer Model [RFC3264] in the sense that if
           an endpoint offers a description of a circuit-switched audio
           stream in addition to a classical RTP-based audio stream, and
           the other endpoint supports only the classical RTP, then both
           endpoints can agree on the RTP-based audio stream, according
           to the rules in SDP offer/answer [RFC3264], and communication
           can still be possible.

   REQ-7:  There MUST be a mechanism that helps an endpoint to correlate
           an incoming PSTN call with the one negotiated in SDP, as
           opposed to another incoming call that is not related to that.


4.  Overview of Operation

   The mechanism defined in this memo extends SDP and allows describing
   a PSTN circuit-switched media stream in SDP.  New tokens are
   registered in the "c=" and "m=" lines to be able to describe a PSTN
   circuit-switched bearer.  These SDP extensions are described in
   Section 5.2.  Since circuit-switched bearers are a sort of
   connection-oriented media streams, the mechanism re-uses the
   connection-oriented extensions defined in RFC 4145 [RFC4145] to
   negotiate the active and passive sides of a connection setup.  This
   is further described in Section 5.3.2.

   Additionally, this memo provides optional means for negotiating an
   audio stream that can be established over any of two separate
   bearers: a regular IP bearer using RTP [RFC3550] and a circuit-
   switched bearer.  The endpoints agree on a given bearer and establish
   the media stream.  This is achieved by using the SDP capability
   negotiation [I-D.ietf-mmusic-sdp-capability-negotiation] and the SDP
   Media Capabilities Negotiation
   [I-D.ietf-mmusic-sdp-media-capabilities], for which this memo also
   include an extension to express the "c=" line as a capability.  This
   is further described in Section 5.4.1.

4.1.  Example Call Flow

   Consider the example presented in Figure 1.  In this example, Alice
   is located in an environment where she has access to both IP and
   circuit-switched bearers for communicating with other endpoints.
   Alice issues an SDP Offer containing two alternative audio media
   stream descriptions: one that uses a circuit-switched connection, and
   the other uses an IP bearer and RTP.






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                 Alice                                 Bob
                   | (1) SDP Offer (RTP and PSTN audio) |
                   |----------------------------------->|
                   |                                    |
                   | (2) SDP Answer (PSTN audio)        |
                   |<-----------------------------------|
                   |                                    |
                   |   PSTN call setup                  |
                   |<-----------------------------------|
                   |                                    |
                   |                                    |
                   |<===== media over PSTN bearer =====>|
                   |                                    |

                          Figure 1: Example Flow

   Bob receives the SDP Offer and determines that he is located in an
   environment where the IP based bearer is not suitable for real-time
   audio media, but he has PSTN circuit-switched bearer available for
   audio.  Bob sends back an SDP Answer where he selects the circuit-
   switched media stream description.

   During the offer-answer exchange Alice and Bob also agree the
   direction in which the circuit-switched connection should be
   established.  The exchange also contained identifiers or references
   that can be used on the circuit-switched network for addressing the
   other endpoint, as well as identifying that the incoming circuit-
   switched connection establishment is related to the ongoing session
   between Alice and Bob.

   Bob establishes a circuit-switched connection towards Alice using
   whatever mechanisms are defined for the network type in question.
   When receiving the incoming circuit-switched connection attempt,
   Alice is able to determine that the attempt is related to the session
   she has with Bob.

   Alice accepts the circuit-switched connection, and the circuit-
   switched connection setup is completed.  Bob and Alice can now use
   the circuit-switched connection for two-way audio media.

   If, for some reason Bob would like to reject an offered stream, he
   would set the port number of the specific stream to zero, as
   specified in RFC3264 [RFC3264].  Also, if Bob does not understand
   some of the SDP attributes specified in this document, he should
   ignore such parameters, as specified in RFC4566 [RFC4566].






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5.  Protocol Description

5.1.  Level of Compliance

   Implementations according to this specification MUST implement the
   SDP extensions described in Section 5.2, MUST implement the
   considerations discussed in Section 5.3, and MAY implement the
   mechanisms for offering alternative media streams discussed in
   Section 5.4.1.

5.2.  Extensions to SDP

   This section provides the syntax and semantics of the extensions
   required for providing a description of circuit-switched streams in
   SDP.

5.2.1.  Connection Data

   According to SDP [RFC4566], the connection data line in SDP has the
   following syntax:

      c=<nettype> <addrtype> <connection-address>

   where <nettype> indicates the network type, <addrtype> indicates the
   address type, and the <connection-address> is the connection address,
   which is dependent on the address type.

   At the moment, the only network type defined is "IN", which indicates
   Internet network type.  The address types "IP4" and "IP6" indicate
   the type of IP addresses.

   This memo defines a new network type for describing a circuit-
   switched network type in the PSTN.  The mnemonic "PSTN" is used for
   this network type.

   For the address type, we initially consider the possibility of
   describing E.164 telephone numbers.  We define a new "E164" address
   type.  When used, the "E164" address type indicates that the
   connection address contains a telephone number represented according
   to the ITU-T E.164 [ITU.E164.1991] specification.

   There are cases, though, when the endpoint is merely aware of a
   circuit-switched bearer, without having further information about the
   address type or the E.164 number allocated to it.  In these cases a
   dash "-"is used to indicate an unknown address type or connection
   address.  This makes the connection data line be according to the SDP
   syntax.




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      Note that <addrtype> and/or <connection-address> should not be
      omitted without being set to a "-" since this would violate basic
      syntax of SDP [RFC4566].

   The following are examples of the extension to the connection data
   line:

      c=PSTN E164 +15551234

      c=PSTN - -

5.2.2.  Media Descriptions

   According to SDP [RFC4566], the media descriptions line in SDP has
   the following syntax:

      m=<media> <port> <proto> <fmt> ...

   The <media> sub-field carries the media type.  Since this document
   deals with establishing an audio bearer, the currently defined
   "audio" media type is used.

   The <port> sub-field is the transport port to which the media stream
   is sent.  Circuit-switched access lacks the concept of a port number,
   and therefore the <port> sub-field is set to a dash "-".

   The <proto> sub-field is the transport protocol.  The circuit-
   switched bearer uses whatever transport protocol it has available.
   This subfield SHOULD be set to the mnemonic "PSTN" to be
   syntactically correct with SDP [RFC4566] and to indicate the usage of
   circuit-switched protocols in the PSTN.

   The <fmt> sub-field is the media format description.  In the
   classical usage of SDP to describe RTP-based media streams, when the
   <proto> sub-field is set to "RTP/AVP" or "RTP/SAVP", the <fmt> sub-
   field contains the payload types as defined in the RTP audio profile
   [RFC3551].

   In the case of circuit-switched descriptions in the PSTN, RTP is not
   really used.  Normally, the <fmt> sub-field carries the RTP payload
   numbers, but since the circuit-switched connection setup uses codec
   negotiation that is specific to used technology, listing the payload
   numbers or codecs is not required.  Therefore, the endpoint MUST
   include a single dash "-" in the <fmt> sub-field when describe a
   circuit-switched connection.

   Example of a media description for circuit-switched audio streams is:




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      m=audio - PSTN -

5.2.3.  Correlating the PSTN Circuit-Switched Bearer with SDP

   The endpoints should be able to correlate the circuit-switched
   connection with the session negotiated with SDP to avoid ringing for
   an incoming circuit-switched call that is related to the session
   controlled with SDP (and SIP).

   Several alternatives exist for performing this correlation.  This
   memo provides two mutually non-exclusive correlation mechanisms.
   Other correlation mechanisms might exist as well, and their usage
   will be specified when need arises.  The first mechanism is based on
   the exchange of PSTN caller-ID between the endpoints, which is also
   made available as the Calling Party ID in the circuit-switched
   signaling.  The second mechanism is based on the inclusion in SDP of
   the value of the User-to-User Information Element that is part of the
   call setup signaling in the PSTN.  Implementations MAY use any of
   these mechanisms and MAY use both mechanisms simultaneously.

5.2.3.1.  Caller-ID Correlation Mechanism

   The caller-ID correlation mechanisms consists of an exchange of the
   calling party number in E.164 format in SDP, followed by the
   availability of the Calling Party Number information element in the
   call setup signaling of the circuit switched connection.

   An endpoint that is feasible to become the active party for setting
   up the circuit-switched bearer SHOULD include its E.164 number in the
   <connection-address> field of the "c=" line.  If SDP capability
   negotiation framework is used, the endpoint SHOULD also include its
   E.164 number in the "a=ccap" capability line.  The 'ccap' capability
   attribute and its usage in the SDP capability negotiation framework
   is defined in Section 5.4.1.  An endpoint that acts as the passive
   party for setting up the circuit-switch bearer MAY include its E.164
   number in the same corresponding lines, although these are not used
   for correlation.

   Examples of inclusion of E.164 numbers in the "c=" and "a=ccap" lines
   are:

      c=PSTN E164 +15551234

      a=ccap:1 PSTN E164 +15551234

   Please note that there are no warranties that this correlation
   mechanism works or is even available, due a number of problems:




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   o  The endpoint might not be aware of its own E.164 number, in which
      case it cannot populate the SDP appropriately.

   o  The Calling Party Number information element in the circuit-
      switched signaling might not be available, e.g., due to policy
      restrictions of the network operator or caller restriction due to
      privacy.

   o  The Calling Party Number information element in the circuit-
      switched signaling might be available, but the digit
      representation of the E.164 number might differ from the one
      expressed in the SDP.  For example, one can be represented in
      international format and the other might only contain the
      significant national digits.  Therefore, an implementation may
      consider only some of the rightmost digits from the E.164 number
      for correlation.  For example, the numbers +358-1-555-12345 and
      01-555-12345 could be considered as one and the same number.  This
      is also the behavior of some phones, which try to display the
      caller's name if it is stored in the phonebook.

5.2.3.2.  User-User Information Element Correlation Mechanism

   A second correlation mechanism is based on indicating in SDP the
   User-User Information Element that is part of the call setup
   signaling of the circuit-switched bearer.  The User-User Information
   Element is specified in ITU-T Q.931 [ITU.Q931.1998] and 3GPP TS
   24.008 [3GPP.24.008], among others.  The User-User Information
   Element has a maximum size of 35 or 131 octets, depending on the
   actual message of the PSTN protocol where it is included.

   The mechanism works as follows: An endpoint creates a User-User
   Information Element, according to the requirement of the call setup
   signaling protocol.  The same value is included in the SDP offer or
   SDP answer, in a new attribute called 'corr-id', defined below.  When
   the SDP offer/answer exchange is completed, each endpoint has become
   aware of the value that will be used in the User-User Information
   Element.  The endpoint that initiates the call setup attempt includes
   this value in the User-User Information Element.  The recipient of
   the call setup attempt can extract the User-User Information Element
   and correlate it with the value previously received in the SDP.  If
   both values match, then the call attempt corresponds to that
   indicated in the SDP.

   Note that, for correlation purposes, the value of the User-User
   Information Element is considered as a opaque string and only used
   for correlation purposes.  Typically call signaling protocols impose
   requirements on the creation of User-User Information Element for
   end-user protocol exchange.  The details regarding the generation of



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   the User-User Information Element are outside the scope of this
   specification.

   This specification defines a new SDP attribute, called 'corr-id',
   whose purpose is to include the User-User Information Element that
   the endpoint will include in the call setup attempt.  The 'corr-id'
   attribute has the following format:

      a=corr-id:corr-token

   An endpoint that is feasible to become the active party for setting
   up the PSTN call SHOULD include in the 'corr-id' attribute the value
   of the User-User Information Element that will be used in the PSTN
   call setup attempt.  If both the SDP offerer and the SDP answerer are
   able to become the active party, each one SHOULD include a
   correlation value.  Then the party that becomes active in setting up
   the PSTN circuit-switched call includes this value in the User-User
   information element of the call signaling setup.  The passive party
   is able to inspect the received value of User-User Information
   Element and correlate it with that received in the SDP in the
   'corr-id' attribute.  An endpoint that takes the role of the passive
   party for setting up the circuit-switched bearer MAY include a
   'corr-id' attribute in the SDP, although it is not used for
   correlation purposes.

   Please not that there are no warranties that this correlation
   mechanism works.  On one side, policy restrictions might not make the
   User-User information available end to end in the PSTN.  On the other
   hand, the generation of the User-User Information Element is
   controlled by the PSTN circuit-switched call protocol, which might
   not offer enough freedom for generating different values from one
   endpoint to another one, or from one call to another in the same
   endpoint.  This might result in the same value of the User-User
   Information Element for all calls.

5.3.  Considerations for Usage of Existing SDP

5.3.1.  Originator of the Session

   According to SDP [RFC4566], the origin line in SDP has the following
   syntax:

      o=<username> <sess-id> <sess-version> <nettype> <addrtype>
      <unicast-address>

   Of interest here are the <nettype> and <addrtype> fields, which
   indicate the type of network and type of address, respectively.
   Typically, this field carries the IP address of the originator of the



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   session.  Even if the SDP was used to negotiate a circuit-switched
   media stream, the originator is using SDP over an IP bearer.
   Therefore, <nettype> and <addrtype> fields in the "o=" line should be
   populated with the IP address identifying the source of the
   signaling.

5.3.2.  Determining the Direction of the Circuit-Switched Connection
        Setup

   The circuit-switched connection can typically be initiated by either
   endpoint.  In order to avoid a situation where both endpoints attempt
   to initiate a connection simultaneously, the direction in which the
   circuit-switched connection is set up should be negotiated during the
   Offer/Answer exchange.

   The framework defined in RFC 4145 [RFC4145] allows the endpoints to
   agree which endpoint acts as the active endpoint when initiating a
   TCP connection.  While RFC 4145 [RFC4145] was originally designed for
   establishing TCP connections, this specification uses the concepts in
   RFC 4145 [RFC4145] for establishing circuit-switched connections.

   RFC 4145 [RFC4145] defines two new attributes in SDP: 'setup' and
   'connection'.  The 'setup' attribute indicates which of the endpoints
   should initiate the connection establishment of the PSTN circuit-
   switched bearer.  Four values are defined in Section 4 of RFC 4145
   [RFC4145]: "active", "passive", "actpass", "holdconn".  Please refer
   to Section 4 of RFC 4145 [RFC4145] for a detailed description of this
   attribute.

   The 'connection' attribute indicates whether a new connection is
   needed, or an existing connection is reused.  The attribute can take
   the values "new" or "existing".  Please refer to Section 5 of RFC
   4145 [RFC4145] for a detailed description of this attribute.

   Implementations according to this specification MUST support the
   'setup' and 'connection' attributes specified in RFC 4145 [RFC4145],
   but applied to circuit-switched bearers in the PSTN.

   In order to establish a circuit-switched connection in the PSTN, the
   initiating endpoint needs to know the address (E.164 number) of the
   other endpoint.  Therefore, if an endpoint wants to be able to
   receive incoming circuit-switched calls, it must know its E.164
   number and must indicate it in SDP.  As a consequence, an endpoint
   that is not aware of its own E.164 number cannot take the role of the
   passive side with respect the establishment of the circuit-switched
   connection.

   If the SDP capability negotiation mechanism is used the endpoint



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   SHOULD include the 'setup' and 'connection' attributes as
   capabilities of the PSTN circuit-switched bearer description, unless
   these two attributes are defined at session-level affecting all
   media-level descriptions.  This implies that there would be "a=acap"
   lines that describe the 'setup' and 'connection' attributes.  For
   example:

      a=acap:1 setup:active a=acap:2 connection:new

5.4.  Extensions to and Usage of the SDP Capability Negotiation
      Framework

5.4.1.  Offering alternative media streams

   In many cases where circuit-switched audio streams are described in
   SDP it is foreseen that circuit-switched bearers will be offered as
   an alternative to regular IP bearers for the same media stream.
   Therefore, it is reasonable to provide a mechanism to define circuit-
   switched bearers as an alternative to an RTP-based IP bearers.

   To offer an audio media stream using alternatively RTP or circuit-
   switched bearers, this specification reuses some of the capability
   attributes defined in SDP Capability Negotiation framework
   [I-D.ietf-mmusic-sdp-capability-negotiation] as well as in SDP Media
   Capabilities Negotiation [I-D.ietf-mmusic-sdp-media-capabilities].
   Additionally, this memo defines a new capability attribute 'ccap' in
   SDP that allows to express a connection address as a capability.

   Section 5.4 describes the extensions to and usage of the SDP
   Capability Negotiation [I-D.ietf-mmusic-sdp-capability-negotiation]
   and SDP Media Capabilities Negotiation
   [I-D.ietf-mmusic-sdp-media-capabilities] for offering alternative
   PSTN circuit-switched media streams.  Section 5.4.2 describes the
   usage of 'corr-id', 'setup', and 'connection' attributes in
   conjunction with the SDP Capability Negotiation framework
   [I-D.ietf-mmusic-sdp-capability-negotiation].

   The 'mcap' media capability attribute defined in the SDP Media
   Capabilities Negotiation [I-D.ietf-mmusic-sdp-media-capabilities]
   lists media formats as capabilities in the form a media type and one
   or more subtypes.

   An example provided in [I-D.ietf-mmusic-sdp-media-capabilities] lists
   four audio media subtypes which are numbered consecutively (starting
   from 1 in this example).






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      a=mcap:1 audio g729 iLBC PCMU g729

   Similarly, we can use the 'mcap' capability attribute to indicate
   media capabilities that correspond to the "m=" line defined in
   Section 5.2.2.

      a=mcap:1 audio -

   Here, we declare a media capability with no specific subtype.  The
   media capability has nevertheless a number associated with it as
   specified in SDP Media Capabilities Negotiation
   [I-D.ietf-mmusic-sdp-media-capabilities].  In this example the
   associated number is 1.

   Transport Protocols can be expressed as capabilities with the 'tcap'
   capability attribute defined in SDP Capability Negotiation
   [I-D.ietf-mmusic-sdp-capability-negotiation].  The 'tcap' capability
   attribute lists one or more <proto> elements, defined in SDP
   [RFC4566].

   An example of transport protocol capability indicating "PSTN"
   transport protocol defined in this document would thus be:

      a=tcap:1 PSTN

   In this document we define a new capability attribute: the connection
   address capability attribute, 'ccap'.  The connection address
   capability lists connection addresses as capabilities, and is defined
   as follows:

      a=ccap:<c-cap-num> <c-cap-attr> *[ <c-cap-attr>]

   where <c-cap-num> is an integer between 1 and 2^31-1 (both included)
   used to number the connection address capability attribute.

   The <c-cap-attr> field consists of network type, address type and a
   connection address, as specified for a "c=" line in SDP [RFC4566].
   As an example, to list <nettype> value of "PSTN", <addrtype> value of
   "E164", and a <connection-address> value of "+15551234" as a
   connection capability attribute, we get:

      a=ccap:1 PSTN E164 +15551234

   The 'pcfg' attribute is defined in the SDP Capabilities Negotiation
   [I-D.ietf-mmusic-sdp-capability-negotiation].  We extend this
   attribute to be able to convey one or more connection capability
   numbers.




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   The 'pcfg' attribute has he following definition:

      a=pcfg: <config-number> [<pot-cfg-list>]

   We extend the <pot-cfg-list> field to according to the following
   definition:

   pot-connection-config  = connection-config-list
   connection-config-list = "c=" c-cap-list *(BAR c-cap-list)
   c-cap-list             = c-cap-num *("," c-cap-num)
   c-cap-num              = 1*DIGIT
   ; BAR defined in SDP capabilities
   ; negotiation

              Figure 2: Syntax of the pot-cfg-list Extension

   Each potential connection configuration is a comma-separated list of
   connection capability numbers where 'c-cap-num' refers to connection
   capability numbers defined explicitly by 'ccap' attributes and hence
   must be between 1 and 2^31-1 (both included).  Alternative potential
   connection configurations are separated by a vertical bar ("|").

   The SDP Capability Negotiation Framework provides a 'creq' attribute
   that lists the extensions to the framework that are required to be
   supported by the recipient of the SDP.  The attribute lists one or
   more option tags that are required for the endpoints to do
   capabilities negotiation.  SDP Capabilities Negotiation Framework
   [I-D.ietf-mmusic-sdp-capability-negotiation] defines the base option
   "cap-v0".  SDP media capabilities negotiation
   [I-D.ietf-mmusic-sdp-media-capabilities] defines the "med-v0" option
   tag that indicates a requirement for media capabilities.

   This memo defines a new extension to the SDP Capabilities Negotiation
   Framework [I-D.ietf-mmusic-sdp-capability-negotiation].
   Consequently, this memo defines a new option tag "pstn-v0" to the
   'creq' attribute.  Implementations according to this specification
   MUST include the "pstn-V0" and "med-v0" option tags in the 'creq'
   attribute when they use the SDP capability negotiation framework for
   negotiating circuit-switched media streams in the PSTN.  Note that
   the base option tag "cap-v0" need not be signaled.

   An example SDP consisting of two alternative media stream is as
   follows:








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   m=audio 49170 RTP/AVP 0 8 3
   c=IN IP4 192.0.2.1
   a=creq:med-v0,pstn-v0
   a=mcap:1 PCMU/8000/1
   a=mcap:2 PCMA/8000/1
   a=mcap:3 GSM/8000/1
   a=mcap:4 -
   a=tcap:1 RTP/AVP PSTN
   a=ccap:1 IN IP4 192.0.2.1
   a=ccap:2 PSTN E164 +15551234
   a=pcfg:1 t=1 m=1,2,3 c=1
   a=pcfg:2 t=2 m=4 c=2

       Figure 3: Detail of SDP offer with alternative media streams

   In the example of Figure 3, the SDP defines a few media capabilities
   "a=mcap", numbered 1, 2, and 3, that specify PCMU, PCMA, and GSM
   format, respectively.  A forth media capability "a=mcap" specifies an
   unknown format (used for the PSTN connection).  A transport
   capability 1 that defines RTP/AVP, and the transport capability
   number 2 defines the PSTN protocol type.  There are two connection
   capabilities "a=ccap" that define the IP and PSTN connection data,
   respectively.  Two potential configurations are supplied, numbered 1
   and 2.  Potential configuration number 1, which has higher priority,
   defines the IP connectivity with RTP by selecting the same parameters
   that compose the definition in the classical "m=" and "c=" lines.
   Potential configuration number 2 defines a PSTN connectivity by
   selecting the appropriate transport, media, and connection
   capabilities.

   An example SDP answer to the offer presented in Figure 3 where PSTN
   audio has been selected as the actual configuration is as follows:

   m=audio - PSTN -
   c=PSTN E164 +15550987654
   a=acfg:2 t=2 m=4 c=2

     Figure 4: Detail of SDP answer with PSTN circuit-switched bearer
                                 selected

   The answer contains the "a=acfg" line selected configuration.  The
   answer carries the accepted configuration in the "m=" and "c=" lines.

5.4.2.  Using Additional Attributes with the SDP Capability Negotiation
        Framework

   Section 5.2.3.2 describes a correlation mechanism based on the usage
   of the User-User Information Element of the PSTN signaling protocol



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   together with a new 'corr-id' attribute.  When the SDP capability
   negotiation framework is used in conjunction with the User-User
   Information Element, the endpoint MUST include the 'corr-id'
   attribute as a capability of the PSTN circuit-switched bearer
   description.  This implies that there would be an "a=acap" line that
   describes the 'corr-id' attribute.  For example:

      a=acap:1 corr-id:2890W284hAT452612908awudfjang908

   Section 5.3.2 describes the usage of the connection-oriented media
   attributes 'setup' and 'connection'.  When 'setup' and 'connection'
   attributes are used in conjunction with the SDP Capability
   Negotiation framework both attributes SHOULD be Included in the SDP
   as "a=acap" capability attributes.  An exception to the rule is
   achieved when these attributes are equally applicable to the RTP and
   PSTN descriptions, in which case, they can be included in their
   regular format rather than as capability descriptions.

5.5.  Formal Syntax

   The following is the formal Augmented Backus-Naur Form (ABNF)
   [RFC5234] syntax that supports the extensions defined in his
   specification.  The syntax is built above the SDP [RFC4566] grammar
   and the SDP capability negotiation
   [I-D.ietf-mmusic-sdp-capability-negotiation].  Implementations
   according to this specification MUST implement this syntax.

























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   ;subrules for ccap and corr-id attribute
   attribute          = corr-id-attr / ccap-attr
                             ; attribute defined in RFC 4566

   corr-id-attr       = "corr-id:" corr-id-value
   corr-id-value      =  1*32(ALPHA/DIGIT)

   ccap- attr         =  "ccap:" c-cap-num SP c-cap-attr
   c-cap-num =           1*DIGIT
   c-cap-attr =          connection-field
                         ; connection-field defined in RFC 4566

   ; subrules for the potential configuration attribute
   ; according to SDP capability negotiation, included
   ; here to get into context (thus, commented)
   ;
   ; a=pcfg: <config-number> [<pot-cfg-list>]
   ;     pot-cfg-list = pot-config *(1*WSP pot-config)
   ;     pot-config   = attribute-config-list /
   ;                    transport-protocol-config-list /
   ;                    extension-config-list
   ;
   ; extension to the pot-config to include a "c=" line
   ;
   pot-config         = pot-conn-config
                             ; pot-config defined in SDP
                             ; capability negotiation

   pot-conn-config    = "c=" c-cap-list *(BAR c-cap-list)
                             ; BAR defined in SDP
                             ; capability negotiation

   c-cap-list         = c-cap-num *("," c-cap-num)

                  Figure 5: Syntax of the SDP extensions


6.  SDP Examples













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6.1.  Basic SDP example: Single Circuit-Switched Audio Stream

                  Alice                               Bob
                    | (1) SDP Offer (PSTN audio)       |
                    |--------------------------------->|
                    |                                  |
                    | (2) SDP Answer (PSTN audio)      |
                    |<---------------------------------|
                    |                                  |
                    |   PSTN call setup                |
                    |<---------------------------------|
                    |                                  |
                    |<==== media over PSTN bearer ====>|
                    |                                  |

                           Figure 6: Basic flow

   Figure 6 shows a basic example that describes a single audio stream
   over a circuit-switched bearer.  The SDP offer is show in Figure 7.
   The endpoint describes a PSTN circuit-switched bearer in the "m=" and
   "c=" line where it also indicates its E.164 number.  Additionally, it
   expresses that it can initiate the circuit-switched connection or be
   the recipient of it.  The SDP offer also includes a correlation
   identifier that this endpoint will be inserting the User-User
   Information Element of the PSTN call setup if eventually this
   endpoint initiates the PSTN call.

   v=0
   o=jdoe 2890844526 2890842807 IN IP4 192.0.2.5
   s=
   t=0 0
   m=audio - PSTN -
   c=PSTN E164 +15551234
   a=setup:actpass
   a=connection:new
   a=corr-id:2890W284hAT452612908awudfjang908

                          Figure 7: SDP offer (1)

6.2.  PSTN Circuit-Switched Audio Bearer as an Alternative to RTP

   Figure 9 provides an example of SDP consisting of two alternative
   audio media streams, one using RTP over an IP bearer, the other using
   PSTN bearer.  The SDP offerer describes the PCMU, PCMA, and GSM
   payload types for RTP usage.  It also indicates that can initiate or
   receive the PSTN connection.





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                 Alice                                 Bob
                   | (1) SDP Offer (PSTN and RTP audio) |
                   |----------------------------------->|
                   |                                    |
                   | (2) SDP Answer (PSTN audio)        |
                   |<-----------------------------------|
                   |                                    |
                   | (3) SDP Offer (PSTN audio)         |
                   |----------------------------------->|
                   |                                    |
                   | (4) SDP Answer (PSTN audio)        |
                   |<-----------------------------------|
                   |                                    |
                   |   PSTN call setup                  |
                   |<-----------------------------------|
                   |                                    |
                   |<===== media over PSTN bearer =====>|
                   |                                    |

    Figure 8: Flow describing alternative PSTN Circuit-Switched bearer
                    and RTP bearer for an audio stream

   Figure 8 shows an example where the initial SDP offer (1) describes a
   single audio media stream that can be either setup over RTP or over
   the PSTN.  This is done using the SDP capability negotiation
   [I-D.ietf-mmusic-sdp-capability-negotiation].  The SDP answerer (2)
   selects the PSTN bearer.  According to SDP capability negotiation
   [I-D.ietf-mmusic-sdp-capability-negotiation], once an alternative
   media stream is selected, a new SDP offer (3) and SDP answer (4) are
   required ), mainly to help intermediaries which do not implement the
   capability negotiation mechanism.




















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   v=0
   o=jdoe 2890844526 2890842807 IN IP4 192.0.2.5
   s=
   t=0 0
   m=audio 49170 RTP/AVP 0 8 3
   c=IN IP4 192.0.2.5
   a=creq:med-v0,pstn-v0
   a=mcap:1 PCMU/8000/1
   a=mcap:2 PCMA/8000/1
   a=mcap:3 GSM/8000/1
   a=mcap:4 -
   a=tcap:1 RTP/AVP PSTN
   a=ccap:1 IN IP4 192.0.2.1
   a=ccap:2 PSTN E164 +15551234
   a=acap:1 corr-id:2890W284hAT452612908awudfjang908
   a=acap:2 setup:actpass
   a=acap:3 connection:new
   a=pcfg:1 t=1 m=1,2,3 c=1
   a=pcfg:2 t=2 m=4 c=2 a=1,2,3

     Figure 9: Example of an SDP offer with alternative media streams

   The SDP answerer replies with the SDP of Figure 10 where the PSTN
   audio stream is selected.

   v=0
   o=- 2890973824 2890987289 IN IP4 192.0.2.7
   s=
   t=0 0
   m=audio - PSTN -
   c=PSTN - -
   a=acfg:2
   a=setup:active
   a=connection:new
   a=corr-id:2890W284hAT452612908awudfjang908

   Figure 10: Example of an SDP answer with PSTN Circuit-Switched bearer
                                 selected


7.  IANA Considerations

   This document instructs IANA to register a number of SDP tokens
   according to the following data.







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7.1.  Registration of new corr-id SDP attribute

      Contact.  Miguel Garcia <miguel.garcia@nsn.com>

      Phone: +358 71400 4000

      Attribute name: corr-id

      Long-form attribute name: PSTN Correlation Identifier

      Type of attribute: media level only

      This attribute is subject to the charset attribute

      Description: This attribute provides the Correlation Identifier
      used in PSTN signaling

      Specification: RFC XXXX

7.2.  Registration of a new "nettype" value

   This memo provides instructions to IANA to register a new "nettype"
   in the Session Description Protocol Parameters registry [1].  The
   registration data, according to RFC 4566 [RFC4566] follows.

   Type            SDP Name                     Reference
   ----            ------------------           ---------
   nettype         PSTN                         [RFCxxxx]

7.3.  Registration of new "addrtype" values

   This memo provides instructions to IANA to register a new "addrtype"
   in the Session Description Protocol Parameters registry [1].  The
   registration data, according to RFC 4566 [RFC4566] follows.

   Type            SDP Name                     Reference
   ----            ------------------           ---------
   addrtype        E164                         [RFCxxxx]
                   -                            [RFCxxxx]

7.4.  Registration of a new "proto" value

   This memo provides instructions to IANA to register a new "proto" in
   the Session Description Protocol Parameters registry [1].  The
   registration data, according to RFC 4566 [RFC4566] follows.

   Type            SDP Name                     Reference
   --------------  ---------------------------  ---------



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   proto           PSTN                         [RFCxxxx]

7.5.  Registration of a new Capability Negotiation Option Tag

   This memo instructs IANA to register a new option tag in the SDP
   Capability Negotiation Option Tag registry, according to the
   following data:.

      Option Tag name: pstn-v0

      Reference: [RFCxxxx]


8.  Security Considerations

   This document provides an extension on top of RFC 4566 [RFC4566], RFC
   3264 [RFC3264], SDP capability negotiation
   [I-D.ietf-mmusic-sdp-capability-negotiation], SDP Media Capabilities
   Negotiation [I-D.ietf-mmusic-sdp-media-capabilities].  As such, the
   security considerations of those documents apply.


9.  Acknowledgments

   The authors want to thank Flemming Andreasen, Thomas Belling, Jari
   Mutikainen, Miikka Poikselka, Jonathan Rosenberg, and Ingemar
   Johansson for providing their insight and comments on this document.


10.  References

10.1.  Normative References

   [I-D.ietf-mmusic-sdp-capability-negotiation]
              Andreasen, F., "SDP Capability Negotiation",
              draft-ietf-mmusic-sdp-capability-negotiation-08 (work in
              progress), December 2007.

   [I-D.ietf-mmusic-sdp-media-capabilities]
              Gilman, R. and F. Andreasen, "SDP media capabilities
              Negotiation", draft-ietf-mmusic-sdp-media-capabilities-03
              (work in progress), February 2008.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC3108]  Kumar, R. and M. Mostafa, "Conventions for the use of the
              Session Description Protocol (SDP) for ATM Bearer



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              Connections", RFC 3108, May 2001.

   [RFC3264]  Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
              with Session Description Protocol (SDP)", RFC 3264,
              June 2002.

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

   [RFC4566]  Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
              Description Protocol", RFC 4566, July 2006.

   [RFC5234]  Crocker, D. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234, January 2008.

10.2.  Informative References

   [3GPP.24.008]
              3GPP, "Mobile radio interface Layer 3 specification; Core
              network protocols; Stage 3", 3GPP TS 24.008 3.20.0,
              December 2005.

   [ITU.E164.1991]
              International Telecommunications Union, "The International
              Public Telecommunication Numbering Plan", ITU-
              T Recommendation E.164, 1991.

   [ITU.Q931.1998]
              "Digital Subscriber Signalling System No. 1 (DSS 1) - ISDN
              User - Network Interface Layer 3 Specification for Basic
              Call Control", ISO Standard 9594-1, May 1998.

   [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.

   [RFC3550]  Schulzrinne, H., Casner, S., Frederick, R., and V.
              Jacobson, "RTP: A Transport Protocol for Real-Time
              Applications", STD 64, RFC 3550, July 2003.

   [RFC3551]  Schulzrinne, H. and S. Casner, "RTP Profile for Audio and
              Video Conferences with Minimal Control", STD 65, RFC 3551,
              July 2003.

   [RFC4975]  Campbell, B., Mahy, R., and C. Jennings, "The Message
              Session Relay Protocol (MSRP)", RFC 4975, September 2007.



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URIs

   [1]  <http://www.iana.org/assignments/sdp-parameters>


Authors' Addresses

   Miguel Garcia-Martin
   Nokia Siemens Networks
   P.O. Box 6
   Nokia Siemens Networks, FIN  02022
   Finland

   Phone: +358-71400-4000
   Email: miguel.garcia@nsn.com


   Simo Veikkolainen
   Nokia Siemens Networks
   P.O. Box 6
   Nokia Siemens Networks, FIN  02022
   Finland

   Phone: +358 50 486 4463
   Email: simo.veikkolainen@nsn.com


























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Full Copyright Statement

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   WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.


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