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

MMUSIC WG                                               M. Garcia-Martin
Internet-Draft                                                  Ericsson
Intended status: Standards Track                         S. Veikkolainen
Expires: May 3, 2009                                               Nokia
                                                        October 30, 2008


Session Description Protocol (SDP) Extension For Setting Up Audio Media
 Streams Over Circuit-Switched Bearers In The Public Switched Telephone
                             Network (PSTN)
                     draft-garcia-mmusic-sdp-cs-02

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
   have been or will be disclosed, and any of which he or she becomes
   aware will be disclosed, in accordance with Section 6 of BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on May 3, 2009.

Abstract

   This memo describes use cases, requirements, and protocol extensions
   for using the Session Description Protocol (SDP) Offer/Answer model
   for establishing audio media streams over circuit-switched bearers in
   the Public Switched Telephone Network (PSTN).








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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 . . . . . . . . . 12
       5.3.1.  Originator of the Session  . . . . . . . . . . . . . . 12
       5.3.2.  Contact Information  . . . . . . . . . . . . . . . . . 12
       5.3.3.  Determining the Direction for Setting Up the
               Circuit-Switched Bearer  . . . . . . . . . . . . . . . 12
     5.4.  Formal Syntax  . . . . . . . . . . . . . . . . . . . . . . 13
   6.  SDP Examples . . . . . . . . . . . . . . . . . . . . . . . . . 14
     6.1.  Basic SDP Example: Single Circuit-Switched Audio Stream  . 14
     6.2.  Advanced SDP Example: Alternative IP and
           Circuit-Switched Audio Streams . . . . . . . . . . . . . . 15
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 16
     7.1.  Registration of a New 'corr-id' SDP Attribute  . . . . . . 17
     7.2.  Registration of a New "nettype" value  . . . . . . . . . . 17
     7.3.  Registration of New "addrtype" values  . . . . . . . . . . 17
     7.4.  Registration of a New "proto" value  . . . . . . . . . . . 17
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 18
   9.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 18
   10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 18
     10.1. Normative References . . . . . . . . . . . . . . . . . . . 18
     10.2. Informative References . . . . . . . . . . . . . . . . . . 18
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 20
   Intellectual Property and Copyright Statements . . . . . . . . . . 21












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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 illustrated with 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, however, 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
   sensitive to jitter and 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.

   Other use cases exists.  For example, and endpoint might have at its
   disposal circuit-switch and packet-switched connectivity, but the
   audio codecs are not the same in both access networks.  Consider that
   the circuit-switched audio stream supports narrow-bandwidth codecs,
   while the packet-switched access allows any other audio codec
   implemented in the endpoint.  In this case, it might be beneficial
   for the endpoint to describe different codecs for each access type
   and get an agreement on the bearer together with the remote endpoint.

   There are additional use cases related to third party call control
   where the session setup time is improved when the circuit-switched
   bearer in the PSTN is described together with one or more codecs.

   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 audio media stream over circuit-switched bearers.

   REQ-1:  A mechanism for endpoints to negotiate and agree on an audio
           media stream established over a circuit-switched bearer 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.





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   REQ-3:  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-4:  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.)

   REQ-5:  There MUST be a mechanism that helps an endpoint to correlate
           an incoming circuit-switched call with the one negotiated in
           SDP, as opposed to another incoming call that is not related
           to that.

   REQ-6:  It must be possible for endpoints to advertise different list
           of audio codecs in the circuit-switched audio stream from
           those used in a packet-switched audio stream.

   REQ-7:  It must be possible for endpoints to not advertise the list
           of available codecs for circuit-switched audio streams.


4.  Overview of Operation

   The mechanism defined in this memo extends SDP and allows describing
   an audio media stream established over a circuit-switched bearer.
   New tokens are registered in the "c=" and "m=" lines to be able to
   describe an audio media stream over a 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 5.3.3.

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 decides that the circuit-switched bearer offers a better
   perceived quality of service for voice, and issues an SDP Offer
   containing the description of an audio media stream over circuit-
   switched bearer.








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                 Alice                                 Bob
                   | (1) SDP Offer (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.  However he also has PSTN circuit-switched bearer
   available for audio.  Bob generates an SDP answer containing a
   description of the audio media stream over a circuit-switched bearer.

   During the offer-answer exchange Alice and Bob also agree the
   direction in which the circuit-switched connection should be
   established.  The exchange also contains 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 bearer establishment is related to the ongoing session
   between Alice and Bob.

   Bob establishes a circuit-switched bearer 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 is just establishing with Bob.

   Alice accepts the circuit-switched connection; the circuit-switched
   bearer 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 the 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 would
   ignore them, 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, and MUST implement the
   considerations discussed in Section 5.3.

5.2.  Extensions to SDP

   This section provides the syntax and semantics of the extensions
   required for providing a description of audio media streams over
   circuit-switched bearers 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 bearer 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] recommendation.

   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 existing "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 the discard port "9".

   According to RFC 3264 [RFC3264], a port number of zero in the offer
   of a unicast stream indicates that the stream is offered but must not
   be used.  If a port number of zero is present in the answer of a
   unicast stream, it indicates that the stream is rejected.  These
   rules are still valid when the media line in SDP represents a
   circuit-switched bearer.

   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, RTP is not really used.
   Rather than specifying the RTP audio profile payload type, we use the
   <fmt> sub-field to indicate the list of available media types over



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   the circuit-switched bearer.  Therefore, the <fmt> sub-field MAY
   indicate one or more available audio codecs for a circuit-switched
   audio stream.  We use the classical RTP audio media types, even when
   applied to PSTN circuit-switched bearers, the media type merely
   represents an audio codec.

   However, in some cases, the endpoint is not able to determine the
   list of available codecs for circuit-switched audio streams.  In this
   case, in order to be syntactically compliant with SDP [RFC4566], the
   endpoint MUST include a single dash "-" in the <fmt> sub-field.

   As per RFC 4566 [RFC4566], the media format descriptions are listed
   in priority order.

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

      m=audio 9 PSTN 3 0 8

      m=audio 9 PSTN -

5.2.3.  Correlating the PSTN Circuit-Switched Bearer with SDP

   The endpoints should be able to correlate the circuit-switched bearer
   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.  An endpoint that acts



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   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.

   Example of inclusion of E.164 number in the "c=" line is:

      c=PSTN E164 +15551234

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

   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 the same number.  This is also
      the behavior of some cellular phones, which correlate the incoming
      calling party with a number stored in the phone book, for the
      purpose of displaying the caller's name.

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



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   Element of the call establishment message of the PSTN protocol.  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
   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 note 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.




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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
   session.  Even if the SDP was used to negotiate an audio media stream
   transported over a circuit-switched bearer, 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.  Contact Information

   SDP [RFC4566] defines the "p=" line which may include the phone
   number of the person reponsible for the conference.  Even though this
   line can carry a phone number, it is not suited for the purpose of
   defining a connection address for the media.  Therefore, we have
   selected to define the PSTN specific connection addresses in the "c="
   line.

5.3.3.  Determining the Direction for Setting Up the Circuit-Switched
        Bearer

   Either endpoint can initiate the establishment of the circuit-
   switched bearer.  In order to avoid a situation where both endpoints
   attempt to initiate a connection simultaneously, the direction in
   which the circuit-switched bearer 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, it is easily extrapolated to the
   connection establishment of circuit-switched bearers.  This
   specification uses the concepts specified in RFC 4145 [RFC4145] for
   agreeing on the direction of establishment of a circuit-switched
   bearer.

   RFC 4145 [RFC4145] defines two new attributes in SDP: 'setup' and
   'connection'.  The 'setup' attribute indicates which of the endpoints



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   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.

5.4.  Formal Syntax

   The following is the formal Augmented Backus-Naur Form (ABNF)
   [RFC5234] syntax that supports the extensions defined in this
   specification.  The syntax is built above the SDP [RFC4566] grammar.
   Implementations according to this specification MUST be compliant
   with this syntax.

   Figure 2 shows the formal syntax of the extensions defined in this
   memo.

















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   ;extension to the connection field originally specified in RFC 4566
   connection-field   =  [%x63 "=" nettype SP addrtype SP
                         connection-address CRLF]
                         ;nettype and addrtype are defined in RFC 4566

   connection-address =  multicast-address / unicast-address /
                         e164-address / "-"
                         ; multicast-address and unicast-address are
                         ; defined in RFC 4566

   e164-address       =  ["+"] 1*15DIGIT
                         ; DIGIT is specified in RFC 5234



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

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


                  Figure 2: Syntax of the SDP extensions


6.  SDP Examples

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 3: Basic flow

   Figure 3 shows a basic example that describes a single audio media
   stream over a circuit-switched bearer.  The SDP offer is show in



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   Figure 4.  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 9 PSTN -
   c=PSTN E164 +15551234
   a=setup:actpass
   a=connection:new
   a=corr-id:2890W284hAT452612908awudfjang908

                          Figure 4: SDP offer (1)

6.2.  Advanced SDP Example: Alternative IP and Circuit-Switched Audio
      Streams

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

                        Figure 5: Alternative media

   Figure 5 shows an example of negotiating audio media streams over IP
   or circuit-switched bearers.  Using the mechanisms described in SDP
   Capability Negotiation Framework
   [I-D.ietf-mmusic-sdp-capability-negotiation] and extensions thereof
   (SDP media capabilities Negotiation
   [I-D.ietf-mmusic-sdp-media-capabilities] and SDP Miscellaneous
   Capabilities [I-D.garcia-mmusic-sdp-misc-cap]) it is possible to
   construct an SDP offer where audio media can be offered alternatively
   over IP or circuit-switched bearer.



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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,ccap-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 6: SDP offer with alternative media (1)

   Upon receiving the SDP offer descibed in Figure 6, Bob decided to
   select the circuit-switched bearer and generates the answer described
   in Figure 7

   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 7: SDP answer with circuit-switched media (2)


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 a New 'corr-id' SDP Attribute

      Contact: Miguel Garcia <miguel.a.garcia@ericsson.com>

      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
   --------------  ---------------------------  ---------
   proto           PSTN                         [RFCxxxx]




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8.  Security Considerations

   This document provides an extension on top of RFC 4566 [RFC4566], and
   RFC 3264 [RFC3264].  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, Ingemar Johansson,
   Christer Holmberg, and Alf Heidermark for providing their insight and
   comments on this document.


10.  References

10.1.  Normative References

   [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
              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.

   [I-D.garcia-mmusic-sdp-misc-cap]



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              Garcia, M., Veikkolainen, S., and R. Gilman,
              "Miscellaneous Capabilities Negotiation in the Session
              Description Protocol  (SDP)",
              draft-garcia-mmusic-sdp-misc-cap-00 (work in progress),
              October 2008.

   [I-D.ietf-mmusic-sdp-capability-negotiation]
              Andreasen, F., "SDP Capability Negotiation",
              draft-ietf-mmusic-sdp-capability-negotiation-09 (work in
              progress), July 2008.

   [I-D.ietf-mmusic-sdp-media-capabilities]
              Gilman, R., Even, R., and F. Andreasen, "SDP media
              capabilities Negotiation",
              draft-ietf-mmusic-sdp-media-capabilities-05 (work in
              progress), July 2008.

   [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.

URIs

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





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Authors' Addresses

   Miguel A. Garcia-Martin
   Ericsson
   Calle Via de los Poblados 13
   Madrid, ES  28033
   Spain

   Email: miguel.a.garcia@ericsson.com


   Simo Veikkolainen
   Nokia
   P.O. Box 407
   NOKIA GROUP, FI  00045
   Finland

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
































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Full Copyright Statement

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