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rfc:rfc7195

Internet Engineering Task Force (IETF) M. Garcia-Martin Request for Comments: 7195 Ericsson Category: Standards Track S. Veikkolainen ISSN: 2070-1721 Nokia

                                                              May 2014
          Session Description Protocol (SDP) Extension for

Setting Audio and Video Media Streams over Circuit-Switched Bearers in

            the Public Switched Telephone Network (PSTN)

Abstract

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

Status of This Memo

 This is an Internet Standards Track document.
 This document is a product of the Internet Engineering Task Force
 (IETF).  It represents the consensus of the IETF community.  It has
 received public review and has been approved for publication by the
 Internet Engineering Steering Group (IESG).  Further information on
 Internet Standards is available in Section 2 of RFC 5741.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 http://www.rfc-editor.org/info/rfc7195.

Copyright Notice

 Copyright (c) 2014 IETF Trust and the persons identified as the
 document authors.  All rights reserved.
 This document is subject to BCP 78 and the IETF Trust's Legal
 Provisions Relating to IETF Documents
 (http://trustee.ietf.org/license-info) in effect on the date of
 publication of this document.  Please review these documents
 carefully, as they describe your rights and restrictions with respect
 to this document.  Code Components extracted from this document must
 include Simplified BSD License text as described in Section 4.e of
 the Trust Legal Provisions and are provided without warranty as
 described in the Simplified BSD License.

Garcia-Martin & Veikkolainen Standards Track [Page 1] RFC 7195 PSTN Circuit-Switched Bearers in SDP May 2014

Table of Contents

 1. Introduction ....................................................3
 2. Conventions Used in This Document ...............................4
 3. Requirements ....................................................5
 4. Overview of Operation ...........................................5
    4.1. Example Call Flow ..........................................6
 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 ..................................9
         5.2.3. Correlating the PSTN Circuit-Switched
                Bearer with SDP ....................................10
                5.2.3.1. The "cs-correlation" Attribute ............11
                5.2.3.2. Caller ID Correlation Mechanism ...........12
                5.2.3.3. User-User Information Element
                         Correlation Mechanism .....................13
                5.2.3.4. DTMF Correlation Mechanism ................14
                5.2.3.5. External Correlation Mechanism ............15
                5.2.3.6. Extensions to Correlation Mechanisms ......16
    5.3. Negotiating the Correlation Mechanisms ....................17
         5.3.1. Determining the Direction of the
                Circuit-Switched Bearer Setup ......................17
         5.3.2. Populating the "cs-correlation" Attribute ..........18
         5.3.3. Considerations for Correlations ....................18
    5.4. Considerations for Usage of Existing SDP ..................19
         5.4.1. Originator of the Session ..........................19
         5.4.2. Contact Information ................................20
    5.5. Considerations for Usage of Third Party Call
         Control (3PCC) ............................................20
    5.6. Offer/Answer Mode Extensions ..............................20
         5.6.1. Generating the Initial Offer .......................21
         5.6.2. Generating the Answer ..............................23
         5.6.3. Offerer Processing the Answer ......................26
         5.6.4. Modifying the Session ..............................27
    5.7. Formal Syntax .............................................28
 6. Examples .......................................................30
    6.1. Single PSTN Audio Stream ..................................30
    6.2. Advanced SDP Example: Circuit-Switched Audio and
         Video Streams .............................................32
 7. Security Considerations ........................................33
 8. IANA Considerations ............................................35
    8.1. Registration of the New "cs-correlation" SDP Attribute ....35
    8.2. Registration of a New "nettype" Value .....................36
    8.3. Registration of a New "addrtype" Value ....................36
    8.4. Registration of a New "proto" Value .......................36
 9. Acknowledgments ................................................37

Garcia-Martin & Veikkolainen Standards Track [Page 2] RFC 7195 PSTN Circuit-Switched Bearers in SDP May 2014

 10. References ....................................................37
    10.1. Normative References .....................................37
    10.2. Informative References ...................................38

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 media transport protocols other
 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 the 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 or video 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 or video is
 received through the classical circuit-switched bearer.
 This document addresses only the use of circuit-switched bearers in
 the PSTN, not a generic circuit-switched network.  The mechanisms
 presented below require a call signaling protocol of the PSTN to be
 used (such as ITU-T Q.931 [ITU.Q931.1998] or 3GPP TS 24.008
 [TS.24.008]).

Garcia-Martin & Veikkolainen Standards Track [Page 3] RFC 7195 PSTN Circuit-Switched Bearers in SDP May 2014

 Setting up a signaling relationship in the IP domain instead of just
 setting up a circuit-switched call also offers 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 or
 video media streams over to the high-speed bearer.  This implies a
 new exchange of SDP offer/answer that leads to a renegotiation of the
 media streams.
 Other use cases exist.  For example, an endpoint might have at its
 disposal circuit-switched and packet-switched connectivity, but the
 same audio or video codecs are not feasible for both access networks.
 For example, the circuit-switched audio or video stream supports
 narrow-bandwidth codecs, while the packet-switched access allows any
 other audio or video 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
 examples of circuit-switched audio or video streams in SDP.  Sections
 7 and 8 contain the Security and IANA considerations, respectively.

2. Conventions Used in This Document

 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
 "SHOULD", "SHOULD NOT", "RECOMMENDED", "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.

Garcia-Martin & Veikkolainen Standards Track [Page 4] RFC 7195 PSTN Circuit-Switched Bearers in SDP May 2014

3. Requirements

 This section presents the general requirements that are specific for
 the audio or video media streams over circuit-switched bearers.
 REQ-1:  A mechanism for endpoints to negotiate and agree on an audio
         or video media stream established over a circuit-switched
         bearer MUST be available.
 REQ-2:  The mechanism MUST allow the endpoints to combine circuit-
         switched audio or video media streams with other
         complementary media streams, for example, text messaging.
 REQ-3:  The mechanism MUST allow the endpoint to negotiate the
         direction of the circuit-switched bearer, i.e., which
         endpoint is active when initiating the circuit-switched
         bearer.
 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 bearer with the one negotiated
         in SDP, as opposed to another incoming call that is not
         related to that.  In case correlation by programmatic means
         is not possible, correlation may also be performed by the
         human user.
 REQ-6:  It MUST be possible for endpoints to advertise different
         lists of audio or video codecs in the circuit-switched audio
         or video stream from those used in a packet-switched audio or
         video stream.
 REQ-7:  It MUST be possible for endpoints to not advertise the list
         of available codecs for circuit-switched audio or video
         streams.

4. Overview of Operation

 The mechanism defined in this memo extends SDP [RFC4566] and allows
 describing an audio or video media stream established over a circuit-
 switched bearer.  A new network type ("PSTN") and a new protocol type
 ("PSTN") are defined for the "c=" and "m=" lines to be able to
 describe a media stream over a circuit-switched bearer.  These SDP
 extensions are described in Section 5.2.  Since circuit-switched
 bearers are connection-oriented media streams, the mechanism reuses

Garcia-Martin & Veikkolainen Standards Track [Page 5] RFC 7195 PSTN Circuit-Switched Bearers in SDP May 2014

 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 5.3.1.

4.1. Example Call Flow

 Consider the example presented in Figure 1.  In this example,
 Endpoint A is located in an environment where it has access to both
 IP and circuit-switched bearers for communicating with other
 endpoints.  Endpoint A 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 a
 circuit-switched bearer.
  Endpoint A                        Endpoint B
    | (1) SDP offer (PSTN audio)         |
    |----------------------------------->|
    |                                    |
    | (2) SDP answer (PSTN audio)        |
    |<-----------------------------------|
    |                                    |
    |   PSTN call setup                  |
    |<-----------------------------------|
    |                                    |
    |                                    |
    |<===== media over PSTN bearer =====>|
    |                                    |
             Figure 1: Example Flow
 Endpoint B receives the SDP offer and determines that it is located
 in an environment where the IP-based bearer is not suitable for real-
 time audio media.  However, Endpoint B also has a PSTN circuit-
 switched bearer available for audio.  Endpoint B generates an SDP
 answer containing a description of the audio media stream over a
 circuit-switched bearer.
 During the offer/answer exchange, Endpoints A and B also agree upon
 the direction in which the circuit-switched bearer should be
 established.  In this example, Endpoint B becomes the active party;
 in other words, it establishes the circuit-switched call to the other
 endpoint.  The offer/answer exchange contains identifiers or
 references that can be used on the circuit-switched network for
 addressing the other endpoint, as well as information that is used to
 determine that the incoming circuit-switched bearer establishment is
 related to the ongoing session between the two endpoints.

Garcia-Martin & Veikkolainen Standards Track [Page 6] RFC 7195 PSTN Circuit-Switched Bearers in SDP May 2014

 Endpoint B establishes a circuit-switched bearer towards Endpoint A
 using whatever mechanisms are defined for the network type in
 question.  When receiving the incoming circuit-switched connection
 attempt, Endpoint A is able to determine that the attempt is related
 to the session it is just establishing with B.
 Endpoint A accepts the circuit-switched connection; the circuit-
 switched bearer setup is completed.  The two endpoints can now use
 the circuit-switched connection for two-way audio media.
 If, for some reason, Endpoint B would like to reject the offered
 stream, it would set the port number of the specific stream to zero,
 as specified in RFC 3264 [RFC3264].  Also, if B does not understand
 some of the SDP attributes specified in this document, it would
 ignore them, as specified in RFC 4566 [RFC4566].

5. Protocol Description

5.1. Level of Compliance

 Implementations that are compliant with this specification MUST
 implement the SDP extensions described in Section 5.2 and MUST
 implement the considerations discussed in Sections 5.3, 5.4, and 5.6.

5.2. Extensions to SDP

 This section provides the syntax and semantics of the extensions
 required for providing a description of audio or video 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 <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.

Garcia-Martin & Veikkolainen Standards Track [Page 7] RFC 7195 PSTN Circuit-Switched Bearers in SDP May 2014

 For the address type, we initially considered the possibility of
 describing E.164 telephone numbers.  We define a new "E164" address
 type to be used within the context of a "PSTN" network type.  The
 "E164" address type indicates that the connection address contains an
 E.164 number represented according to the ITU-T E.164 [ITU.E164.2010]
 recommendation.
 It is a common convention that an international E.164 number contains
 a leading '+' sign.  For consistency's sake, we also require the
 E.164 telephone is prepended with a '+', even if that is not
 necessary for routing of the call in the PSTN network.
 There are cases, though, when the endpoint is merely aware of a
 circuit-switched bearer, without having further information about the
 E.164 number allocated to it.  In these cases, a dash ("-") is used
 to indicate an unknown connection address.  This makes the connection
 data line consistent with SDP syntax.
 Please note that the "E164" address type defined in this memo is
 exclusively defined to be used in conjunction with the "PSTN" network
 type in accordance with regular offer/answer procedures [RFC4566].
    Note: RFC 3108 [RFC3108] also defines address type "E.164".  This
    definition is distinct from the one defined by this memo and shall
    not be used with <nettype> "PSTN".
 This memo exclusively uses the international representation of E.164
 numbers, i.e., those including a country code and, as described
 above, prepended with a '+' sign.  Implementations conforming to this
 specification and using the "E164" address type together with the
 "PSTN" network type MUST use the 'global-number-digits' construction
 specified in RFC 3966 [RFC3966] for representing international E.164
 numbers.  This representation requires the presence of the '+' sign
 and additionally allows for the presence of one or more 'visual-
 separator' constructions for easier human readability (see
 Section 5.7).
 Note that <connection-address> MUST NOT be omitted when unknown since
 this would violate basic syntax of SDP [RFC4566].  In such cases, it
 MUST be set to a "-".
 The following are examples of the extension to the connection data
 line:
    c=PSTN E164 +441134960123
    c=PSTN E164 -

Garcia-Martin & Veikkolainen Standards Track [Page 8] RFC 7195 PSTN Circuit-Switched Bearers in SDP May 2014

 When the <addrtype> is E164, the connection address is defined as
 follows:
 o  an international E.164 number (prepended with a '+' sign)
 o  the value "-", signifying that the address is unknown
 o  any other value resulting from the production rule of connection-
    address in RFC 4566 [RFC4566], but in all cases any value
    encountered will be ignored.

5.2.2. Media Descriptions

 According to SDP [RFC4566], the media description line in SDP has the
 following syntax:
    m=<media> <port> <proto> <fmt> ...
 The <media> subfield carries the media type.  For establishing an
 audio bearer, the existing "audio" media type is used.  For
 establishing a video bearer, the existing "video" media type is used.
 The <port> subfield is the transport port to which the media stream
 is sent.  Circuit-switched access lacks the concept of a port number;
 therefore, the <port> subfield does not carry any meaningful value.
 In order to be compliant with SDP syntax, implementations SHOULD set
 the <port> subfield to the discard port value "9" and MUST ignore it
 on reception.
 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> subfield 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> subfield is the media format description.  In the classical
 usage of SDP to describe RTP-based media streams, when the <proto>
 subfield is set to "RTP/AVP" or "RTP/SAVP", the <fmt> subfield
 contains the payload types as defined in the RTP audio profile
 [RFC3551].

Garcia-Martin & Veikkolainen Standards Track [Page 9] RFC 7195 PSTN Circuit-Switched Bearers in SDP May 2014

 When "RTP/AVP" is used in the <proto> field, the <fmt> subfield
 contains the RTP payload type numbers.  We use the <fmt> subfield to
 indicate the list of available codecs over the circuit-switched
 bearer, by reusing the conventions and payload type numbers defined
 for RTP / AVP.  The RTP audio and video media types, when applied to
 PSTN circuit-switched bearers, represent merely an audio or video
 codec.  If the endpoint is able to determine the list of available
 codecs for circuit-switched media streams, it MUST use the
 corresponding payload type numbers in the <fmt> subfield.
 In some cases, the endpoint is not able to determine the list of
 available codecs for circuit-switched media streams.  In this case,
 in order to be syntactically compliant with SDP [RFC4566], the
 endpoint MUST include a single dash ("-") in the <fmt> subfield.
 As per RFC 4566 [RFC4566], the media format descriptions are listed
 in priority order.
 Examples of media descriptions for circuit-switched audio streams
 are:
    m=audio 9 PSTN 3 0 8
    m=audio 9 PSTN -
 Similarly, an example of a media description for circuit-switched
 video stream is:
    m=video 9 PSTN 34
    m=video 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 in order to avoid ringing for an
 incoming circuit-switched bearer that is related to the session
 controlled with SDP (and SIP).
 Several alternatives exist for performing this correlation.  This
 memo provides three mutually non-exclusive correlation mechanisms.
 Additionally, we define a fourth mechanism where correlation may be
 performed by external means, typically by the human user, in case
 using other correlation mechanisms is not possible or does not
 succeed.  Other correlation mechanisms may exist, and their usage
 will be specified when need arises.

Garcia-Martin & Veikkolainen Standards Track [Page 10] RFC 7195 PSTN Circuit-Switched Bearers in SDP May 2014

 All mechanisms share the same principle: some unique information is
 sent in the SDP and in the circuit-switched signaling protocol.  If
 these pieces of information match, then the circuit-switched bearer
 is part of the session described in the SDP exchange.  Otherwise,
 there is no guarantee that the circuit-switched bearer is related to
 such session.
 The first mechanism is based on the exchange of PSTN Caller ID
 between the endpoints.  The Caller ID is also available as the
 Calling Party Number in the circuit-switched signaling.
 The second mechanism is based on the inclusion in SDP of a value that
 is also sent in the User-User Information Element that is part of the
 bearer setup signaling in the PSTN.
 The third mechanism is based on sending in SDP a string that
 represents Dual-Tone Multi-Frequency (DTMF) digits that will be later
 sent right after the circuit-switched bearer is established.
 The fourth correlation mechanism declares support for cases where
 correlation is done by external means.  Typically, this means that
 the decision is left to the human user.  This is how some current
 conferencing systems operate: after logging on to the conference, the
 system calls back to the user's phone number to establish audio
 communications, and it is up to the human user to accept or reject
 the incoming call.  By declaring explicit support for this mechanism,
 endpoints can use it only when such a possibility exists.
 Endpoints may opt to implement any combination of the correlation
 mechanisms specified in Sections 5.2.3.2, 5.2.3.3, 5.2.3.4, and
 5.2.3.5, including the option to implement none at all.

5.2.3.1. The "cs-correlation" Attribute

 In order to provide support for the correlation mechanisms, we define
 a new media-level SDP attribute called "cs-correlation".  There MUST
 be at most one "cs-correlation" attribute per media description.
 This "cs-correlation" attribute MAY contain zero or more subfields --
 "callerid", "uuie", "dtmf", or "external" to specify additional
 information required by the Caller ID, User-User Information Element,
 DTMF, or external correlation mechanisms, respectively.  The list of
 correlation mechanisms may be extended by other specifications; see
 Section 5.2.3.6 for more details.
 The following sections provide more detailed information about these
 subfields.

Garcia-Martin & Veikkolainen Standards Track [Page 11] RFC 7195 PSTN Circuit-Switched Bearers in SDP May 2014

 The values "callerid", "uuie", "dtmf", and "external" refer to the
 correlation mechanisms defined in Sections 5.2.3.2, 5.2.3.3, 5.2.3.4,
 and 5.2.3.5, respectively.  The formal Augmented Backus-Naur Format
 (ABNF) syntax of the "cs-correlation" attribute is presented in
 Section 5.7.

5.2.3.2. Caller ID Correlation Mechanism

 The Caller ID correlation mechanism consists of an exchange of the
 Calling Party Number as an international E.164 number in SDP,
 followed by the availability of the Calling Party Number Information
 Element in the call setup signaling of the circuit-switched
 connection.  If both pieces of information match, the circuit-
 switched bearer is correlated to the session described in SDP.
 An example of inclusion of an international E.164 number in the
 "cs-correlation" attribute is:
    a=cs-correlation:callerid:+441134960123
 The presence of the "callerid" subfield indicates that the endpoint
 supports use of the Calling Party Number as a means of correlating a
 PSTN call with the session being negotiated.  The "callerid" subfield
 MAY be accompanied by the international E.164 number of the party
 inserting the parameter.
    Note that there are no guarantees that this correlation mechanism
    works or is even available, due a number of problems:
  • The endpoint might not be aware of its own E.164 number, in

which case it cannot populate the SDP appropriately.

  • 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.
  • 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, due to, e.g., lack of country code.  To
       mitigate this problem, implementations should consider only
       some of the rightmost digits from the E.164 number for
       correlation.  For example, the numbers +44-113-496-0123 and
       0113-496-0123 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,

Garcia-Martin & Veikkolainen Standards Track [Page 12] RFC 7195 PSTN Circuit-Switched Bearers in SDP May 2014

       for the purpose of displaying the caller's name.  Please refer
       to ITU-T E.164 recommendation [ITU.E164.2010] for consideration
       of the relevant number of digits to consider.

5.2.3.3. User-User Information Element Correlation Mechanism

 A second correlation mechanism is based on including in SDP a string
 that represents 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 [TS.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 and the
 network settings.
 The mechanism works as follows.  An endpoint creates a User-User
 Information Element, according to the requirements of the call setup
 signaling protocol.  The same value is included in the SDP offer or
 SDP answer, in the "uuie" subfield of the "cs-correlation" attribute.
 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 of the call setup 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 setup attempt corresponds to that
 indicated in the SDP.
 According to ITU-T Q.931 [ITU.Q931.1998], the User-User Information
 Element (UUIE) identifier is composed of a first octet identifying
 this as a User-User Information Element, a second octet containing
 the length of the user-user contents, a third octet containing a
 Protocol Discriminator, and a value of up to 32 or 128 octets
 (depending on network settings) containing the actual User
 Information (see Figure 4-36 in [ITU.Q931.1998]).  The first two
 octets of the UUIE MUST NOT be used for correlation; only the octets
 carrying the Protocol Discriminator and the User Information value
 are input to the creation of the value of the "uuie" subfield in the
 "cs-correlation" attribute.  Therefore, the value of the "uuie"
 subfield in the "cs-correlation" attribute MUST start with the
 Protocol Discriminator octet, followed by the User Information
 octets.  The value of the Protocol Discriminator octet is not
 specified in this document; it is expected that organizations using
 this technology will allocate a suitable value for the Protocol
 Discriminator.

Garcia-Martin & Veikkolainen Standards Track [Page 13] RFC 7195 PSTN Circuit-Switched Bearers in SDP May 2014

 Once the binary value of the "uuie" subfield in the "cs-correlation"
 attribute is created, it MUST be base 16 (also known as "hex")
 encoded before it is inserted in SDP.  Please refer to RFC 4648
 [RFC4648] for a detailed description of base 16 encoding.  The
 resulting encoded value needs to have an even number of hexadecimal
 digits and MUST be considered invalid if it has an odd number.
    Note: The encoding of the "uuie" subfield of the "cs-correlation"
    attribute is largely inspired by the encoding of the same value in
    the User-to-User header field in SIP, according to "A Mechanism
    for Transporting User to User Call Control Information in SIP"
    [SIP-UUI].
 As an example, an endpoint willing to send a UUIE containing a
 Protocol Discriminator with the hexadecimal value of %x56 and an
 hexadecimal User Information value of %xA390F3D2B7310023 would
 include an "a=cs-correlation" attribute line as follows:
    a=cs-correlation:uuie:56A390F3D2B7310023
 Note that the value of the User-User Information Element is
 considered as an opaque string and only used for correlation
 purposes.  Typically, call signaling protocols impose requirements on
 the creation of a 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.
 Please note that there are no guarantees 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.

5.2.3.4. DTMF Correlation Mechanism

 We introduce a third mechanism for correlating the circuit-switched
 bearer with the session described with SDP.  This is based on
 agreeing on a sequence of digits that are negotiated in the SDP
 offer/answer exchange and sent as DTMF tones as described in ITU-T
 Recommendation Q.23 [ITU.Q23.1988] over the circuit-switched bearer
 once this bearer is established.  If the DTMF digit sequence received
 through the circuit-switched bearer matches the digit string
 negotiated in the SDP, the circuit-switched bearer is correlated with

Garcia-Martin & Veikkolainen Standards Track [Page 14] RFC 7195 PSTN Circuit-Switched Bearers in SDP May 2014

 the session described in the SDP.  The mechanism is similar to many
 voice conferencing systems that require the user to enter a PIN code
 using DTMF tones in order to be accepted in a voice conference.
 The mechanism works as follows.  An endpoint selects a DTMF digit
 sequence.  The same sequence is included in the SDP offer or SDP
 answer, in a "dtmf" subfield of the "cs-correlation" attribute.  When
 the SDP offer/answer exchange is completed, each endpoint has become
 aware of the DTMF sequence that will be sent right after the circuit-
 switched bearer is set up.  The endpoint that initiates the call
 setup attempt sends the DTMF digits according to the procedures
 defined for the circuit-switched bearer technology used.  The
 recipient (passive side of the bearer setup) of the call setup
 attempt collects the digits and compares them with the value
 previously received in the SDP.  If the digits match, then the call
 setup attempt corresponds to that indicated in the SDP.
    Note: Implementations are advised to select a number of DTMF
    digits that provide enough assurance that the call is related but
    do not prolong the bearer setup time unnecessarily.  A number of 5
    to 10 digits is a good compromise.
 As an example, an endpoint willing to send DTMF tone sequence "14D*3"
 would include an "a=cs-correlation" attribute line as follows:
    a=cs-correlation:dtmf:14D*3
 If the endpoints successfully agree on the usage of the DTMF digit
 correlation mechanism but the passive side does not receive any DTMF
 digits after successful circuit-switched bearer setup or receives a
 set of DTMF digits that do not match the value of the "dtmf"
 attribute (including receiving too many digits), the passive side
 SHOULD consider that this DTMF mechanism has failed to correlate the
 incoming call.

5.2.3.5. External Correlation Mechanism

 The fourth correlation mechanism relies on external means for
 correlating the incoming call to the session.  Since endpoints can
 select which correlation mechanisms they support, it may happen that
 no other common correlation mechanism is found or that the selected
 correlation mechanism does not succeed due to the required feature
 not being supported by the underlying PSTN network.  In these cases,
 the human user can make the decision to accept or reject the incoming
 call, thus "correlating" the call with the session.  Since not all
 endpoints are operated by a human user and since there may be no

Garcia-Martin & Veikkolainen Standards Track [Page 15] RFC 7195 PSTN Circuit-Switched Bearers in SDP May 2014

 other external means implemented by the endpoint for the correlation
 function, we explicitly define support for such an external
 correlation mechanism.
 Endpoints wishing to use this external correlation mechanism would
 use the "external" subfield in the "cs-correlation" attribute.
 Unlike the other three correlation mechanisms, the "external"
 subfield does not accept a value.  The following is an example of an
 "a=cs-correlation" attribute line:
    a=cs-correlation:external
 Endpoints that are willing to only use the three explicit correlation
 mechanisms defined in this document ("callerid", "uuie", and/or
 "dtmf") would not include the "external" mechanism in the
 offer/answer exchange.
 The external correlation mechanism typically relies on the human user
 to make the decision on whether or not the call is related to the
 ongoing session.  After the user accepts the call, that bearer is
 considered as related to the session.  There is a small chance that
 the user receives at the same time another circuit-switched call that
 is not related to the ongoing session.  The user may reject this call
 if he is able to determine (e.g., based on the calling line
 identification) that the call is not related to the session and
 continue waiting for another call attempt.  If the user accepts the
 incoming circuit-switched call, but it turns out to be not related to
 the session, the endpoints need to rely on the human user to take
 appropriate action (typically, the user would hang up).

5.2.3.6. Extensions to Correlation Mechanisms

 New values for the "cs-correlation" attribute may be specified.  The
 registration policy for new values is "Specification Required"; see
 Section 8.  Any such specification MUST include a description of how
 the SDP offer/answer mechanism is used to negotiate the use of the
 new values, taking into account how endpoints determine which side
 will become active or passive (see Section 5.3 for more details).
 If, during the offer/answer negotiation, either endpoint encounters
 an unknown value in the "cs-correlation" attribute, it MUST consider
 that mechanism as unsupported and MUST NOT include that value in
 subsequent offer/answer negotiation.

Garcia-Martin & Veikkolainen Standards Track [Page 16] RFC 7195 PSTN Circuit-Switched Bearers in SDP May 2014

5.3. Negotiating the Correlation Mechanisms

 The four correlation mechanisms presented above (based on Called
 Party Number, User-User Information Element, DTMF digit sending, and
 external) are non-exclusive and can be used independently of each
 other.  In order to know how to populate the "cs-correlation"
 attribute, the endpoints need to agree which endpoint will become the
 active party, i.e., the one that will set up the circuit-switched
 bearer.

5.3.1. Determining the Direction of the Circuit-Switched Bearer Setup

 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 MUST 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 can be 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
 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", and "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 that are compliant with this specification MUST
 support the "setup" and "connection" attributes specified in RFC 4145
 [RFC4145], but applied to circuit-switched bearers in the PSTN.
 We define the active party as the one that initiates the circuit-
 switched bearer after the offer/answer exchange.  The passive party
 is the one receiving the circuit-switched bearer.  Either party may
 indicate its desire to become the active or passive party during the
 offer/answer exchange using the procedures described in Section 5.6.

Garcia-Martin & Veikkolainen Standards Track [Page 17] RFC 7195 PSTN Circuit-Switched Bearers in SDP May 2014

5.3.2. Populating the "cs-correlation" Attribute

 By defining values for the subfields in the "cs-correlation"
 attribute, the endpoint indicates that it is willing to become the
 active party and that it can use those values in the Calling Party
 Number, in the User-User Information Element, or as DTMF tones during
 the circuit-switched bearer setup.
 Thus, the following rules apply:
 o  An endpoint that can only become the active party in the circuit-
    switched bearer setup MUST include all correlation mechanisms it
    supports in the "cs-correlation" attribute and MUST also specify
    values for the "callerid", "uuie", and "dtmf" subfields.  Notice
    that the "external" subfield does not accept a value.
 o  An endpoint that can only become the passive party in the circuit-
    switched bearer setup MUST include all correlation mechanisms it
    supports in the "cs-correlation" attribute but MUST NOT specify
    values for the subfields.
 o  An endpoint that is willing to become either the active or passive
    party (by including the "a=setup:actpass" attribute in the offer)
    MUST include all correlation mechanisms it supports in the
    "cs-correlation" attribute and MUST also specify values for the
    "callerid", "uuie", and "dtmf" subfields.  Notice that the
    "external" subfield does not accept a value.

5.3.3. Considerations for Correlations

 Passive endpoints should expect an incoming circuit-switched (CS)
 call for setting up the audio bearer.  Passive endpoints MAY suppress
 the incoming CS alert during certain time periods.  Additional
 restrictions can be applied, such as the passive endpoint not
 alerting incoming calls originated from the number that was observed
 during the offer/answer negotiation.
 There may be cases when an endpoint is not willing to include one or
 more correlation mechanisms in the "a=cs-correlation" attribute line
 even if it supports it.  For example, some correlation mechanisms can
 be omitted if the endpoint is certain that the PSTN network does not
 support carrying the correlation identifier.  Also, since using the
 DTMF-based correlation mechanism requires the call to be accepted
 before DTMF tones can be sent, some endpoints may enforce a policy
 restricting this due to, for example, cost associated with received
 calls, making the DTMF-based mechanism unusable.

Garcia-Martin & Veikkolainen Standards Track [Page 18] RFC 7195 PSTN Circuit-Switched Bearers in SDP May 2014

 Note that it cannot be guaranteed that the correlation mechanisms
 relying on caller identification, User-User Information Element, and
 DTMF sending will succeed even if the usage of those was agreed
 beforehand.  This is due to the fact that correlation mechanisms
 require support from the circuit-switched bearer technology used.
 Therefore, even a single positive indication using any of these
 mechanisms SHOULD be interpreted by the passive endpoint so that the
 circuit-switched bearer establishment is related to the ongoing
 session, even if the other correlation mechanisms fail.
 If, after successfully negotiating any of the "callerid", "uuie", or
 "dtmf" correlation mechanisms in the SDP offer/answer exchange, an
 endpoint receives an incoming establishment of a circuit-switched
 bearer with no correlation information present, the endpoint first
 checks whether or not the offer/answer exchange was also used to
 successfully negotiate the "external" correlation mechanism.  If it
 was, the endpoint should let the decision be made by external means,
 typically the human user.  If the "external" correlation mechanism
 was not successfully negotiated, the endpoint should treat the call
 as unrelated to the ongoing session in the IP domain.

5.4. Considerations for Usage of Existing SDP

5.4.1. 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 or video
 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.

Garcia-Martin & Veikkolainen Standards Track [Page 19] RFC 7195 PSTN Circuit-Switched Bearers in SDP May 2014

5.4.2. Contact Information

 SDP [RFC4566] defines the "p=" line, which may include the phone
 number of the person responsible 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.

5.5. Considerations for Usage of Third Party Call Control (3PCC)

 "Best Current Practices for Third Party Call Control (3PCC) in the
 Session Initiation Protocol (SIP)" [RFC3725] outlines several flows
 that are possible in third party call control scenarios and
 recommends some flows for specific situations.
 One of the assumptions in [RFC3725] is that an SDP offer may include
 a "black hole" connection address, which has the property that
 packets sent to it will never leave the host that sent them.  For
 IPv4, this "black hole" connection address is 0.0.0.0 or a domain
 name within the .invalid DNS top level domain.
 When using an E.164 address scheme in the context of third party call
 control, when the User Agent needs to indicate an unknown phone
 number, it MUST populate the <addrtype> of the SDP "c=" line with a
 "-" string.
    Note: This may result in the recipient of the initial offer
    rejecting such offer if the recipient of the offer was not aware
    of its own E.164 number.  Consequently, it will not be possible to
    establish a circuit-switched bearer, since neither party is aware
    of its E.164 number.

5.6. Offer/Answer Mode Extensions

 In this section, we define extensions to the offer/answer model
 defined in "An Offer/Answer Model with the Session Description
 Protocol (SDP)" [RFC3264] to allow for PSTN addresses to be used with
 the offer/answer model.

Garcia-Martin & Veikkolainen Standards Track [Page 20] RFC 7195 PSTN Circuit-Switched Bearers in SDP May 2014

5.6.1. Generating the Initial Offer

 The offerer, wishing to use PSTN audio or video stream, MUST populate
 the "c=" and "m=" lines as follows.
 The endpoint MUST set the <nettype> in the "c=" line to "PSTN" and
 the <addrtype> to "E164".  Furthermore, the endpoint SHOULD set the
 <connection-address> field to its own international E.164 number
 (with a leading "+").  If the endpoint is not aware of its own E.164
 number, it MUST set the <connection-address> to "-".
 In the "m=" line, the endpoint MUST set the <media> subfield to
 "audio" or "video", depending on the media type, and the <proto>
 subfield to "PSTN".  The <port> subfield SHOULD be set to "9" (the
 discard port).  The values "audio" or "video" in the <media> subfield
 MUST NOT be set by the endpoint unless it has knowledge that these
 bearer types are available on the circuit-switched network.
 The <fmt> subfield carries the payload type number(s) the endpoint is
 wishing to use.  Payload type numbers in this case refer to the
 codecs that the endpoint wishes to use on the PSTN media stream.  For
 example, if the endpoint wishes to use the GSM codec, it would add
 payload type number 3 in the list of codecs.  The list of payload
 types MUST only contain those codecs the endpoint is able to use on
 the PSTN bearer.  In case the endpoint is not aware of the codecs
 available for the circuit-switched media streams, it MUST include a
 dash ("-") in the <fmt> subfield.
 The mapping table of static payload types numbers to payload types is
 initially specified in [RFC3551] and maintained by IANA.  For dynamic
 payload types, the endpoint MUST define the set of valid encoding
 names and related parameters using the "a=rtpmap" attribute line.
 See Section 6 of RFC 4566 [RFC4566] for details.
 When generating the offer, the offerer MUST include an
 "a=cs-correlation" attribute line in the SDP offer.  The offerer MUST
 NOT include more than one "cs-correlation" attribute per media
 description.  The "a=cs-correlation" line SHOULD contain an
 enumeration of all the correlation mechanisms supported by the
 offerer, in the format of subfields.  See Section 5.3.3 for more
 information on usage of the correlation mechanisms.
 The current list of subfields include "callerid", "uuie", "dtmf", and
 "external", and they refer to the correlation mechanisms defined in
 Sections 5.2.3.2, 5.2.3.3, 5.2.3.4, and 5.2.3.5, respectively.

Garcia-Martin & Veikkolainen Standards Track [Page 21] RFC 7195 PSTN Circuit-Switched Bearers in SDP May 2014

 If the offerer supports any of the correlation mechanisms defined in
 this memo and is willing to become the active party, the offerer MUST
 add the "callerid", "uuie", "dtmf", and/or "external" subfields and
 MUST specify values for them as follows:
 o  The international E.164 number as the value in the "callerid"
    subfield.
 o  The contents of the User-User Information Element as the value of
    the "uuie" subfield.
 o  The DTMF tone string as the value of the "dtmf" subfield.
 o  The endpoint MUST NOT specify any value for the "external"
    subfield.
 If the offerer is only able to become the passive party in the
 circuit-switched bearer setup, it MUST add at least one of the
 possible correlation mechanisms but MUST NOT specify values for those
 subfields.
 For example, if the offerer is willing to use the User-User
 Information Element and DTMF digit-sending mechanisms but can only
 become the passive party, and is also able to let the human user
 decide whether the correlation should be done or not, it includes the
 following lines in the SDP:
    a=cs-correlation:uuie dtmf external
    a=setup:passive
 If, on the other hand, the offerer is willing to use the User-User
 Information Element and the DTMF correlation mechanisms and is able
 to become the active or passive side, and is also able to let the
 human user decide whether the correlation should be done or not, it
 includes the following lines in the SDP:
    a=cs-correlation:uuie:56A390F3D2B7310023 dtmf:14D*3 external
    a=setup:actpass
 The negotiation of the value of the "setup" attribute takes place as
 defined in Section 4.1 of RFC 4145 [RFC4145].
 The offerer states which role or roles it is willing to perform; the
 answerer, taking the offerer's willingness into consideration,
 chooses which roles both endpoints will actually perform during the
 circuit-switched bearer setup.

Garcia-Martin & Veikkolainen Standards Track [Page 22] RFC 7195 PSTN Circuit-Switched Bearers in SDP May 2014

 By "active" endpoint, we refer to an endpoint that will establish the
 circuit-switched bearer; by "passive" endpoint, we refer to an
 endpoint that will receive a circuit-switched bearer.
 If an offerer does not know its international E.164 number, it MUST
 set the "setup" attribute to the value "active".  If the offerer
 knows its international E.164 number, it SHOULD set the value to
 either "actpass" or "passive".
 Also "holdconn" is a permissible value in the "setup" attribute.  It
 indicates that the connection should not be established for the time
 being.
 The offerer uses the "connection" attribute to decide whether a new
 circuit-switched bearer is to be established or not.  For the initial
 offer, the offerer MUST use value "new".

5.6.2. Generating the Answer

 If the offer contained a circuit-switched audio or video stream, the
 answerer first determines whether it is able to accept and use such
 streams on the circuit-switched network.  If the answerer does not
 support or is not willing to use circuit-switched media for the
 session, it MUST construct an answer where the port number for such
 media stream(s) is set to zero, according to Section 6 of [RFC3264].
 If the answerer is willing to use circuit-switched media for the
 session, it MUST ignore the received port number (unless the port
 number is set to zero).
 If the offer included a "-" as the payload type number, it indicates
 that the offerer is not willing or able to define any specific
 payload type.  Most often, a "-" is expected to be used instead of
 the payload type when the endpoint is not aware of or not willing to
 define the codecs that will eventually be used on the circuit-
 switched bearer.  The circuit-switched signaling protocols have their
 own means of negotiating or indicating the codecs; therefore, an
 answerer SHOULD accept such offers and SHOULD set the payload type to
 "-" in the answer.
 If the answerer explicitly wants to specify a codec for the circuit-
 switched media, it MAY set the respective payload numbers in the
 <fmt> subfield in the answer.  This behavior, however, is NOT
 RECOMMENDED.
 When receiving the offer, the answerer MUST determine whether it
 becomes the active or passive party.

Garcia-Martin & Veikkolainen Standards Track [Page 23] RFC 7195 PSTN Circuit-Switched Bearers in SDP May 2014

 If the SDP in the offer indicates that the offerer is only able to
 become the active party, the answerer needs to determine whether it
 is able to become the passive party.  If this is not possible, e.g.,
 due to the answerer not knowing its international E.164 number, the
 answerer MUST reject the circuit-switched media by setting the port
 number to zero on the answer.  If the answerer is aware of its
 international E.164 number, it MUST include the "setup" attribute in
 the answer and set it to value "passive" or "holdconn".  The answerer
 MUST also include its E.164 number in the "c=" line.
 If the SDP in the offer indicates that the offerer is only able to
 become the passive party, the answerer MUST verify that the offerer's
 E.164 number is included in the "c=" line of the offer.  If the
 number is included, the answerer MUST include the "setup" attribute
 in the answer and set it to value "active" or "holdconn".  If the
 number is not included, the recipient of the offer is not willing to
 establish a connection the E.164 based on a priori knowledge of cost,
 or other reasons, call establishment is not possible, and the
 answerer MUST reject the circuit-switched media by setting the port
 number to zero in the answer.
 If the SDP in the offer indicates that the offerer is able to become
 either the active or passive party, the answerer determines which
 role it will take.  If the offer includes an international E.164
 number in the "c=" line, the answerer SHOULD become the active party.
 If the answerer does not become the active party and if the answerer
 is aware of its E.164 number, it MUST become the passive party.  If
 the answerer does not become the active or the passive party, it MUST
 reject the circuit-switched media by setting the port number to zero
 in the answer.
 For each media description where the offer includes a
 "cs-correlation" attribute, the answerer MUST select from the offer
 those correlation mechanisms it supports and include in the answer
 one "a=cs-correlation" attribute line containing those mechanisms it
 is willing to use.  The answerer MUST only add one "cs-correlation"
 attribute in those media descriptions where also the offer included a
 "cs-correlation" attribute.  The answerer MUST NOT add any mechanisms
 that were not included in the offer.  If there is more than one
 "cs-correlation" attribute per media description in the offer, the
 answerer MUST discard all but the first for any media description.
 Also, the answerer MUST discard all unknown "cs-correlation"
 attribute values.
 If the answerer becomes the active party, it MUST add a value to any
 of the possible subfields.

Garcia-Martin & Veikkolainen Standards Track [Page 24] RFC 7195 PSTN Circuit-Switched Bearers in SDP May 2014

 If the answerer becomes the passive party, it MUST NOT add any values
 to the subfields in the "cs-correlation" attribute.
 After generating and sending the answer, if the answerer became the
 active party, it
 o  MUST extract the E.164 number from the "c=" line of the offer and
    MUST establish a circuit-switched bearer to that address.
 o  if the SDP answer contained a value for the "callerid" subfield,
    MUST set the Calling Party Number Information Element to that
    number.
 o  if the SDP answer contained a value for the "uuie" subfield, MUST
    send the User-User Information Element according to the rules
    defined for the circuit-switched technology used and set the value
    of the Information Element to that received in the SDP offer.
 o  if the SDP answer contained a value for the "dtmf" subfield, MUST
    send those DTMF digits according to the circuit-switched
    technology used.
 If, on the other hand, the answerer became the passive party, it
 o  MUST be prepared to receive a circuit-switched bearer,
 o  if the offer contained a value for the "callerid" subfield, MUST
    compare that value to the Calling Party Number Information Element
    of the circuit-switched bearer.  If the received Calling Party
    Number Information Element matches the value of the "callerid"
    subfield, the call SHOULD be treated as correlated to the ongoing
    session.
 o  if the offer contained a value for the "dtmf" subfield, MUST be
    prepared to receive and collect DTMF digits once the circuit-
    switched bearer is set up.  The answerer MUST compare the received
    DTMF digits to the value of the "dtmf" subfield.  If the received
    DTMF digits match the value of the "dtmf" subfield in the
    "cs-correlation" attribute, the call SHOULD be treated as
    correlated to the ongoing session.
 o  if the offer contained a value for the "uuie" subfield, MUST be
    prepared to receive a User-User Information Element once the
    circuit-switched bearer is set up.  The answerer MUST compare the
    received UUIE to the value of the "uuie" subfield.  If the value
    of the received UUIE matches the value of the "uuie" subfield, the
    call SHOULD be treated as correlated to the ongoing session.

Garcia-Martin & Veikkolainen Standards Track [Page 25] RFC 7195 PSTN Circuit-Switched Bearers in SDP May 2014

 o  if the offer contained an "external" subfield, MUST be prepared to
    receive a circuit-switched call and use the external means
    (typically, the human user) for accepting or rejecting the call.
 If the answerer becomes the active party, generates an SDP answer,
 and then it finds out that the circuit-switched call cannot be
 established, then the answerer MUST create a new SDP offer where the
 circuit-switched stream is removed from the session (actually, by
 setting the corresponding port in the "m=" line to zero) and send it
 to its counterpart.  This is to synchronize both parties (and
 potential intermediaries) on the state of the session.

5.6.3. Offerer Processing the Answer

 When receiving the answer, if the SDP does not contain an
 "a=cs-correlation" attribute line, the offerer should take that as an
 indication that the other party does not support or is not willing to
 use the procedures defined in the document for this session and MUST
 revert to normal processing of SDP.
 When receiving the answer, the offerer MUST first determine whether
 it becomes the active or passive party, as described in
 Section 5.3.1.
 If the offerer becomes the active party, it
 o  MUST extract the E.164 number from the "c=" line and MUST
    establish a circuit-switched bearer to that address.
 o  if the SDP answer contained a value for the "uuie" subfield, MUST
    send the User-User Information Element according to the rules
    defined for the circuit-switched technology used and set the value
    of the Information Element to that received in the SDP answer.
 o  if the SDP answer contained a value for the "dtmf" subfield, MUST
    send those DTMF digits according to the circuit-switched
    technology used.
 If the offerer becomes the passive party:
 o  It MUST be prepared to receive a circuit-switched bearer.
 o  Note that if delivery of the answer is delayed for some reason,
    the circuit-switched call attempt may arrive at the offerer before
    the answer has been processed.  In this case, since the
    correlation mechanisms are negotiated as part of the offer/answer
    exchange, the answerer cannot know whether or not the incoming

Garcia-Martin & Veikkolainen Standards Track [Page 26] RFC 7195 PSTN Circuit-Switched Bearers in SDP May 2014

    circuit-switched call attempt is correlated with the session being
    negotiated; thus, the offerer SHOULD answer the circuit-switched
    call attempt only after it has received and processed the answer.
 o  If the answer contained a value for the "dtmf" subfield, the
    offerer MUST be prepared to receive and collect DTMF digits once
    the circuit-switched bearer is set up.  The offerer SHOULD compare
    the received DTMF digits to the value of the "dtmf" subfield.  If
    the received DTMF digits match the value of the "dtmf" subfield in
    the "cs-correlation" attribute, the call SHOULD be treated as
    correlated to the ongoing session.
 o  If the answer contained a value for the "uuie" subfield, the
    offerer MUST be prepared to receive a User-User Information
    Element once the circuit-switched bearer is set up.  The offerer
    SHOULD compare the received UUIE to the value of the "uuie"
    subfield.  If the value of the received UUIE matches the value of
    the "uuie" subfield, the call SHOULD be treated as correlated to
    the ongoing session.
 o  If the answer contained an "external" subfield, the offerer MUST
    be prepared to receive a circuit-switched call and use the
    external means (typically, the human user) for accepting or
    rejecting the call.
 According the "An Offer/Answer Model with the Session Description
 Protocol (SDP)" [RFC3264], the offerer needs to be ready to receive
 media as soon as the offer has been sent.  It may happen that the
 answerer, if it became the active party, will initiate a circuit-
 switched bearer setup that will arrive at the offerer before the
 answer has arrived.  However, the offerer needs to receive the answer
 and examine the information about the correlation mechanisms in order
 to successfully perform correlation of the circuit-switched call to
 the session.  Therefore, if the offerer receives an incoming circuit-
 switched call, it MUST NOT accept the call before the answer has been
 received.  If no answer is received during an implementation-specific
 time, the offerer MUST either modify the session according to
 [RFC3264] or terminate it according to the session signaling
 procedures in question (for terminating a SIP session, see Section 15
 of [RFC3261]).

5.6.4. Modifying the Session

 If, at a later time, one of the parties wishes to modify the session,
 e.g., by adding a new media stream or by changing properties used on
 an existing stream, it may do so via the mechanisms defined in "An
 Offer/Answer Model with the Session Description Protocol (SDP)"
 [RFC3264].

Garcia-Martin & Veikkolainen Standards Track [Page 27] RFC 7195 PSTN Circuit-Switched Bearers in SDP May 2014

 If there is an existing circuit-switched bearer between the endpoints
 and the offerer wants to reuse that, the offerer MUST set the value
 of the "connection" attribute to "existing".
 If either party removes the circuit-switched media from the session
 (by setting the port number to zero), it MUST terminate the circuit-
 switched bearer using whatever mechanism is appropriate for the
 technology in question.
 If either party wishes to drop and reestablish an existing call, that
 party MUST first remove the circuit-switched media from the session
 by setting the port number to zero and then use another offer/answer
 exchange where it MUST set the "connection" attribute to "new".  If
 the media types are different (for example, a different codec will be
 used for the circuit-switched bearer), the media descriptions for
 terminating the existing bearer and the new bearer can be in the same
 offer.
 If either party would like to remove existing RTP-based media from
 the session and replace that with a circuit-switched bearer, it would
 create a new offer to add the circuit-switched media as described in
 Section 5.6.1 above, replacing the RTP-based media description with
 the circuit-switched media description, as specified in RFC 3264
 [RFC3264].
 Once the offer/answer exchange is done, but the circuit-switched
 bearer is not yet established, there may be a period of time when no
 media is available.  Also, it may happen that correlating the
 circuit-switched call fails for reasons discussed in Section 5.3.3.
 In this case, even if the offer/answer exchange was successful,
 endpoints are not able to receive or send media.  It is up to the
 implementation to decide the behavior in this case; if nothing else
 is done, the user most likely hangs up after a while if there is no
 other media in the session.  Note that this may also happen when
 switching from one RTP media to another RTP media (for example, when
 firewall blocks the new media stream).
 If either party would like to remove existing circuit-switched media
 from the session and replace that with RTP-based media, it would
 modify the media description as per the procedures defined in RFC
 3264 [RFC3264].  The endpoint MUST then terminate the circuit-
 switched bearer using whatever mechanism is appropriate for the
 technology in question.

5.7. Formal Syntax

 The following is the formal Augmented Backus-Naur Form (ABNF)
 [RFC5234] syntax that supports the extensions defined in this

Garcia-Martin & Veikkolainen Standards Track [Page 28] RFC 7195 PSTN Circuit-Switched Bearers in SDP May 2014

 specification.  The syntax is built above the SDP [RFC4566] and the
 tel URI [RFC3966] grammars.  Implementations that are compliant with
 this specification MUST be compliant with this syntax.
 Figure 2 shows the formal syntax of the extensions defined in this
 memo.
         ; extension to the connection field originally specified
         ; in RFC 4566
         connection-field   =  [%x63 "=" nettype SP addrtype SP
         connection-address CRLF]
         ; CRLF defined in RFC 5234
         ;nettype and addrtype are defined in RFC 4566
         connection-address =/  global-number-digits / "-"
         ; global-number-digits specified in RFC 3966
         ;subrules for correlation attribute
         attribute          =/ cs-correlation-attr
         ; attribute defined in RFC 4566
         cs-correlation-attr = "cs-correlation:" corr-mechanisms
         corr-mechanisms    = corr-mech *(SP corr-mech)
         corr-mech          = caller-id-mech / uuie-mech /
                              dtmf-mech / external-mech /
                              ext-mech
         caller-id-mech     = "callerid" [":" caller-id-value]
         caller-id-value    = "+" 1*15DIGIT
         ; DIGIT defined in RFC 5234
         uuie-mech          = "uuie" [":" uuie-value]
         uuie-value         = 1*65(HEXDIG HEXDIG)
                              ;This represents up to 130 HEXDIG
                              ; (65 octets)
                              ;HEXDIG defined in RFC 5234
                              ;HEXDIG defined as 0-9, A-F
         dtmf-mech          = "dtmf" [":" dtmf-value]
         dtmf-value         = 1*32(DIGIT / %x41-44 / %x23 / %x2A )
                              ;0-9, A-D, '#' and '*'
         external-mech      = "external"
         ext-mech           = ext-mech-name [":" ext-mech-value]
         ext-mech-name      = token
         ext-mech-value     = token
         ; token is specified in RFC 4566
                Figure 2: Syntax of the SDP Extensions

Garcia-Martin & Veikkolainen Standards Track [Page 29] RFC 7195 PSTN Circuit-Switched Bearers in SDP May 2014

6. Examples

 In the examples below, where an SDP line is too long to be displayed
 as a single line, a breaking character "\" indicates continuation in
 the following line.  Note that this character is included for display
 purposes only.  Implementations MUST write a single line without
 breaks.

6.1. Single PSTN Audio Stream

          Endpoint A                        Endpoint B
            |                                  |
            | (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.  Endpoint A generates an SDP
 offer, which is shown in Figure 4.  The offer describes a PSTN
 circuit-switched bearer in the "m=" and "c=" line where it also
 indicates its international E.164 number format.  Additionally,
 Endpoint A expresses that it can initiate the circuit-switched bearer
 or be the recipient of it in the "a=setup" attribute line.  The SDP
 offer also includes correlation identifiers that this endpoint will
 insert in the Calling Party Number and/or User-User Information
 Element of the PSTN call setup if eventually this endpoint initiates
 the PSTN call.  Endpoint A also includes "external" as one
 correlation mechanism, indicating that it can use the human user to
 perform correlation in case other mechanisms fail.

Garcia-Martin & Veikkolainen Standards Track [Page 30] RFC 7195 PSTN Circuit-Switched Bearers in SDP May 2014

         v=0
         o=alice 2890844526 2890842807 IN IP4 192.0.2.5
         s=
         t=0 0
         m=audio 9 PSTN -
         c=PSTN E164 +441134960123
         a=setup:actpass
         a=connection:new
         a=cs-correlation:callerid:+441134960123 \
           uuie:56A390F3D2B7310023 external
                        Figure 4: SDP Offer (1)
 Endpoint B generates an SDP answer (Figure 5), describing a PSTN
 audio media on port 9 without information on the media subtype on the
 "m=" line.  The "c=" line contains B's international E.164 number.
 In the "a=setup" line, Endpoint B indicates that it is willing to
 become the active endpoint when establishing the PSTN call, and it
 also includes the "a=cs-correlation" attribute line containing the
 values it is going to include in the Calling Party Number and User-
 User Information Element of the PSTN call establishment.  Endpoint B
 is also able to perform correlation by external means, in case other
 correlation mechanisms fail.
       v=0
       o=- 2890973824 2890987289 IN IP4 192.0.2.7
       s=
       t=0 0
       m=audio 9 PSTN -
       c=PSTN E164 +441134960124
       a=setup:active
       a=connection:new
       a=cs-correlation:callerid:+441134960124 \
         uuie:74B9027A869D7966A2 external
           Figure 5: SDP Answer with Circuit-Switched Media
 When Endpoint A receives the answer, it examines that B is willing to
 become the active endpoint when setting up the PSTN call.  Endpoint A
 temporarily stores B's E.164 number and the User-User IE value of the
 "cs-correlation" attribute and waits for a circuit-switched bearer
 establishment.
 Endpoint B initiates a circuit-switched bearer using whatever
 circuit-switched technology is available for it.  The Called Party
 Number is set to A's number, and the Calling Party Number is set to
 B's own number.  Endpoint B also sets the User-User Information
 Element value to the one contained in the SDP answer.

Garcia-Martin & Veikkolainen Standards Track [Page 31] RFC 7195 PSTN Circuit-Switched Bearers in SDP May 2014

 When Endpoint A receives the circuit-switched bearer establishment,
 it examines the UUIE and the Calling Party Number and, by comparing
 those received during the offer/answer exchange, determines that the
 call is related to the SDP session.
 It may also be that neither the UUIE nor the Calling Party Number is
 received by the called party, or the format of the Calling Party
 Number is changed by the PSTN.  Implementations may still accept such
 call establishment attempts as being related to the session that was
 established in the IP network.  As it cannot be guaranteed that the
 values used for correlation are always passed intact through the
 network, they should be treated as additional hints that the circuit-
 switched bearer is actually related to the session.

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

  Endpoint A                                Endpoint B
    |                                            |
    | (1) SDP offer (PSTN audio and video)       |
    |------------------------------------------->|
    |                                            |
    | (2) SDP answer (PSTN audio)                |
    |<-------------------------------------------|
    |                                            |
    |   PSTN call setup                          |
    |<-------------------------------------------|
    |                                            |
    |<======== media over PSTN bearer ==========>|
    |                                            |
          Figure 6: Circuit-Switched Audio and Video Streams
 Figure 6 shows an example of negotiating audio and video media
 streams over circuit-switched bearers.

Garcia-Martin & Veikkolainen Standards Track [Page 32] RFC 7195 PSTN Circuit-Switched Bearers in SDP May 2014

       v=0
       o=alice 2890844526 2890842807 IN IP4 192.0.2.5
       s=
       t=0 0
       a=setup:actpass
       a=connection:new
       c=PSTN E164 +441134960123
       m=audio 9 PSTN -
       a=cs-correlation:dtmf:1234536
       m=video 9 PSTN 34
       a=rtpmap:34 H263/90000
       a=cs-correlation:callerid:+441134960123
     Figure 7: SDP Offer with Circuit-Switched Audio and Video (1)
 Upon receiving the SDP offer described in Figure 7, Endpoint B
 rejects the video stream as the device does not currently support
 video, but it accepts the circuit-switched audio stream.  As Endpoint
 A indicated that it is able to become either the active or passive
 party, Endpoint B gets to select which role it would like to take.
 Since the offer contained the international E.164 number of Endpoint
 A, Endpoint B decides that it becomes the active party in setting up
 the circuit-switched bearer.  B includes a new value in the "dtmf"
 subfield of the "cs-correlation" attribute, which it is going to send
 as DTMF tones once the bearer setup is complete.  The answer is
 described in Figure 8.
       v=0
       o=- 2890973824 2890987289 IN IP4 192.0.2.7
       s=
       t=0 0
       a=setup:active
       a=connection:new
       c=PSTN E164 +441134960124
       m=audio 9 PSTN -
       a=cs-correlation:dtmf:654321
       m=video 0 PSTN 34
       a=cs-correlation:callerid:+441134960124
    Figure 8: SDP Answer with Circuit-Switched Audio and Video (2)

7. Security Considerations

 This document provides an extension to RFC 4566 [RFC4566] and RFC
 3264 [RFC3264].  As such, the security considerations of those
 documents apply.

Garcia-Martin & Veikkolainen Standards Track [Page 33] RFC 7195 PSTN Circuit-Switched Bearers in SDP May 2014

 This memo provides mechanisms to agree on a correlation identifier or
 identifiers that are used to evaluate whether an incoming circuit-
 switched bearer is related to an ongoing session in the IP domain.
 If an attacker replicates the correlation identifier and establishes
 a call within the time window the receiving endpoint is expecting a
 call, the attacker may be able to hijack the circuit-switched bearer.
 These types of attacks are not specific to the mechanisms presented
 in this memo.  For example, Caller ID spoofing is a well-known attack
 in the PSTN.  Users are advised to use the same caution before
 revealing sensitive information as they would on any other phone
 call.  Furthermore, users are advised that mechanisms that may be in
 use in the IP domain for securing the media, like Secure RTP (SRTP)
 [RFC3711], are not available in the CS domain.
 For the purposes of establishing a circuit-switched bearer, the
 active endpoint needs to know the passive endpoint's phone number.
 Phone numbers are sensitive information, and some people may choose
 not to reveal their phone numbers when calling using supplementary
 services like Calling Line Identification Restriction (CLIR) in GSM.
 Implementations should take the caller's preferences regarding
 calling line identification into account if possible, by restricting
 the inclusion of the phone number in the SDP "c=" line if the caller
 has chosen to use CLIR.  If this is not possible, implementations may
 present a prompt informing the user that their phone number may be
 transmitted to the other party.
 As with IP addresses, if there is a desire to protect the SDP
 containing phone numbers carried in SIP, implementers are advised to
 follow the security mechanisms defined in [RFC3261].
 It is possible that an attacker creates a circuit-switched session
 whereby the attacked endpoint should dial a circuit-switched number,
 perhaps even a premium-rate telephone number.  To mitigate the
 consequences of this attack, endpoints MUST authenticate and trust
 remote endpoints users who try to remain passive in the circuit-
 switched connection establishment.  It is RECOMMENDED that endpoints
 have local policies precluding the active establishment of circuit-
 switched connections to certain numbers (e.g., international,
 premium, and long distance).  Additionally, it is strongly
 RECOMMENDED that the end user is asked for consent prior to the
 endpoint initiating a circuit-switched connection.

Garcia-Martin & Veikkolainen Standards Track [Page 34] RFC 7195 PSTN Circuit-Switched Bearers in SDP May 2014

8. IANA Considerations

 IANA has registered a number of SDP tokens according to the following
 data.

8.1. Registration of the New "cs-correlation" SDP Attribute

    Contact: Miguel Garcia <miguel.a.garcia@ericsson.com>
    Attribute name: cs-correlation
    Long-form attribute name: PSTN Correlation Identifier
    Type of attribute: media level only
    Subject to charset: No
    Description: This attribute provides the Correlation Identifier
    used in PSTN signaling
    Appropriate values: see Section 5.2.3.1
    Specification: RFC 7195
 The IANA has created a subregistry for the "cs-correlation" attribute
 under the "Session Description Protocol (SDP) Parameters" registry.
 The initial values for the subregistry are presented in the
 following; IANA has registered these values accordingly:
 Value of "cs-correlation" attribute Reference Description
 ----------------------------------- --------- -------------------
 callerid                            RFC 7195  Caller ID
 uuie                                RFC 7195  User-User
                                               Information Element
 dtmf                                RFC 7195  Dual-Tone
                                               Multi-Frequency
 external                            RFC 7195  External
 As per the terminology in [RFC5226], the registration policy for new
 values of the "cs-correlation" attribute is "Specification Required".

Garcia-Martin & Veikkolainen Standards Track [Page 35] RFC 7195 PSTN Circuit-Switched Bearers in SDP May 2014

8.2. Registration of a New "nettype" Value

 IANA has registered a new "nettype" in the "Session Description
 Protocol (SDP) Parameters" registry [IANA].  The registration data,
 according to RFC 4566 [RFC4566], is as follows.
 Type             SDP Name             Reference
 --------------   ------------------   ---------
 nettype          PSTN                 RFC 7195

8.3. Registration of a New "addrtype" Value

 IANA has registered a new "addrtype" in the "Session Description
 Protocol (SDP) Parameters" registry [IANA].  The registration data,
 according to RFC 4566 [RFC4566], is as follows.
 Type             SDP Name             Reference
 --------------   ------------------   ---------
 addrtype         E164                 RFC 7195
 Note: This document defines the "E164" addrtype in the context of the
 "PSTN" nettype only.  RFC 3108 [RFC3108] also defines address type
 "E.164".  This definition is distinct from the one defined by this
 memo and shall not be used with <nettype> "PSTN".

8.4. Registration of a New "proto" Value

 IANA has registered a new "proto" in the "Session Description
 Protocol (SDP) Parameters" registry [IANA].  The registration data,
 according to RFC 4566 [RFC4566], is as follows.
 Type             SDP Name             Reference
 --------------   ------------------   ---------
 proto            PSTN                 RFC 7195
 The related "fmt" namespace reuses the conventions and payload type
 number defined for RTP/AVP.  In this document, the RTP audio and
 video media types, when applied to PSTN circuit-switched bearers,
 represent merely an audio or video codec in its native format
 directly on top of a single PSTN bearer.
 In some cases, the endpoint is not able to determine the list of
 available codecs for circuit-switched media streams.  In this case,
 in order to be syntactically compliant with SDP [RFC4566], the
 endpoint MUST include a single dash ("-") in the <fmt> subfield.

Garcia-Martin & Veikkolainen Standards Track [Page 36] RFC 7195 PSTN Circuit-Switched Bearers in SDP May 2014

9. Acknowledgments

 The authors want to thank Paul Kyzivat, Flemming Andreasen, Thomas
 Belling, John Elwell, Jari Mutikainen, Miikka Poikselka, Jonathan
 Rosenberg, Ingemar Johansson, Christer Holmberg, Alf Heidermark, Tom
 Taylor, Thomas Belling, Keith Drage, and Andrew Allen for providing
 their insight and comments on this document.

10. References

10.1. Normative References

 [ITU.Q931.1998]
             International Telecommunications Union, "Digital
             Subscriber Signalling System No. 1 - ISDN User-Network
             Interface Layer 3 Specification for Basic Call Control",
             ITU-T Recommendation Q931, May 1998.
 [RFC2119]   Bradner, S., "Key words for use in RFCs to Indicate
             Requirement Levels", BCP 14, RFC 2119, March 1997.
 [RFC3264]   Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
             with Session Description Protocol (SDP)", RFC 3264, June
             2002.
 [RFC3966]   Schulzrinne, H., "The tel URI for Telephone Numbers", RFC
             3966, December 2004.
 [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.
 [RFC4648]   Josefsson, S., "The Base16, Base32, and Base64 Data
             Encodings", RFC 4648, October 2006.
 [RFC5226]   Narten, T. and H. Alvestrand, "Guidelines for Writing an
             IANA Considerations Section in RFCs", BCP 26, RFC 5226,
             May 2008.
 [RFC5234]   Crocker, D. and P. Overell, "Augmented BNF for Syntax
             Specifications: ABNF", STD 68, RFC 5234, January 2008.

Garcia-Martin & Veikkolainen Standards Track [Page 37] RFC 7195 PSTN Circuit-Switched Bearers in SDP May 2014

10.2. Informative References

 [IANA]      IANA, "Session Description Protocol (SDP) Parameters
             Registry", <http://www.iana.org/assignments/
             sdp-parameters>.
 [ITU.E164.2010]
             International Telecommunications Union, "The
             International Public Telecommunication Numbering Plan",
             ITU-T Recommendation E.164, 2010.
 [ITU.Q23.1988]
             International Telecommunications Union, "Technical
             features of push-button telephone sets", ITU-T Technical
             Recommendation Q.23, 1988.
 [RFC3108]   Kumar, R. and M. Mostafa, "Conventions for the use of the
             Session Description Protocol (SDP) for ATM Bearer
             Connections", RFC 3108, May 2001.
 [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.
 [RFC3711]   Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
             Norrman, "The Secure Real-time Transport Protocol
             (SRTP)", RFC 3711, March 2004.
 [RFC3725]   Rosenberg, J., Peterson, J., Schulzrinne, H., and G.
             Camarillo, "Best Current Practices for Third Party Call
             Control (3pcc) in the Session Initiation Protocol (SIP)",
             BCP 85, RFC 3725, April 2004.
 [RFC4975]   Campbell, B., Mahy, R., and C. Jennings, "The Message
             Session Relay Protocol (MSRP)", RFC 4975, September 2007.
 [SIP-UUI]   Johnston, A. and J. Rafferty, "A Mechanism for
             Transporting User to User Call Control Information in
             SIP", Work in Progress, April 2014.

Garcia-Martin & Veikkolainen Standards Track [Page 38] RFC 7195 PSTN Circuit-Switched Bearers in SDP May 2014

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

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 226
 NOKIA GROUP, FI  00045
 Finland
 Phone: +358 50 486 4463
 EMail: simo.veikkolainen@nokia.com

Garcia-Martin & Veikkolainen Standards Track [Page 39]

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