MMUSIC WG M. Garcia-Martin Internet-Draft Ericsson Intended status: Standards Track S. Veikkolainen Expires: February 23, 2011 Nokia August 22, 2010 Session Description Protocol (SDP) Extension For Setting Up Audio and Video Media Streams Over Circuit-Switched Bearers In The Public Switched Telephone Network (PSTN) draft-ietf-mmusic-sdp-cs-04 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 Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at http://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." This Internet-Draft will expire on February 23, 2011. Copyright Notice Copyright (c) 2010 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 Garcia-Martin & Veikkolainen Expires February 23, 2011 [Page 1] Internet-Draft PSTN Circuit-Switched Bearers in SDP August 2010 the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. This document may contain material from IETF Documents or IETF Contributions published or made publicly available before November 10, 2008. The person(s) controlling the copyright in some of this material may not have granted the IETF Trust the right to allow modifications of such material outside the IETF Standards Process. Without obtaining an adequate license from the person(s) controlling the copyright in such materials, this document may not be modified outside the IETF Standards Process, and derivative works of it may not be created outside the IETF Standards Process, except to format it for publication as an RFC or to translate it into languages other than English. Garcia-Martin & Veikkolainen Expires February 23, 2011 [Page 2] Internet-Draft PSTN Circuit-Switched Bearers in SDP August 2010 Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4 2. Conventions Used in This Document . . . . . . . . . . . . . . 5 3. Requirements . . . . . . . . . . . . . . . . . . . . . . . . . 5 4. Overview of Operation . . . . . . . . . . . . . . . . . . . . 6 4.1. Example Call Flow . . . . . . . . . . . . . . . . . . . . 6 5. Protocol Description . . . . . . . . . . . . . . . . . . . . . 8 5.1. Level of Compliance . . . . . . . . . . . . . . . . . . . 8 5.2. Extensions to SDP . . . . . . . . . . . . . . . . . . . . 8 5.2.1. Connection Data . . . . . . . . . . . . . . . . . . . 8 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 . . . . . . . . . 11 5.2.3.3. User-User Information Element Correlation Mechanism . . . . . . . . . . . . . . . . . . . . 12 5.2.3.4. DTMF Correlation Mechanism . . . . . . . . . . . . 13 5.2.3.5. Negotiating the used correlation mechanisms . . . 15 5.3. Considerations for Usage of Existing SDP . . . . . . . . . 17 5.3.1. Originator of the Session . . . . . . . . . . . . . . 17 5.3.2. Contact information . . . . . . . . . . . . . . . . . 17 5.3.3. Determining the Direction of the Circuit-Switched Connection Setup . . . . . . . . . . . . . . . . . . . 17 5.4. Formal Syntax . . . . . . . . . . . . . . . . . . . . . . 18 6. SDP Examples . . . . . . . . . . . . . . . . . . . . . . . . . 19 6.1. Basic SDP example: Single Circuit-Switched Audio Stream . 20 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 20 7.1. Registration of new correlation SDP attribute . . . . . . 21 7.2. Registration of a new "nettype" value . . . . . . . . . . 21 7.3. Registration of new "addrtype" values . . . . . . . . . . 21 7.4. Registration of a new "proto" value . . . . . . . . . . . 21 8. Security Considerations . . . . . . . . . . . . . . . . . . . 22 9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 22 10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 22 10.1. Normative References . . . . . . . . . . . . . . . . . . . 22 10.2. Informative References . . . . . . . . . . . . . . . . . . 23 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 24 Garcia-Martin & Veikkolainen Expires February 23, 2011 [Page 3] Internet-Draft PSTN Circuit-Switched Bearers in SDP August 2010 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 or video 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 or video 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 or video media streams over to the high-speed bearer. This implies a new Garcia-Martin & Veikkolainen Expires February 23, 2011 [Page 4] Internet-Draft PSTN Circuit-Switched Bearers in SDP August 2010 exchange 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 or video codecs are not the same in both access networks. Consider that 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 an example of descriptions of circuit-switched audio or video 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 or video media stream 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. Garcia-Martin & Veikkolainen Expires February 23, 2011 [Page 5] Internet-Draft PSTN Circuit-Switched Bearers in SDP August 2010 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 bearer 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 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 and allows describing an audio or video media stream established over a circuit-switched bearer. New tokens are registered in 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 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. Garcia-Martin & Veikkolainen Expires February 23, 2011 [Page 6] Internet-Draft PSTN Circuit-Switched Bearers in SDP August 2010 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]. Garcia-Martin & Veikkolainen Expires February 23, 2011 [Page 7] Internet-Draft PSTN Circuit-Switched Bearers in SDP August 2010 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 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= where indicates the network type, indicates the address type, and the 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. Garcia-Martin & Veikkolainen Expires February 23, 2011 [Page 8] Internet-Draft PSTN Circuit-Switched Bearers in SDP August 2010 Note that and/or 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= ... The sub-field 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 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 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 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 sub-field is the media format description. In the classical usage of SDP to describe RTP-based media streams, when the sub-field is set to "RTP/AVP" or "RTP/SAVP", the 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/video profile payload type, we use the sub-field to indicate the list of available media types Garcia-Martin & Veikkolainen Expires February 23, 2011 [Page 9] Internet-Draft PSTN Circuit-Switched Bearers in SDP August 2010 over the circuit-switched bearer. Therefore, the sub-field MAY indicate one or more available audio or video codecs for a circuit- switched audio or video stream. We use the classical RTP audio and video media types, even when applied to PSTN circuit-switched bearers, the media type merely represents an audio or video codec. However, 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 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 - 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 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. Other correlation mechanisms might exist as well, and their usage will be specified when need arises. All mechanisms share the same principles: 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 ID in the circuit-switched signaling. Garcia-Martin & Veikkolainen Expires February 23, 2011 [Page 10] Internet-Draft PSTN Circuit-Switched Bearers in SDP August 2010 The second mechanism is based on the inclusion in SDP of a value that is also sent in the User-to-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 MultiFrequency (DTMF) digits that will be later sent right after the circuit-switched bearer is established. Implementations MAY use any of these mechanisms and MAY use two or more mechanisms simultaneously. 5.2.3.1. The "cs-correlation" attribute In order to provide support for the correlation mechanisms, we define a new SDP attribute called "cs-correlation". This "cs-correlation" attribute can include any of the "callerid", "uuie", or "dtmf" parameters, which specify additional information required by the Caller-ID, User to User Information, or DTMF correlation mechanisms, respectively. The list of correlation mechanisms may be extended by other specifications. The following sections provide more detailed information of these parameters. The "cs-correlation" attribute has the following format: "a=cs-correlation: "callerid:" | "uuie:" | "dtmf:" The values "callerid", "uuie" and "dtmf" refer to the correlation mechanisms defined in Section 5.2.3.2, Section 5.2.3.3, and Section 5.2.3.4, respectively. The formal Augmented Backus-Naur Format (ABNF) syntax of the "cs-correlation" attribute is presented in Section 5.4. 5.2.3.2. 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. If both pieces of information match, the circuit-switched bearer is correlated to the session described in SDP. An endpoint that is feasible to become the active party for setting up the circuit-switched bearer and is willing to send the Calling Party Number in the PSTN signaling SHOULD add a "callerid" parameter in the "cs-correlation" attribute of the SDP offer or answer, and SHOULD include as the value the E.164 number that will be presented Garcia-Martin & Veikkolainen Expires February 23, 2011 [Page 11] Internet-Draft PSTN Circuit-Switched Bearers in SDP August 2010 in the Calling Party Number in the PSTN signaling. An endpoint that acts as the passive party for setting up the circuit-switch bearer SHOULD add a "callerid" parameter in the "cs- correlation" attribute of the SDP if it supports the mechanism, and MAY include the E.164 number that will be presented in the circuit- switched bearer in the same corresponding lines, although these are not used for correlation. Example of inclusion of E.164 number in the "cs-correlation" attribute is: a=cs-correlation:callerid:+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. To mitigate this problem implementations should 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. 5.2.3.3. User-User Information Element Correlation Mechanism A second correlation mechanism is based on indicating 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 [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. Garcia-Martin & Veikkolainen Expires February 23, 2011 [Page 12] Internet-Draft PSTN Circuit-Switched Bearers in SDP August 2010 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 a "cs-correlation:uuie" 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. 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. An endpoint that is feasible to become the active party for setting up the PSTN call and is willing to send the User-User Information Element in the PSTN signaling SHOULD add a "uuie" parameter in the "cs-correlation" attribute of the SDP offer or answer. This "uuie" parameter SHOULD include the value of the User-User Information Element that will be used in the call setup attempt. An endpoint that takes the role of the passive party for setting up the circuit-switched bearer SHOULD include include a "uuie" parameter in the "cs-correlation" attribute in the SDP, if it supports the UUI mechanism. It MAY also add a value for the "uuie" parameter 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. 5.2.3.4. DTMF Correlation Mechanism We introduce a third mechanism for correlating the circuit-switched bearer with the session controlled with SDP. This is based on Garcia-Martin & Veikkolainen Expires February 23, 2011 [Page 13] Internet-Draft PSTN Circuit-Switched Bearers in SDP August 2010 agreeing on a sequence of digits that are negotiated in the SDP Offer/Answer exchange and sent as Dual Tone Multifrequency (DTMF)tones 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 the session described in the SDP. The mechanism is similar to many voice conferencing systems which 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 "cs-correlation:dtmf" 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 as per 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 correlates 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. An endpoint that is feasible to become the active party for setting up the PSTN call and is willing to send the DTMF digits after circuit-switched bearer cut-through SHOULD include a "dtmf" parameter in the "cs-correlation" attribute of the SDP offer or answer. The value of the "dtmf" parameter SHOULD contain up to 32 randomly selected DTMF digits (numbers 0-9, characters A-D, "#" and "*"). Implementations are advised to select a number of DTMF digits that provide enough assurance that the call is related, but on the other hand do not prolong the bearer setup time unnecessarily. As an example, an endpoint willing to send DTMF tone sequence "14D*3" would include a "cs-correlation" attribute line as follows: a=cs-correlation:dtmf:14D*3 An endpoint that takes the role of the passive party for setting up the circuit-switched bearer SHOULD include include a "dtmf" parameter in the "cs-correlation" attribute in the SDP, if it supports the mechanism. It MAY also add a value for the "dtmf" parameter although it is not used for correlation purposes. Once the circuit-switched bearer is successfully set up, the active side MUST send DTMF digits according to the circuit-switched bearer technology used. The values and number of the DTMF digits MUST match Garcia-Martin & Veikkolainen Expires February 23, 2011 [Page 14] Internet-Draft PSTN Circuit-Switched Bearers in SDP August 2010 those that were agreed during SDP negotiation. The passive side of the circuit-switched connection setup MUST be prepared to receive and collect DTMF digits once the circuit-switched bearer is set up. The received DTMF digits are compared to the value of the "dtmf" parameter of the "cs-correlation" attribute that the the active side sent during SDP offer/answer exchange. If the received DTMF digits match the value of the "dtmf" parameter in the "cs-correlation" attribute, the call SHOULD be treated as correlated to the ongoing session. If the offerer and answerer 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 receving too many digits), the passive side SHOULD treat the circuit-switched bearer as not correlated to the ongoing session. DTMF digits can only be sent once the circuit-switched bearer is set up. In order to suppress alerting for an incoming circuit- switched call, implementations may choose various mechanisms. For example, alerting may be suppressed for a certain time period for incoming call attempts that originate from the number that was observed during the offer/answer negotiation. 5.2.3.5. Negotiating the used correlation mechanisms The three correlation mechanisms presented above (based on called party number, User-User Information Element and DTMF digit sending) are non-exclusive, and can be used independently of each other. In order to agree which correlation mechanisms are supported by each endpoint, we define a negotiation mechanism similar to the one defined for codec negotiation. In some cases an endpoint may support the correlation mechanism, but it is not willing to become the active party in the circuit-switched bearer establishment. If the offerer supports any of the correlation mechanisms defined in this memo, it SHOULD include an attribute line "a=cs-correlation" in the SDP offer. The "a=cs-correlation" line contains an enumeration of the correlation mechanisms supported by the offerer, in the format of parameters. The current list of parameters include "callerid", "uuie" and "dtmf" and they refer to the correlation mechanisms defined in Section 5.2.3.2, Section 5.2.3.3, and Section 5.2.3.4, respectively. For example, if an endpoint is willing to use the Garcia-Martin & Veikkolainen Expires February 23, 2011 [Page 15] Internet-Draft PSTN Circuit-Switched Bearers in SDP August 2010 User-User Information element and DTMF digit sending mechanisms, it includes the following line to the SDP: a=cs-correlation:uuie dtmf The answerer, when generating the answer, SHOULD select those correlation mechanisms it supports, and include an "a=cs-correlation" attribute line in the answer containing those mechanisms it supports. The answerer MUST NOT add any mechanism which was not included in the offer. If the answer does not contain an "a=cs-correlation" attribute line, the offerer MUST interpret this as an indication that the anwerer does not support any of the correlation mechanisms for this session. If, in addition to supporting any of the correlation mechanisms, an endpoint is willing to assume the role of the active party in establishing the circuit-switched bearer, it MUST add a parameter value to the supported mechanisms. For example, if the endpoint supports and is willing to send the User-User Information element and DTMF digits, it includes the following line to the SDP offer: a=cs-correlation:uuie:2890W284hAT452612908awudfjang908 dtmf:14D*3 The answerer SHOULD select those correlation mechanisms it supports and is willing to use, and include respective parameter values. If the answerer supports but is not willing to use some of the mechanisms (for example, due to not being able to become the active endpoint when setting up the circuit-switched bearer), it SHOULD include the respective parameter, but MUST NOT add a value to the parameter. Note that, as stated above, it cannot be guaranteed that any given correlation mechanism will succeed even if the usage of those was agreed beforehand. This is due to the fact that the 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 negotiating one or more correlation mechanisms in the SDP offer/answer exchange, an endpoint receives a circuit-switched call with no correlation information present, the endpoint has two choices: it can either treat the call as unrelated, or treat the call as related to the ongoing session in the IP domain. Garcia-Martin & Veikkolainen Expires February 23, 2011 [Page 16] Internet-Draft PSTN Circuit-Switched Bearers in SDP August 2010 An endpoint may for example specify a time window after SDP offer/ answer exchange during which received calls are treated as correlated even if the signalling in the circuit-switched domain does not carry any correlation information. In this case, there is a chance that the call is erroneously treated as related to the ongoing session. An endpoint may also choose to always treat an incoming call as unrelated if the signalling in the circuit-switched domain does not carry any correlation information. In this case, there is a chance that the call is erroneously treated as unrelated. Since, in these cases, no correlation information can be deduced from the signalling, it is up to the implementation to decide how to behave. One option is also to let the user decide whether to accept the call as related, or to treat the call as unrelated. 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= Of interest here are the and 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, and 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 of the Circuit-Switched Connection Setup Either endpoint can initiate the establishment of the circuit- Garcia-Martin & Veikkolainen Expires February 23, 2011 [Page 17] Internet-Draft PSTN Circuit-Switched Bearers in SDP August 2010 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 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. Garcia-Martin & Veikkolainen Expires February 23, 2011 [Page 18] Internet-Draft PSTN Circuit-Switched Bearers in SDP August 2010 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] ;nettype and addrtype are defined in RFC 4566 connection-address /= e164-address / "-" e164-address = ["+"] 1*15DIGIT ; DIGIT is specified in RFC 5234 ;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 / ext-mech caller-id-mech = "callerid" [":" caller-id-value] caller-id-value = ["+"] 1*DIGIT uuie-mech = "uuie" [":" uuie-value] uuie-value = 1*32(ALPHA/DIGIT) dtmf-mech = "dtmf" [":" dtmf-value] dtmf-value = 1*32(DIGIT / %x41-44 / %x23 / %x2A ) ;0-9, A-D, '#' and '*' ext-mech = token ; token is specified in RFC4566 Figure 2: Syntax of the SDP extensions 6. SDP Examples Garcia-Martin & Veikkolainen Expires February 23, 2011 [Page 19] Internet-Draft PSTN Circuit-Switched Bearers in SDP August 2010 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 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=cs-correlation:uuie:2890W284hAT452612908awudfjang908 Figure 4: SDP offer (1) 7. IANA Considerations This document instructs IANA to register a number of SDP tokens according to the following data. Garcia-Martin & Veikkolainen Expires February 23, 2011 [Page 20] Internet-Draft PSTN Circuit-Switched Bearers in SDP August 2010 7.1. Registration of new correlation SDP attribute Contact: Miguel Garcia Attribute name: cs-correlation 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] Garcia-Martin & Veikkolainen Expires February 23, 2011 [Page 21] Internet-Draft PSTN Circuit-Switched Bearers in SDP August 2010 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. This memo provides mechanisms to agree on a correlation identifier or identifiers that are used to evaluate whether an incoming circuit- switched call is related to an ongoing session in the IP domain. If an attacker replicates the correlation identifer 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 call. 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. 9. Acknowledgments The authors want to thank Flemming Andreasen, Thomas Belling, John Elwell, 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. Garcia-Martin & Veikkolainen Expires February 23, 2011 [Page 22] Internet-Draft PSTN Circuit-Switched Bearers in SDP August 2010 [RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session Description Protocol", RFC 4566, July 2006. [RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", STD 68, RFC 5234, January 2008. 10.2. Informative References [3GPP.24.008] 3GPP, "Mobile radio interface Layer 3 specification; Core network protocols; Stage 3", 3GPP TS 24.008 3.20.0, December 2005. [ITU.E164.1991] International Telecommunications Union, "The International Public Telecommunication Numbering Plan", ITU- T Recommendation E.164, 1991. [ITU.Q931.1998] "Digital Subscriber Signalling System No. 1 (DSS 1) - ISDN User - Network Interface Layer 3 Specification for Basic Call Control", ISO Standard 9594-1, May 1998. [RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A., Peterson, J., Sparks, R., Handley, M., and E. Schooler, "SIP: Session Initiation Protocol", RFC 3261, June 2002. [RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson, "RTP: A Transport Protocol for Real-Time Applications", STD 64, RFC 3550, July 2003. [RFC3551] Schulzrinne, H. and S. Casner, "RTP Profile for Audio and Video Conferences with Minimal Control", STD 65, RFC 3551, July 2003. [RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K. Norrman, "The Secure Real-time Transport Protocol (SRTP)", RFC 3711, March 2004. [RFC4975] Campbell, B., Mahy, R., and C. Jennings, "The Message Session Relay Protocol (MSRP)", RFC 4975, September 2007. URIs [1] Garcia-Martin & Veikkolainen Expires February 23, 2011 [Page 23] Internet-Draft PSTN Circuit-Switched Bearers in SDP August 2010 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 Garcia-Martin & Veikkolainen Expires February 23, 2011 [Page 24]