MMUSIC WG M. Garcia-Martin
Internet-Draft Ericsson
Intended status: Standards Track S. Veikkolainen
Expires: April 11, 2011 Nokia
October 08, 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-05
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
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This Internet-Draft will expire on April 11, 2011.
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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. Example . . . . . . . . . . . . . . . . . . . . . . . . . . . 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
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1. Introduction
The Session Description Protocol (SDP) [RFC4566] is intended for
describing multimedia sessions for the purposes of session
announcement, session invitation, and other forms of multimedia
session initiation. SDP is most commonly used for describing media
streams that are transported over the Real-Time Transport Protocol
(RTP) [RFC3550], using the profiles for audio and video media defined
in RTP Profile for Audio and Video Conferences with Minimal Control
[RFC3551].
However, SDP can be used to describe other transport protocols than
RTP. Previous work includes SDP conventions for describing ATM
bearer connections [RFC3108] and the Message Session Relay Protocol
[RFC4975].
SDP is commonly carried in Session Initiation Protocol (SIP)
[RFC3261] messages in order to agree on a common media description
among the endpoints. An Offer/Answer Model with Session Description
Protocol (SDP) [RFC3264] defines a framework by which two endpoints
can exchange SDP media descriptions and come to an agreement as to
which media streams should be used, along with the media related
parameters.
In some scenarios it might be desirable to establish the media stream
over a circuit-switched bearer connection even if the signaling for
the session is carried over an IP bearer. An example of such a
scenario is illustrated with two mobile devices capable of both
circuit-switched and packet-switched communication over a low-
bandwidth radio bearer. The radio bearer may not be suitable for
carrying real-time audio 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
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exchange of SDP Offer/Answer that lead to a re-negotiation of the
media streams.
Other use cases exist. 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.
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REQ-3: The mechanism MUST allow the endpoint to negotiate the
direction of the circuit-switched connection, i.e., which
endpoint is active when initiating the circuit-switched
connection.
REQ-4: The mechanism MUST be independent of the type of the circuit-
switched access (e.g., Integrated Services Digital Network
(ISDN), Global System for Mobile Communication (GSM), etc.)
REQ-5: There MUST be a mechanism that helps an endpoint to correlate
an incoming circuit-switched 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 connection-oriented media streams, the mechanism
re-uses the connection-oriented extensions defined in RFC 4145
[RFC4145] to negotiate the active and passive sides of a connection
setup. This is further described in Section 5.3.3.
4.1. Example Call Flow
Consider the example presented in Figure 1. In this example, Alice
is located in an environment where she has access to both IP and
circuit-switched bearers for communicating with other endpoints.
Alice decides that the circuit-switched bearer offers a better
perceived quality of service for voice, and issues an SDP Offer
containing the description of an audio media stream over circuit-
switched bearer.
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Alice Bob
| (1) SDP Offer (PSTN audio) |
|----------------------------------->|
| |
| (2) SDP Answer (PSTN audio) |
|<-----------------------------------|
| |
| PSTN call setup |
|<-----------------------------------|
| |
| |
|<===== media over PSTN bearer =====>|
| |
Figure 1: Example Flow
Bob receives the SDP offer and determines that he is located in an
environment where the IP based bearer is not suitable for real-time
audio media. However he also has PSTN circuit-switched bearer
available for audio. Bob generates an SDP answer containing a
description of the audio media stream over a circuit-switched bearer.
During the offer-answer exchange Alice and Bob also agree the
direction in which the circuit-switched connection should be
established. The exchange contains identifiers or references that
can be used on the circuit-switched network for addressing the other
endpoint, as well as identifying that the incoming circuit-switched
bearer establishment is related to the ongoing session between Alice
and Bob.
Bob establishes a circuit-switched bearer towards Alice using
whatever mechanisms are defined for the network type in question.
When receiving the incoming circuit-switched connection attempt,
Alice is able to determine that the attempt is related to the session
she is just establishing with Bob.
Alice accepts the circuit-switched connection; the circuit-switched
bearer setup is completed. Bob and Alice can now use the circuit-
switched connection for two-way audio media.
If, for some reason, Bob would like to reject the offered stream, he
would set the port number of the specific stream to zero, as
specified in RFC3264 [RFC3264]. Also, if Bob does not understand
some of the SDP attributes specified in this document, he would
ignore them, as specified in RFC4566 [RFC4566].
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5. Protocol Description
5.1. Level of Compliance
Implementations according to this specification MUST implement the
SDP extensions described in Section 5.2, and MUST implement the
considerations discussed in Section 5.3.
5.2. Extensions to SDP
This section provides the syntax and semantics of the extensions
required for providing a description of audio 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 the <connection-address> is the connection address,
which is dependent on the address type.
At the moment, the only network type defined is "IN", which indicates
Internet network type. The address types "IP4" and "IP6" indicate
the type of IP addresses.
This memo defines a new network type for describing a circuit-
switched bearer network type in the PSTN. The mnemonic "PSTN" is
used for this network type.
For the address type, we initially consider the possibility of
describing E.164 telephone numbers. We define a new "E164" address
type. When used, the "E164" address type indicates that the
connection address contains a telephone number represented according
to the ITU-T E.164 [ITU.E164.1991] recommendation.
There are cases, though, when the endpoint is merely aware of a
circuit-switched bearer, without having further information about the
address type or the E.164 number allocated to it. In these cases a
dash "-" is used to indicate an unknown address type or connection
address. This makes the connection data line be according to the SDP
syntax.
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Note that <addrtype> and/or <connection-address> should not be
omitted without being set to a "-" since this would violate basic
syntax of SDP [RFC4566].
The following are examples of the extension to the connection data
line:
c=PSTN E164 +15551234
c=PSTN - -
5.2.2. Media Descriptions
According to SDP [RFC4566], the media descriptions line in SDP has
the following syntax:
m=<media> <port> <proto> <fmt> ...
The <media> sub-field carries the media type. 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> sub-field is the transport port to which the media stream
is sent. Circuit-switched access lacks the concept of a port number,
and therefore the <port> sub-field is set to the discard port "9".
According to RFC 3264 [RFC3264], a port number of zero in the offer
of a unicast stream indicates that the stream is offered but must not
be used. If a port number of zero is present in the answer of a
unicast stream, it indicates that the stream is rejected. These
rules are still valid when the media line in SDP represents a
circuit-switched bearer.
The <proto> sub-field is the transport protocol. The circuit-
switched bearer uses whatever transport protocol it has available.
This subfield SHOULD be set to the mnemonic "PSTN" to be
syntactically correct with SDP [RFC4566] and to indicate the usage of
circuit-switched protocols in the PSTN.
The <fmt> sub-field is the media format description. In the
classical usage of SDP to describe RTP-based media streams, when the
<proto> sub-field is set to "RTP/AVP" or "RTP/SAVP", the <fmt> sub-
field contains the payload types as defined in the RTP audio profile
[RFC3551].
In the case of circuit-switched descriptions, RTP is not really used.
Rather than specifying the RTP audio/video profile payload type, we
use the <fmt> sub-field to indicate the list of available media types
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over the circuit-switched bearer. Therefore, the <fmt> 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 <fmt> sub-field.
As per RFC 4566 [RFC4566], the media format descriptions are listed
in priority order.
Example of a media description for circuit-switched audio streams is:
m=audio 9 PSTN 3 0 8
m=audio 9 PSTN -
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.
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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:<callerid-value> |
iuie:<uuie-value> |
dtmf:<dtmf-value> |
[extension-name:<extension-value>]
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
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SHOULD include as the value the E.164 number that will be presented
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.
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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
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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 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.
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
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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
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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 answerer
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.
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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 signaling 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 signaling 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 signaling, 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=<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.
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-
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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.
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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 = ext-mech-name[":" ext-mech-value]
ext-mech-name = token
ext-mech-value = token
; token is specified in RFC4566
Figure 2: Syntax of the SDP extensions
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6. Example
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.
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7.1. Registration of new 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
This attribute is subject to the charset attribute
Description: This attribute provides the Correlation Identifier
used in PSTN signaling
Specification: RFC XXXX
7.2. Registration of a new "nettype" value
This memo provides instructions to IANA to register a new "nettype"
in the Session Description Protocol Parameters registry [1]. The
registration data, according to RFC 4566 [RFC4566] follows.
Type SDP Name Reference
---- ------------------ ---------
nettype PSTN [RFCxxxx]
7.3. Registration of new "addrtype" values
This memo provides instructions to IANA to register a new "addrtype"
in the Session Description Protocol Parameters registry [1]. The
registration data, according to RFC 4566 [RFC4566] follows.
Type SDP Name Reference
---- ------------------ ---------
addrtype E164 [RFCxxxx]
- [RFCxxxx]
7.4. Registration of a new "proto" value
This memo provides instructions to IANA to register a new "proto" in
the Session Description Protocol Parameters registry [1]. The
registration data, according to RFC 4566 [RFC4566] follows.
Type SDP Name Reference
-------------- --------------------------- ---------
proto PSTN [RFCxxxx]
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8. Security Considerations
This document provides an extension on top of RFC 4566 [RFC4566], and
RFC 3264 [RFC3264]. As such, the security considerations of those
documents apply.
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.
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[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] <http://www.iana.org/assignments/sdp-parameters>
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Authors' Addresses
Miguel A. Garcia-Martin
Ericsson
Calle Via de los Poblados 13
Madrid, ES 28033
Spain
Email: miguel.a.garcia@ericsson.com
Simo Veikkolainen
Nokia
P.O. Box 407
NOKIA GROUP, FI 00045
Finland
Phone: +358 50 486 4463
Email: simo.veikkolainen@nokia.com
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