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Network Working Group R. Ejzak
Request for Comments: 5009 Alcatel-Lucent
Category: Informational September 2007
Private Header (P-Header) Extension to
the Session Initiation Protocol (SIP) for Authorization of Early Media
Status of This Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Abstract
This document describes a private Session Initiation Protocol (SIP)
header field (P-header) to be used by the European Telecommunications
Standards Institute (ETSI) Telecommunications and Internet-converged
Services and Protocols for Advanced Networks (TISPAN) for the purpose
of authorizing early media flows in Third Generation Partnership
Project (3GPP) IP Multimedia Subsystems (IMS). This header field is
useful in any SIP network that is interconnected with other SIP
networks and needs to control the flow of media in the early dialog
state.
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Table of Contents
1. Introduction ....................................................2
2. Applicability Statement .........................................3
3. Conventions and Acronyms ........................................3
4. Background on Early Media Authorization .........................4
4.1. Backward Early Media .......................................5
4.2. Forward Early Media ........................................5
5. Applicability of RFC 3959 and RFC 3960 ..........................6
6. Overview of Operation ...........................................6
7. Limitations of the P-Early-Media Header Field ...................8
8. The P-Early-Media Header Field ..................................8
8.1. Procedures at the User Agent Client .......................10
8.2. Procedures at the User Agent Server .......................10
8.3. Procedures at the Proxy ...................................11
9. Formal Syntax ..................................................11
10. Security Considerations .......................................11
11. IANA Considerations ...........................................12
11.1. Registration of the "P-Early-Media" SIP Header Field .....12
12. Acknowledgements ..............................................12
13. References ....................................................12
13.1. Normative References .....................................12
13.2. Informative References ...................................13
1. Introduction
This document defines the use of the P-Early-Media header field for
use within SIP [1] messages in certain SIP networks to authorize the
cut-through of backward and/or forward early media when permitted by
the early media policies of the networks involved. The P-Early-Media
header field is intended for use in a SIP network, such as a 3GPP IMS
[13][14] that has the following characteristics: its early media
policy prohibits the exchange of early media between end users; it is
interconnected with other SIP networks that have unknown, untrusted,
or different policies regarding early media; and it has the
capability to "gate" (enable/disable) the flow of early media to/from
user equipment.
Within an isolated SIP network, it is possible to gate early media
associated with all endpoints within the network to enforce a desired
early media policy among network endpoints. However, when a SIP
network is interconnected with other SIP networks, only the boundary
node connected to the external network can determine which early
media policy to apply to a session established between endpoints on
different sides of the boundary. The P-Early-Media header field
provides a means for this boundary node to communicate this early
media policy decision to other nodes within the network.
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2. Applicability Statement
The use of this extension is only applicable inside a "Trust Domain"
as defined in RFC 3325 [6]. Nodes in such a Trust Domain are
explicitly trusted by its users and end-systems to authorize early
media requests only when allowed by early media policy within the
Trust Domain.
This document does NOT offer a general early media authorization
model suitable for inter-domain use or use in the Internet at large.
Furthermore, since the early media requests are not cryptographically
certified, they are subject to forgery, replay, and falsification in
any architecture that does not meet the requirements of the Trust
Domain.
An early media request also lacks an indication of who specifically
is making or modifying the request, and so it must be assumed that
the Trust Domain is making the request. Therefore, the information
is only meaningful when securely received from a node known to be a
member of the Trust Domain.
Although this extension can be used with parallel forking, it does
not improve on the known problems with early media and parallel
forking, as described in RFC 3960 [4], unless one can assume the use
of symmetric RTP.
Despite these limitations, there are sufficiently useful specialized
deployments that meet the assumptions described above, and can accept
the limitations that result, to warrant publication of this
mechanism. An example deployment would be a closed network that
emulates a traditional circuit switched telephone network.
3. Conventions and Acronyms
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 RFC 2119 [2].
The following acronyms are used in this document:
3GPP - the Third Generation Partnership Project
ABNF - Augmented Backus-Naur Form [5]
DTMF - Dual Tone Multi-Frequency
ETSI - European Telecommunications Standards Institute
IMS - Internet Protocol Multimedia Subsystem [13][14]
MIME - Multipurpose Internet Mail Extensions
NAT - Network Address Translation
PSTN - Public Switched Telephone Network
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SDP - Session Description Protocol [7]
SIP - Session Initiation Protocol [1]
TISPAN - Telecommunications and Internet-converged Services and
Protocols for Advanced Networks
UA - User Agent [1]
UAC - User Agent Client [1]
UAS - User Agent Server [1]
4. Background on Early Media Authorization
PSTN networks typically provide call progress information as backward
early media from the terminating switch towards the calling party.
PSTN networks also use forward early media from the calling party
towards the terminating switch under some circumstances for
applications, such as digit collection for secondary dialing. PSTN
networks typically allow backward and/or forward early media since
they are used for the purpose of progressing the call to the answer
state and do not involve the exchange of data between endpoints.
In a SIP network, backward early media flows from the User Agent
Server (UAS) towards the User Agent Client (UAC). Forward early
media flows from the UAC towards the UAS. SIP networks by default
allow both forms of early media, which may carry user data, once the
media path is established. Early media is typically desirable with a
PSTN gateway as UAS, but not with SIP user equipment as UAS.
To prevent the exchange of user data within early media while
allowing early media via PSTN gateways, a SIP network may have a
policy to prohibit backward early media from SIP user equipment and
to prohibit forward media towards SIP user equipment, either of which
may contain user data. A SIP network containing both PSTN gateways
and SIP end devices, for example, can maintain such an early media
policy by gating "off" any early media with a SIP end device acting
as UAS, gating "on" early media with a SIP end device acting as UAC,
and gating "on" early media at each PSTN gateway.
Unfortunately, a SIP network interconnected with another SIP network
may have no means of assuring that the interconnected network is
implementing a compatible early media policy, thus allowing the
exchange of user data within early media under some circumstances.
For example, if a network "A" allows all early media with user
equipment as UAC and an interconnected network "B" allows all early
media with user equipment as UAS, any session established between
user equipment as UAC in "A" and user equipment as UAS in "B" will
allow bidirectional user data exchange as early media. Other
combinations of early media policies may also produce similar
undesirable results.
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The purpose of the extension is to allow a SIP network interconnected
to other SIP networks with different early media policies to
correctly identify and enable authorized early media according to its
policies.
4.1. Backward Early Media
Backward early media in the PSTN typically comprises call progress
information, such as ringing feedback ("ringback"), or announcements
regarding special handling such as forwarding. It may also include
requests for further information, such as a credit card number to be
entered as forward early media in the form of Dual Tone Multi-
Frequency (DTMF) tones or speech. Backward early media of this type
provides information to the calling party strictly for the purpose of
progressing the call and involves no exchange of data between end
users. The usual PSTN charging policy assumes that no data is
exchanged between users until the call has been answered.
A terminating SIP User Agent (UA) outside of the SIP network, on the
other hand, may provide any user data in a backward early media
stream. Thus, if the network implements the usual early media
policy, the network equipment gating the backward early media flow
for the originating UA must distinguish between authorized early
media from a terminating SIP endpoint and unauthorized early media
from another SIP device outside of the network. Given the assumption
of a transitive trust relationship between SIP servers in the
network, this can be accomplished by including some information in a
backward SIP message that identifies the presence of authorized
backward early media. Since it is necessary to verify that this
indication comes from a trusted source, it is necessary for each
server on the path back to the originating UA to be able to verify
the trust relationship with the previous server and to remove such an
indication when it cannot do so. A server on the boundary to an
untrusted SIP network can assure that no indication of authorized
backward early media passes from an external UAS to a UAC within the
network. Thus, the use of a private header field that can be
modified by SIP proxies is to be preferred over the use of a
Multipurpose Internet Mail Extensions (MIME) attachment that cannot
be modified in this way.
4.2. Forward Early Media
Forward early media is less common than backward early media in the
PSTN. It is typically used to collect secondary dialed digits, to
collect credit card numbers, or to collect other DTMF or speech
responses for the purpose of further directing the call. Forward
early media in the PSTN is always directed toward a network server
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for the purpose of progressing a call and involves no exchange of
data between end users.
A terminating SIP UA outside of the SIP network, on the other hand,
may receive any user data in a forward early media stream. Thus, if
the network implements the usual early media policy, the network
equipment gating the forward early media flow for the originating UA
must distinguish between a terminating endpoint that is authorized to
receive forward early media, and another SIP device outside of the
network that is not authorized to receive forward early media
containing user data. This authorization can be accomplished in the
same manner as for backward early media by including some information
in a backward SIP message that identifies that the terminating side
is authorized to receive forward early media.
5. Applicability of RFC 3959 and RFC 3960
The private header extension defined in this document is applicable
to the gateway model defined in RFC 3960 [4], since the PSTN gateway
is the primary requestor of early media in an IMS. For the same
reason, neither the application server model of RFC 3960, nor the
early-session disposition type defined in RFC 3959 [3] is applicable.
The gateway model of RFC 3960 [4] allows for individual networks to
create local policy with respect to the handling of early media, but
does not address the case where a network is interconnected with
other networks with unknown, untrusted, or different early media
policies. Without the kind of information in the P-Early-Media
header field, it is not possible for the network to determine whether
cut-through of early media could lead to the transfer of data between
end-users during session establishment.
Thus, the private header extension in this document is a natural
extension of the gateway model of RFC 3960 [4] that is applicable
within a transitive trust domain.
6. Overview of Operation
This document defines a new P-Early-Media header field for the
purpose of requesting and authorizing requests for backward and/or
forward early media. A UAC capable of recognizing the P-Early-Media
header field may include the header field in an INVITE request. The
P-Early-Media header field in an INVITE request contains the
"supported" parameter.
As members of the Trust Domain, each proxy receiving an INVITE
request must decide whether to insert or delete the P-Early-Media
header field before forwarding.
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A UAS receiving an INVITE request can use the presence of the P-
Early-Media header field in the request to decide whether to request
early media authorization in subsequent messages towards the UAC.
After receiving an incoming INVITE request, the UAS requesting
backward and/or forward early media will include the P-Early-Media
header field in a message towards the UAC within the dialog,
including direction parameter(s) that identify for each media line in
the session whether the early media request is for backward media,
forward media, both, or neither. The UAS can change its request for
early media by including a modified P-Early-Media header field in a
subsequent message towards the UAC within the dialog.
Each proxy in the network receiving the P-Early-Media header field in
a message towards the UAC has the responsibility for assuring that
the early media request comes from an authorized source. If a P-
Early-Media header field arrives from either an untrusted source, a
source not allowed to send backward early media, or a source not
allowed to receive forward early media, then the proxy may remove the
P-Early-Media header field or alter the direction parameter(s) of the
P-Early-Media header field before forwarding the message, based on
local policy.
A proxy in the network not receiving the P-Early-Media header field
in a message towards the UAC may insert one based on local policy.
If the proxy also performs gating of early media, then it uses the
parameter(s) of the P-Early-Media header field to decide whether to
open or close the gates for backward and forward early media flow(s)
between the UAs. The proxy performing gating of early media may also
add a "gated" parameter to the P-Early-Media header field before
forwarding the message so that other gating proxies in the path can
choose to leave open their gates.
If the UAC is a trusted server within the network (e.g., a PSTN
gateway), then the UAC may use the parameter(s) of the P-Early-Media
header field in messages received from the UAS to decide whether to
perform early media gating or cut-through and to decide whether or
not to render backward early media in preference to generating
ringback based on the receipt of a 180 Ringing response.
If the UAC is associated with user equipment, then the network will
have assigned a proxy the task of performing early media gating, so
that the parameter(s) of the P-Early-Media header field received at
such a UAC do not require that the UAC police the early media
flow(s), but they do provide additional information that the UAC may
use to render media.
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The UAC and proxies in the network may also insert, delete, or modify
the P-Early-Media header field in messages towards the UAS within the
dialog according to local policy, but the interpretation of the
header field when used in this way is a matter of local policy and
not defined herein. The use of direction parameter(s) in this header
field could be used to inform the UAS of the final early media
authorization status.
7. Limitations of the P-Early-Media Header Field
The P-Early-Media header field does not apply to any SDP with
Content-Disposition: early-session [3].
When parallel forking occurs, there is no reliable way to correlate
early media authorization in a dialog with the media from the
corresponding endpoint unless one can assume the use of symmetric
RTP, since the SDP messages do not identify the RTP source address of
any media stream. When a UAC or proxy receives multiple early
dialogs and cannot accurately identify the source of each media
stream, it SHOULD use the most restrictive early media authorization
it receives on any of the dialogs to decide the policy to apply
towards all received media. When early media usage is desired for
any reason and one cannot assume the use of symmetric RTP, it is
advisable to disable parallel forking using callerprefs [9].
Although the implementation of media gating is outside the scope of
this extension, note that media gating must be implemented carefully
in the presence of NATs and protocols that aid in NAT traversal.
Media gating may also introduce a potential for media clipping that
is similar to that created during parallel forking or any other
feature that may disable early media, such as custom ringback.
8. The P-Early-Media Header Field
The P-Early-Media header field with the "supported" parameter MAY be
included in an INVITE request to indicate that the UAC or a proxy on
the path recognizes the header field.
A network entity MAY request the authorization of early media or
change a request for authorization of early media by including the
P-Early-Media header field in any message allowed by Table 1 within
the dialog towards the sender of the INVITE request. The P-Early-
Media header field includes one or more direction parameters where
each has one of the values: "sendrecv", "sendonly", "recvonly", or
"inactive", following the convention used for Session Description
Protocol (SDP) [7][8] stream directionality. Each parameter applies,
in order, to the media lines in the corresponding SDP messages
establishing session media. Unrecognized parameters SHALL be
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silently discarded. Non-direction parameters are ignored for
purposes of early media authorization. If there are more direction
parameters than media lines, the excess SHALL be silently discarded.
If there are fewer direction parameters than media lines, the value
of the last direction parameter SHALL apply to all remaining media
lines. A message directed towards the UAC containing a P-Early-Media
header field with no recognized direction parameters SHALL NOT be
interpreted as an early media authorization request.
The parameter value "sendrecv" indicates a request for authorization
of early media associated with the corresponding media line, both
from the UAS towards the UAC and from the UAC towards the UAS (both
backward and forward early media). The value "sendonly" indicates a
request for authorization of early media from the UAS towards the UAC
(backward early media), and not in the other direction. The value
"recvonly" indicates a request for authorization of early media from
the UAC towards the UAS (forward early media), and not in the other
direction. The value "inactive" indicates either a request that no
early media associated with the corresponding media line be
authorized, or a request for revocation of authorization of
previously authorized early media.
The P-Early-Media header field in any message within a dialog towards
the sender of the INVITE request MAY also include the non-direction
parameter "gated" to indicate that a network entity on the path
towards the UAS is already gating the early media, according to the
direction parameter(s). When included in the P-Early-Media header
field, the "gated" parameter SHALL come after all direction
parameters in the parameter list.
When receiving a message directed toward the UAC without the P-
Early-Media header field and no previous early media authorization
request has been received within the dialog, the default early media
authorization depends on local policy and may depend on whether the
header field was included in the INVITE request. After an early
media authorization request has been received within a dialog, and a
subsequent message is received without the P-Early-Media header
field, the previous early media authorization remains unchanged.
The P-Early-Media header field in any message within a dialog towards
the UAS MAY be ignored or interpreted according to local policy.
The P-Early-Media header field does not interact with SDP
offer/answer procedures in any way. Early media authorization is not
influenced by the state of the SDP offer/answer procedures (including
preconditions and directionality) and does not influence the state of
the SDP offer/answer procedures. The P-Early-Media header field may
or may not be present in messages containing SDP. The most recently
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received early media authorization applies to the corresponding media
line in the session established for the dialog until receipt of the
200 OK response to the INVITE request, at which point all media lines
in the session are implicitly authorized. Early media flow in a
particular direction requires that early media in that direction is
authorized, that media flow in that direction is enabled by the SDP
direction attribute for the stream, and that any applicable
preconditions [11] are met. Early media authorization does not
override the SDP direction attribute or preconditions state, and the
SDP direction attribute does not override early media authorization.
Table 1 is an extension of Tables 2 and 3 in RFC 3261 [1] for the P-
Early-Media header field. The column "PRA" is for the PRACK method
[12]. The column "UPD" is for the UPDATE method [10].
Header field where proxy ACK BYE CAN INV OPT REG PRA UPD
________________________________________________________________
P-Early-Media R amr - - - o - - o o
P-Early-Media 18x amr - - - o - - - -
P-Early-Media 2xx amr - - - - - - o o
Table 1: P-Early-Media Header Field
8.1. Procedures at the User Agent Client
A User Agent Client MAY include the P-Early-Media header field with
the "supported" parameter in an INVITE request to indicate that it
recognizes the header field.
A User Agent Client receiving a P-Early-Media header field MAY use
the parameter(s) of the header field to gate or cut-through early
media, and to decide whether to render early media from the UAS to
the UAC in preference to any locally generated ringback triggered by
a 180 Ringing response. If a proxy is providing the early media
gating function for the User Agent Client, then the gateway model of
RFC 3960 [4] for rendering of early media is applicable. A User
Agent Client without a proxy in the network performing early media
gating that receives a P-Early-Media header field SHOULD perform
gating or cut-through of early media according to the parameter(s) of
the header field.
8.2. Procedures at the User Agent Server
A User Agent Server that is requesting authorization to send or
receive early media MAY insert a P-Early-Media header field with
appropriate parameters(s) in any message allowed in table 1 towards
the UAC within the dialog. A User Agent Server MAY request changes
in early media authorization by inserting a P-Early-Media header
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field with appropriate parameter(s) in any subsequent message allowed
in table 1 towards the UAC within the dialog.
If the P-Early-Media header field is not present in the INVITE
request, the User Agent Server MAY choose to suppress early media
authorization requests and MAY choose to execute alternate early
media procedures.
8.3. Procedures at the Proxy
When forwarding an INVITE request, a proxy MAY add, retain, or delete
the P-Early-Media header field, depending on local policy and the
trust relationship with the sender and/or receiver of the request.
When forwarding a message allowed in Table 1 towards the UAC, a proxy
MAY add, modify, or delete a P-Early-Media header field, depending on
local policy and the trust relationship with the sender and/or
receiver of the message. In addition, if the proxy controls the
gating of early media for the User Agent Client, it SHOULD use the
contents of the P-Early-Media header field to gate the early media,
according to the definitions of the header field parameters defined
in clause 8.
9. Formal Syntax
The syntax of the P-Early-Media header field is described below in
ABNF, according to RFC 4234 [5], as an extension to the ABNF for SIP
in RFC 3261 [1]. Note that not all combinations of em-param elements
are semantically valid.
P-Early-Media = "P-Early-Media" HCOLON
[ em-param *(COMMA em-param) ]
em-param = "sendrecv" / "sendonly" / "recvonly"
/ "inactive" / "gated" / "supported" / token
10. Security Considerations
The use of this extension is only applicable inside a "Trust Domain",
as defined in RFC 3325 [6]. This document does NOT offer a general
early media authorization model suitable for inter-domain use or use
in the Internet at large.
There are no confidentiality concerns associated with the P-Early-
Media header field. It is desirable to maintain the integrity of the
direction parameters in the header field across each hop between
servers to avoid the potential for unauthorized use of early media.
It is assumed that the P-Early-Media header field is used within the
context of the 3GPP IMS trust domain or a similar trust domain,
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consisting of a collection of SIP servers maintaining pair wise
security associations.
Within the trust domain of a network it is only necessary to police
the use of the P-Early-Media header field at the boundary to user
equipment served by the network and at the boundary to peer networks.
It is assumed that boundary servers in the trust domain of a network
will have local policy for the treatment of the P-Early-Media header
field as it is sent to or received from any possible server external
to the network. Since boundary servers are free to modify or remove
any P-Early-Media header field in SIP messages forwarded across the
boundary, the integrity of the P-Early-Media header field can be
verified to the extent that the connections to external servers are
secured. The authenticity of the P-Early-Media header field can only
be assured to the extent that the external servers are trusted to
police the authenticity of the header field.
11. IANA Considerations
11.1. Registration of the "P-Early-Media" SIP Header Field
Name of Header field: P-Early-Media
Short form: none
Registrant: Richard Ejzak
ejzak@alcatel-lucent.com
Normative description: Section 8 of this document
12. Acknowledgements
The author would like to thank Miguel Garcia-Martin, Jan Holm,
Sebastien Garcin, Akira Kurokawa, Erick Sasaki, James Calme, Greg
Tevonian, Aki Niemi, Paul Kyzivat, Gonzalo Camarillo, Brett Tate, Jon
Peterson, Alfred Hoenes, and David Black for their significant
contributions made throughout the writing and reviewing of this
document.
13. References
13.1. Normative References
[1] 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.
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[2] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[3] Camarillo, G., "The Early Session Disposition Type for the
Session Initiation Protocol (SIP)", RFC 3959, December 2004.
[4] Camarillo, G. and H. Schulzrinne, "Early Media and Ringing Tone
Generation in the Session Initiation Protocol (SIP)", RFC 3960,
December 2004.
[5] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 4234, October 2005.
[6] Jennings, C., Peterson, J., and M. Watson, "Private Extensions
to the Session Initiation Protocol (SIP) for Asserted Identity
within Trusted Networks", RFC 3325, November 2002.
[7] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 4566, July 2006.
[8] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with
Session Description Protocol (SDP)", RFC 3264, June 2002.
[9] Rosenberg, J., Schulzrinne, H., and P. Kyzivat, "Caller
Preferences for the Session Initiation Protocol (SIP)", RFC
3841, August 2004.
[10] Rosenberg, J., "The Session Initiation Protocol (SIP) UPDATE
Method", RFC 3311, October 2002.
[11] Camarillo, G., Marshall, W., and J. Rosenberg, "Integration of
Resource Management and Session Initiation Protocol (SIP)", RFC
3312, October 2002.
[12] Rosenberg, J. and H. Schulzrinne, "Reliability of Provisional
Responses in Session Initiation Protocol (SIP)", RFC 3262, June
2002.
13.2. Informative References
[13] 3GPP "TS 23.228: IP Multimedia Subsystem (IMS); Stage 2 (Release
7)", 3GPP 23.228, March 2007,
ftp://ftp.3gpp.org/specs/archive/23_series/23.228/.
[14] 3GPP "TS 24.229: IP Multimedia Call Control Protocol based on
SIP and SDP; Stage 3 (Release 7)", 3GPP 24.229, March 2007,
ftp://ftp.3gpp.org/specs/archive/24_series/24.229/.
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ETSI documents can be downloaded from the ETSI Web server,
"http://www.etsi.org/". Any 3GPP document can be downloaded
from the 3GPP Web server, "http://www.3gpp.org/". See
specifications.
Authors Address
Richard Ejzak
Alcatel-Lucent
1960 Lucent Lane
Naperville, IL 60566
USA
Phone: +1 630 979 7036
EMail: ejzak@alcatel-lucent.com
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