Network Working Group Richard Ejzak
INTERNET-DRAFT Lucent Technologies
October 4, 2006
Private Header (P-Header) Extension to the Session Initiation
Protocol (SIP) for Authorization of Early Media
<draft-ejzak-sipping-p-em-auth-02.txt>
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Abstract
This document describes a private Session Initiation Protocol (SIP)
header (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 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.......................................4
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.........................7
8. The P-Early-Media header........................................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...................................10
9. Formal syntax..................................................11
10. Security Considerations.......................................11
11. IANA Considerations...........................................11
11.1. Registration of the "P-Early-Media" SIP header...........11
12. Acknowledgements..............................................11
13. References....................................................12
13.1. Normative References.....................................12
13.2. Informative References...................................12
14. Authors' Addresses............................................13
15. IPR Notice....................................................13
16. Copyright Notice..............................................13
1. Introduction
This document defines the use of the P-Early-Media header for use
within SIP [1] messages in certain SIP networks to authorize the
cut-through of backward and/or forward early media. The P-Early-
Media header is intended for use in a SIP network, such as a 3GPP
IMS, that prohibits the exchange of early media between end users,
that is interconnected with other SIP networks that have unknown,
untrusted or different policies regarding early media, and that 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 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 [9]. 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 whom 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.
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 RFC2119 [1].
The following acronyms are used in this document:
3GPP - the Third Generation Partnership Project
ABNF - Augmented Backus-Naur Form
DTMF - Dual Tone Multi-Frequency
ETSI - European Telecommunications Standards Institute
IMS - Internet Protocol Multimedia Subsystem
MIME - Multipurpose Internet Mail Extensions
NAT - Network Address Translation
PSTN - Public Switched Telephone Network
SDP - Session Description Protocol
SIP - Session Initiation Protocol
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TISPAN - Telecommunications and Internet converged Services and
Protocols for Advanced Networks
UA - User Agent
UAC - User Agent Client
UAS - User Agent Server
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. In a SIP network, backward early media flows from the User
Agent Server (UAS) towards the User Agent Client (UAC). 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. In a SIP network,
forward early media flows from the UAC towards the UAS.
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. On the other hand, 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
appropriately gating 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.
Without this extension, a SIP network interconnected with other SIP
networks provides no mechanism for an originating SIP endpoint
within the network, be it a PSTN gateway or SIP user equipment, from
identifying if the terminating SIP endpoint, which may be located
outside the network, is a SIP endpoint that is authorized either to
send backward early media or to receive forward early media.
4.1. Backward early media
Backward early media in the PSTN typically comprises call progress
information such as ringing, 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
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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 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 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
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.
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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 [7], 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 [6] is
applicable.
The gateway model of RFC 3960 [7] 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, 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 [7] 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 may include the header in an INVITE request. The P-Early-
Media header in an INVITE request contains no parameters.
As members of the Trust Domain, each proxy receiving an INVITE
request must decide whether to insert or delete the P-Early-Media
header before forwarding.
A UAS receiving an INVITE request can use the presence of the P-
Early-Media header 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 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 in a subsequent
message towards the UAC within the dialog.
Each proxy in the network receiving the P-Early-Media header 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 arrives from either an untrusted source, a source not
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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 or alter the direction parameter(s) of the P-Early-
Media header before forwarding the message, based on local policy.
A proxy in the network not receiving the P-Early-Media header 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 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 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 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 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.
The UAC and proxies in the network may also insert, delete or modify
the P-Early-Media header in messages towards the UAS within the
dialog according to local policy, but the interpretation of the
header when used in this way is a matter of local policy and not
defined herein. The use of direction parameter(s) in this header
could be used to inform the UAS of the final early media
authorization status.
7. Limitations of the P-Early-Media header
The P-Early-Media header does not apply to any SDP with Content-
Disposition: early-session [6].
When parallel forking occurs, there is no reliable way to correlate
early media authorization in a dialog with the media from the
corresponding endpoint, 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
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decide the policy to apply towards all received media. When early
media usage is desired for any reason it is advisable to disable
parallel forking using callerprefs [14].
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
The P-Early-Media header with no parameters MAY be included in an
INVITE request to indicate that the UAC or a proxy on the path
recognizes the header.
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 in any message allowed by Table 1 within the
dialog towards the sender of the INVITE request. The P-Early-Media
header 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)
[10] stream directionality. Each parameter applies, in order, to
the media lines in the corresponding SDP messages establishing
session media. Unrecognized parameters SHALL be 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
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.
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The P-Early-Media header 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,
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 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 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, the
previous early media authorization remains unchanged.
The P-Early-Media header in any message within a dialog towards the
UAS MAY be ignored or interpreted according to local policy.
The P-Early-Media header 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 may or
may not be present in messages containing SDP. The most recently
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 [16] 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
[17]. The column "UPD" is for the UPDATE method [15].
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
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8.1. Procedures at the User Agent Client
A User Agent Client MAY include the P-Early-Media header with no
parameters in an INVITE request to indicate that it recognizes the
header.
A User Agent Client receiving a P-Early-Media header MAY use the
parameter(s) of the header 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 [7] 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 SHOULD perform gating or cut-
through of early media according to the parameter(s) of the header.
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 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
with appropriate parameter(s) in any subsequent message allowed in
table 1 towards the UAC within the dialog.
If the P-Early-Media header 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, 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, 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 to gate the early media
according to the definitions of the header parameters defined in
clause 8.
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9. Formal syntax
The syntax of the P-Early-Media header is described below in ABNF
according to RFC 4234 [8], as an extension to the ABNF for SIP in
RFC 3261 [1].
P-Early-Media = "P-Early-Media" HCOLON
[ em-param *(COMMA em-param) ]
em-param = "sendrecv" / "sendonly" / "recvonly"
/ "inactive" / "gated" / token
10. Security Considerations
There are no confidentiality concerns associated with the P-Early-
Media header. It is desirable to maintain the integrity of the
direction parameters in the header across each hop between servers
to avoid the potential for unauthorized use of early media. It is
assumed that the P-Early-Media header is used within the context of
the 3GPP IMS trust domain or a similar trust domain, consisting of a
collection of SIP servers maintaining pair wise security
associations. In an IMS it is only necessary to police the use of
the P-Early-Media header 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 IMS will have local policy for the
treatment of the P-Early-Media header 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 in SIP
messages forwarded across the boundary, the integrity of the P-
Early-Media header can be verified to the extent that the
connections to external servers are secured. The authenticity of
the P-Early-Media header can only be assured to the extent that the
external servers are trusted to police the authenticity of the
header.
11. IANA Considerations
11.1. Registration of the "P-Early-Media" SIP header
Name of Header: P-Early-Media
Short form: none
Registrant: Richard Ejzak
ejzak@lucent.com
Normative description: Section 8 of this document
12. Acknowledgements
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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 and Brett Tate
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.
[2] 3GPP TS 23.228: IP Multimedia Subsystem (IMS); Stage 2
(Release 7), 3GPP 23.228, September 2005,
ftp://ftp.3gpp.org/Specs/archive/23-series/23.228/.
[3] 3GPP TS 24.229: IP Multimedia Call Control Protocol based on
SIP and SDP; Stage 3 (Release 7), 3GPP 24.229, September 2005,
ftp://ftp.3gpp.org/Specs/archive/24-series/24.229/.
[4] 3GPP TS 32.200: Telecommunication Management; Charging
management; Charging principles (Release 7), 3GPP 32.200,
September 2005, ftp://ftp.3gpp.org/Specs/archive/32-
series/32.200/.
[5] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[6] Camarillo, G., The Early Session Disposition Type for the
Session Initiation Protocol (SIP), RFC 3959, December 2004.
[7] Camarillo, G., Early Media and Ringing Tone Generation in the
Session Initiation Protocol (SIP), RFC 3960, December 2004.
[8] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 4234, October 2005.
[9] Jennings, C., Peterson, J. and Watson, M., Private Extensions
to the Session Initiation Protocol (SIP) for Asserted Identity
within Trusted Networks, RFC 3325, November 2002.
[10] Handley, M. and V. Jacobson, "SDP: Session Description
Protocol", RFC 2327, April 1998.
13.2. Informative References
[11] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with
Session Description Protocol (SDP)", RFC 3264, June 2002.
[12] Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson,
"RTP: A Transport Protocol for Real-Time Applications", STD 64,
RFC 3550, July 2003.
[13] Schulzrinne, H. and S. Casner, "RTP Profile for Audio and Video
Conferences with Minimal Control", STD 65, RFC 3551, July 2003.
[14] Rosenberg, J., Schulzrinne, H. and Kyzivat, P., "Caller
Preferences for the Session Initiation Protocol (SIP)", RFC
3841, August 2004.
[15] Rosenberg, J., "The Session Initiation Protocol (SIP) UPDATE
Method", RFC 3311, September 2002.
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[16] Camarillo, G., Marshall, W. and Rosenberg, J., "Integration of
Resource Management and Session Initiation Protocol (SIP)", RFC
3312, October 2002.
[17] Rosenberg, J. and Schulzrinne, H., "Reliability of Provisional
Responses in the Session Initiation Protocol (SIP)", RFC 3262,
June 2002.
ETSI documents can be downloaded from the ETSI web server,
http://www.etsi.org/". Any 3GPP document can be downloaded from the
3GPP webserver, "http://www.3gpp.org/", see specifications.
14. Authors' Addresses
Richard Ejzak
Lucent Technologies
1960 Lucent Lane
Naperville, IL 60566, USA
Phone: +1 630 979 7036
EMail: ejzak@lucent.com
15. IPR Notice
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16. Copyright Notice
Copyright (C) The Internet Society (2006).
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This document is subject to the rights, licenses and restrictions
contained in BCP 78, and except as set forth therein, the authors
retain all their rights.
This document and the information contained herein are provided on
an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE
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THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
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This Internet-Draft expires in April 2007.
RFC Editor Considerations
- The RFC editor is requested to replace all occurrences of XXXX
with the RFC number this document receives.
Ejzak [Page 14]
