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Keywords: APEX, Application Exchange, datagram, service, core, relaying, mesh
Network Working Group G. Klyne
Request for Comments: 3342 Clearswift Corporation
Category: Standards Track M. Rose
Dover Beach Consulting, Inc.
M. Schwartz
Code On The Road, LLC
E. Dixon
H. Franklin
J. Kint
D. New
S. Pead
July 2002
The Application Exchange (APEX) Option Party Pack, Part Deux!
Status of this Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2002). All Rights Reserved.
Abstract
Application Exchange (APEX), at its core, provides a best-effort
application-layer datagram service. Options are used to alter the
semantics of the core service. This memo defines various options to
change the default behavior of APEX's "relaying mesh".
Klyne, et. al. Standards Track [Page 1]
RFC 3342 The Application Exchange (APEX) Party Pack July 2002
Table of Contents
1. The attachOverride Option . . . . . . . . . . . . . . . . . 2
2. The dataTiming Option . . . . . . . . . . . . . . . . . . . 3
2.1 Upper-Bounds on Delivery . . . . . . . . . . . . . . . . . . 4
2.1.1 Final Hop Report . . . . . . . . . . . . . . . . . . . . . . 5
2.1.2 Timing Error Report . . . . . . . . . . . . . . . . . . . . 7
2.2 Reporting on Delayed Delivery . . . . . . . . . . . . . . . 8
2.2.1 Transient Timing Report . . . . . . . . . . . . . . . . . . 9
3. The hold4Endpoint Option . . . . . . . . . . . . . . . . . . 10
4. The dataHopping Option . . . . . . . . . . . . . . . . . . . 13
5. Initial Registrations . . . . . . . . . . . . . . . . . . . 15
5.1 Registration: The attachOverride Option . . . . . . . . . . 15
5.2 Registration: The dataTiming Option . . . . . . . . . . . . 16
5.3 Registration: The hold4Endpoint Option . . . . . . . . . . . 16
5.4 Registration: The dataHopping Option . . . . . . . . . . . . 16
6. The APEX Party Pack DTD . . . . . . . . . . . . . . . . . . 17
7. Security Considerations . . . . . . . . . . . . . . . . . . 18
References . . . . . . . . . . . . . . . . . . . . . . . . . 18
A. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 19
B. IANA Considerations . . . . . . . . . . . . . . . . . . . . 19
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 20
Full Copyright Statement . . . . . . . . . . . . . . . . . . 22
1. The attachOverride Option
Section 5.1 contains the APEX option registration for the
"attachOverride" option.
The default behavior of the APEX relaying mesh, in the absence of
processing options, is to allow at most one application to attach as
a particular endpoint, on a "first come, first served" basis. The
"attachOverride" option provides gives preference to the current
application trying to attach.
If this option is present in the "attach" operation (c.f., Section
4.4.1 of [1]) and if any application is already attached as the
specified endpoint, that endpoint has its attachment terminated
(c.f., Section 4.4.3 of [1]) concurrently with processing of that
"attach" operation. The "code" attribute of the resulting
"terminate" operation is set to 556.
Note that any data being expected by the previously-attached
application may instead be delivered to the last application to
successfully attach. Accordingly, applications should take care to
properly deal with incoming data having unrecognized transaction-
identifiers (c.f., Section 6.1.1 of [1]).
Klyne, et. al. Standards Track [Page 2]
RFC 3342 The Application Exchange (APEX) Party Pack July 2002
This option provides for a new attachment to automatically terminate
any existing attachment for the same endpoint. For example, this
might be helpful when a new attachment is required from a different
device while a previously-used device is still attached e.g.,
+-------+ +-------+
| | -- attach -----> | |
| appl. | | relay |
| #1 | <--------- ok -- | |
+-------+ +-------+
C: <attach endpoint='fred@example.com' transID='1' />
S: <ok />
... some time later appl #2 starts on a different computer ...
+-------+ +-------+
| | <----- attach -- | |
+-------+ | | | appl. |
| | <-- terminate -- | relay | -- ok ---------> | #2 |
| appl. | | | +-------+
| #1 | -- ok ---------> | |
+-------+ +-------+
C: <attach endpoint='fred@example.com' transID='2'>
<option internal='attachOverride' transID='3' />
</attach>
S: <ok />
C: <terminate transID='1' code='556'>overriden</terminate>
S: <ok />
2. The dataTiming Option
Section 5.2 contains the APEX option registration for the
"dataTiming" option. This option contains a "dataTiming" element
(c.f., Section 6).
The default behavior of the APEX relaying mesh is "immediate, best
effort", and expects that all relays and endpoints are able to
process and transfer data without delay -- in the absence of
processing options, if a relay is unavailable, then data is silently
dropped. The "dataTiming" option provides for controlled queuing
delays in processing, whilst providing reasonable deterministic
behavior for the originator.
Klyne, et. al. Standards Track [Page 3]
RFC 3342 The Application Exchange (APEX) Party Pack July 2002
There are two types of delays addressed by the "dataTiming" option:
o delays in transit through the relaying mesh, possibly due to
intermittent or slow connections, or congested relays; and,
o delays because the intended endpoint is not available to receive
the data, when used in conjunction with the hold4Endpoint option
(Section 3).
Accordingly, the "dataTiming" option allows for:
o data to be discarded if not delivered within a finite amount of
time as specified using the "noLaterThan" attribute (Section 2.1);
o a "statusResponse" message (c.f., Section 5.1 of [1]) to be
generated if data is not delivered within a known amount of time
as specified using the "reportAfter" attribute (Section 2.2); and,
o an upper limit on the amount of time for the "statusResponse"
message to be delivered using the "returnTrip" attribute (Section
2.1.1), after which the sender may presume the message to be lost.
This option does not provide any functionality with respect to the
priority of the data. Nor does this option have any effect on other
parts of the relaying process.
Further, note that because this option is processed on a per-hop
basis, the originator must set the "targetHop" attribute to the value
"all" and the "mustUnderstand" attribute to the value "true".
2.1 Upper-Bounds on Delivery
The "noLaterThan" attribute of the "dataTiming" option provides for
control over delays that may occur in transit through the relaying
mesh or to the recipient endpoint.
If this option is present in the "data" operation (c.f., Section
4.4.4 of [1]) and the value of the "noLaterThan" attribute is non-
zero, then:
o For Step 5.2 of Section 4.4.4.1 of [1]:
Immediately prior to sending the data to the next relay, the value
of the "noLaterThan" attribute is adjusted to reflect the
processing time of the data at the local relay (e.g., the time
required to determine the next relay, to successfully issue a
"bind" operation, and then be ready to immediately issue a "data"
operation).
Klyne, et. al. Standards Track [Page 4]
RFC 3342 The Application Exchange (APEX) Party Pack July 2002
If the value of the "noLaterThan" attribute becomes less than or
equal to zero, an error in processing has occurred, the data
element is not sent to the next relay, and if the "reportErrors"
attribute is true, the APEX report service is invoked to send a
timing error report.
o For Step 5.3 of Section 4.4.4.1 of [1]:
If the relay does not receive an "ok" element from the recipient
endpoint within the number of milli-seconds indicated by the value
of the "noLaterThan" attribute, an error in processing has
occurred, and if the "reportErrors" attribute is true, the APEX
report service is invoked to send a timing error report.
Otherwise, if the data is successfully transmitted to the
recipient, and the "returnTrip" attribute is non-zero, the APEX
report service is invoked to send a final hop report.
Note that in some cases, a relay may be able to predict this outcome
without actually connecting to the next relay; if so, a timing error
report may be sent without connecting to the next relay.
2.1.1 Final Hop Report
If the APEX report service (c.f., Section 6.2 of [1]) is invoked to
send a final hop report, it issues a data operation with:
o its originator identifying the report service associated with the
issuing relay
o its recipient identifying the endpoint address of the originator
associated with the "dataTiming" option
o a new "dataTiming" option having:
* its "noLaterThan" attribute equal to the "returnTrip" attribute
of the original "dataTiming" option
* and no other attributes present
o its content consisting of a "statusResponse" element having:
* its "transID" attribute equal to the "transID" attribute of the
"dataTiming" option
* and identifying the original recipient with a permanent success
indicator
Klyne, et. al. Standards Track [Page 5]
RFC 3342 The Application Exchange (APEX) Party Pack July 2002
For example:
+-------+ +-------+
| | -- data -------> | |
| relay | | appl. |
| | <--------- ok -- | #2 |
+-------+ +-------+
C: <data content='cid:1@example.com'>
<originator identity='fred@example.com' />
<recipient identity='barney@example.com' />
<option internal='dataTiming' targetHop='all'
mustUnderstand='true' transID='86'>
<dataTiming noLaterThan='10000' returnTrip='20000' />
</option>
</data>
S: <ok />
+-------+ +-------+
| | <------- data -- | |
| appl. | | relay |
| #1 | -- ok ---------> | |
+-------+ +-------+
C: <data content='#Content'>
<originator identity='apex=report@example.com' />
<recipient identity='fred@example.com' />
<option internal='dataTiming' targetHop='all'
mustUnderstand='true' transID='99'>
<dataTiming noLaterThan='20000' />
</option>
<data-content Name='Content'>
<statusResponse transID='86'>
<destination identity='barney@example.com'>
<reply code='250' />
</destination>
</statusResponse>
</data-content>
</data>
S: <ok />
Klyne, et. al. Standards Track [Page 6]
RFC 3342 The Application Exchange (APEX) Party Pack July 2002
2.1.2 Timing Error Report
If the APEX report service (c.f., Section 6.2 of [1]) is invoked to
send a timing error report, it issues a data operation with:
o its originator identifying the report service associated with the
issuing relay
o its recipient identifying the endpoint address of the originator
associated with the "dataTiming" option
o its content consisting of a "statusResponse" element having:
* its "transID" attribute equal to the "transID" attribute of the
"dataTiming" option
* and identifying the original recipient with a permanent failure
indicator
Klyne, et. al. Standards Track [Page 7]
RFC 3342 The Application Exchange (APEX) Party Pack July 2002
For example:
+-------+ +-------+
| | -- data -------> | |
| appl. | | relay |
| | <--------- ok -- | |
+-------+ +-------+
C: <data content='cid:1@example.com'>
<originator identity='fred@example.com' />
<recipient identity='barney@example.com' />
<option internal='dataTiming' targetHop='all'
mustUnderstand='true' transID='86'>
<dataTiming noLaterThan='6000' reportErrors='true' />
</option>
</data>
S: <ok />
... some time later ...
+-------+ +-------+
| | <------- data -- | |
| appl. | | relay |
| | -- ok ---------> | |
+-------+ +-------+
C: <data content='#Content'>
<originator identity='apex=report@example.com' />
<recipient identity='fred@example.com' />
<data-content Name='Content'>
<statusResponse transID='86'>
<destination identity='barney@example.com'>
<reply code='550' />
</destination>
</statusResponse>
</data-content>
</data>
S: <ok />
2.2 Reporting on Delayed Delivery
The "reportAfter" attribute of the "dataTiming" option provides for
the originator to be notified if delivery is delayed beyond a
specified time. Delivery of the data is not affected. Note that if
the value of the "noLaterThan" attribute is non-zero, then it
provides the operational upper-bounds for the "reportAfter"
attribute.
Klyne, et. al. Standards Track [Page 8]
RFC 3342 The Application Exchange (APEX) Party Pack July 2002
If this option is present in the "data" operation (c.f., Section
4.4.4 of [1]) and the value of the "reportAfter" attribute is non-
zero, then:
o For Step 5.2 of Section 4.4.4.1 of [1]:
Immediately prior to sending the data to the next relay, the value
of the "reportAfter" attribute is adjusted to reflect the
processing time of the data at the local relay (e.g., the time
required to determine the next relay, to successfully issue a
"bind" operation, and then be ready to immediately issue a "data"
operation).
If the value of the "reportAfter" attribute becomes less than or
equal to zero, then its value is set to zero and the APEX report
service is invoked to send a transient timing report; regardless,
the data element is sent to the next relay.
o For Step 5.3 of Section 4.4.4.1 of [1]:
If the relay does not receive an "ok" element from the recipient
endpoint within the number of milli-seconds indicated by the value
of the "reportAfter" attribute, then its value is set to zero and
the APEX report service is invoked to send a transient timing
report.
2.2.1 Transient Timing Report
If the APEX report service (c.f., Section 6.2 of [1]) is invoked to
send a transient timing report, it issues a data operation with:
o its originator identifying the report service associated with the
issuing relay
o its recipient identifying the endpoint address of the originator
associated with the "dataTiming" option
o its content consisting of a "statusResponse" element having:
* its "transID" attribute equal to the "transID" attribute of the
"dataTiming" option
* and identifying the original recipient with a transient success
indicator
Klyne, et. al. Standards Track [Page 9]
RFC 3342 The Application Exchange (APEX) Party Pack July 2002
For example:
+-------+ +-------+
| | -- data -------> | |
| appl. | | relay |
| #1 | <--------- ok -- | |
+-------+ +-------+
C: <data content='cid:1@example.com'>
<originator identity='fred@example.com' />
<recipient identity='barney@example.com' />
<option internal='dataTiming' targetHop='all'
mustUnderstand='true' transID='86'>
<dataTiming reportAfter='60000' />
</option>
</data>
S: <ok />
... some time later ...
+-------+ +-------+
| | <------- data -- | |
| relay | | relay |
| #n-1 | -- ok ---------> | #n |
+-------+ +-------+
C: <data content='#Content'>
<originator identity='apex=report@example.com' />
<recipient identity='fred@example.com' />
<data-content Name='Content'>
<statusResponse transID='86'>
<destination identity='barney@example.com'>
<reply code='350' />
</destination>
</statusResponse>
</data-content>
</data>
S: <ok />
3. The hold4Endpoint Option
Section 5.3 contains the APEX option registration for the
"hold4Endpoint" option.
The default behavior of the APEX relaying mesh, in the absence of
processing options, is to silently drop data for a recipient if its
endpoint isn't attached. The "hold4Endpoint" option provides for
data to be queued if the recipient endpoint is not attached.
Klyne, et. al. Standards Track [Page 10]
RFC 3342 The Application Exchange (APEX) Party Pack July 2002
If this option is present in the "data" operation (c.f., Section
4.4.4 of [1]), and the value of the "hold4Endpoint" attribute is true
then:
o For Step 5.3 of Section 4.4.4.1 of [1]:
If the recipient's endpoint is not currently attached, the relay
will queue the data waiting for an application to attach as that
endpoint.
Note that in the absence of an upper-bounds on delivery, such as
limits provided by the dataTiming option (Section 2), the data will
be queued indefinitely for the endpoint.
Klyne, et. al. Standards Track [Page 11]
RFC 3342 The Application Exchange (APEX) Party Pack July 2002
For example:
+-------+ +-------+
| | -- data -------> | |
| appl. | | relay |
| #1 | <--------- ok -- | |
+-------+ +-------+
C: <data content='cid:1@example.com'>
<originator identity='fred@example.com' />
<recipient identity='barney@example.com' />
<option internal='hold4Endpoint' />
<option internal='dataTiming' targetHop='all'
mustUnderstand='true' transID='86'>
<dataTiming noLaterThan='60000' />
</option>
</data>
S: <ok />
... some time later the recipient's endpoint attaches ...
+-------+ +-------+
| | <----- attach -- | |
| | | |
| | -- ok ---------> | |
| relay | | appl. |
| | -- data -------> | #2 |
| | | |
| | <--------- ok -- | |
+-------+ +-------+
C: <attach endpoint='barney@example.com' transID='2'>
<option internal='attachOverride' transID='3' />
</attach>
S: <ok />
C: <data content='cid:1@example.com'>
<originator identity='fred@example.com' />
<recipient identity='barney@example.com' />
<option internal='hold4Endpoint' />
<option internal='dataTiming' targetHop='all'
mustUnderstand='true' transID='86'>
<dataTiming noLaterThan='18000' />
</option>
</data>
S: <ok />
Klyne, et. al. Standards Track [Page 12]
RFC 3342 The Application Exchange (APEX) Party Pack July 2002
4. The dataHopping Option
To detect misconfigurations that cause forwarding loops in the APEX
relaying mesh, the APEX pubsub service re-introduces a mechanism
similar to the IP TTL [2] mechanism, in the form of an APEX option.
Section 5.4 contains the APEX option registration for the
"dataHopping" option.
If this option is present in the "data" operation (c.f., Section
4.4.4 of [1]) and the value of the "noMoreThan" attribute is non-
zero, then:
o For Step 5.2 of Section 4.4.4.1 of [1]:
Immediately prior to sending the data to the next relay, the value
of the "noMoreThan" attribute is reduced by 1.
If the value of the "noMoreThan" attribute becomes less than or
equal to zero, an error in processing has occurred, the data
element is not sent to the next relay, and if the "reportErrors"
attribute is true, the APEX report service is invoked to send an
error report.
Further, note that because this option is processed on a per-hop
basis, the originator must set the "targetHop" attribute to the value
"all" and the "mustUnderstand" attribute to the value "true".
If the APEX report service (c.f., Section 6.2 of [1]) is invoked to
send an error report, it issues a data operation with:
o its originator identifying the report service associated with the
issuing relay
o its recipient identifying the endpoint address of the originator
associated with the "dataHopping" option
o its content consisting of a "statusResponse" element having:
* its "transID" attribute equal to the "transID" attribute of the
"dataHopping" option
* and identifying the original recipient with a permanent failure
indicator
Klyne, et. al. Standards Track [Page 13]
RFC 3342 The Application Exchange (APEX) Party Pack July 2002
For example:
+-------+ +-------+
| | -- data -------> | |
| appl. | | relay |
| | <--------- ok -- | #1 |
+-------+ +-------+
C: <data content='cid:1@example.com'>
<originator identity='appl=pubsub/topic=fred@example.com' />
<recipient identity='barney@example.com' />
<option internal='dataHopping' targetHop='all'
mustUnderstand='true' transID='86'>
<dataHopping noMoreThan='2' reportErrors='true' />
</option>
</data>
S: <ok />
+-------+ +-------+
| | -- data -------> | |
| relay | | relay |
| #1 | <--------- ok -- | #2 |
+-------+ +-------+
C: <data content='cid:1@example.com'>
<originator identity='appl=pubsub/topic=fred@example.com' />
<recipient identity='barney@example.com' />
<option internal='dataHopping' targetHop='all'
mustUnderstand='true' transID='86'>
<dataHopping noMoreThan='1' reportErrors='true' />
</option>
</data>
S: <ok />
Klyne, et. al. Standards Track [Page 14]
RFC 3342 The Application Exchange (APEX) Party Pack July 2002
relay #2 determines that further relaying is necessary:
+-------+ +-------+
| | <------- data -- | |
| relay | | relay |
| #1 | -- ok ---------> | #2 |
+-------+ +-------+
C: <data content='#Content'>
<originator identity='apex=report@example.com' />
<recipient identity='appl=pubsub/topic=fred@example.com' />
<data-content Name='Content'>
<statusResponse transID='86'>
<destination identity='barney@example.com'>
<reply code='550' />
</destination>
</statusResponse>
</data-content>
</data>
S: <ok />
5. Initial Registrations
The APEX option registration template is defined in Section 7.1 of
[1].
5.1 Registration: The attachOverride Option
Option Identification: attachOverride
Present in: APEX's "attach" element
Contains: nothing
Processing Rules: c.f., Section 1
Contact Information: c.f., the "Authors' Addresses" section of this
memo
Klyne, et. al. Standards Track [Page 15]
RFC 3342 The Application Exchange (APEX) Party Pack July 2002
5.2 Registration: The dataTiming Option
Option Identification: dataTiming
Present in: APEX's "data" element
Contains: dataTiming (c.f., Section 6)
Processing Rules: c.f., Section 2
Contact Information: c.f., the "Authors' Addresses" section of this
memo
5.3 Registration: The hold4Endpoint Option
Option Identification: hold4Endpoint
Present in: APEX's "data" element
Contains: nothing
Processing Rules: c.f., Section 3
Contact Information: c.f., the "Authors' Addresses" section of this
memo
5.4 Registration: The dataHopping Option
Option Identification: dataHopping
Present in: APEX's "data" element
Contains: dataHopping (c.f., Section 6)
Processing Rules: c.f., Section 4
Contact Information: c.f., the "Authors' Addresses" section of this
memo
Klyne, et. al. Standards Track [Page 16]
RFC 3342 The Application Exchange (APEX) Party Pack July 2002
6. The APEX Party Pack DTD
<!--
DTD for the APEX option party pack, as of 2001-05-14
Refer to this DTD as:
<!ENTITY % APEXPARTY PUBLIC "-//IETF//DTD APEX PARTY//EN" "">
%APEXPARTY;
-->
<!ENTITY % APEXCORE PUBLIC "-//IETF//DTD APEX CORE//EN"
%APEXCORE;
<!--
DTD data types:
entity syntax/reference example
====== ================ =======
hopcount
HOPS 0..255 17
milli-seconds
MILLISECS 0..2147483647 60000
-->
<!ENTITY % HOPS "CDATA">
<!ENTITY % MILLISECS
"CDATA">
<!ELEMENT dataHopping EMPTY>
<!ATTLIST dataHopping
noMoreThan %HOPS; "0"
reportErrors
(true|false) "false">
<!ELEMENT dataTiming EMPTY>
<!ATTLIST dataTiming
noLaterThan %MILLISECS; "0"
returnTrip %MILLISECS; "0"
reportAfter %MILLISECS; "0"
reportErrors
(true|false) "false">
Klyne, et. al. Standards Track [Page 17]
RFC 3342 The Application Exchange (APEX) Party Pack July 2002
7. Security Considerations
Consult [1]'s Section 11 for a discussion of security issues.
In addition:
o The dataTiming option (Section 2) may be used to expose private
network topology. Accordingly, an administrator may wish to
choose to disable this option except at the ingress/egress points
for its administrative domain.
o The hold4Endpoint option (Section 3) may be used to facilitate
denial-of-service attacks. Accordingly, an administrator may wish
to impose administrative limits on this attribute (e.g., always
require that the "dataTiming" option also be present with a
short-lived "noLaterThan" attribute).
References
[1] Rose, M., Klyne, G. and D. Crocker, "The Application Exchange
Core", RFC 3340, July 2002.
[2] Postel, J., "Internet Protocol", STD 5, RFC 791, September
1981.
[3] Newman, D., "Deliver By SMTP Service Extension", RFC 2852, June
2000.
Klyne, et. al. Standards Track [Page 18]
RFC 3342 The Application Exchange (APEX) Party Pack July 2002
Appendix A. Acknowledgements
The authors gratefully acknowledge the contributions of Chris Newman
and Bob Wyman. Further, the dataTiming option is similar in function
to "Deliver By" SMTP service extension defined by Dan Newman in [3].
Appendix B. IANA Considerations
The IANA completed the registrations specified in Section 5.
Klyne, et. al. Standards Track [Page 19]
RFC 3342 The Application Exchange (APEX) Party Pack July 2002
Authors' Addresses
Graham Klyne
Clearswift Corporation
1310 Waterside
Arlington Business Park
Theale, Reading RG7 4SA
UK
Phone: +44 11 8903 8903
EMail: Graham.Klyne@MIMEsweeper.com
Marshall T. Rose
Dover Beach Consulting, Inc.
POB 255268
Sacramento, CA 95865-5268
US
Phone: +1 916 483 8878
EMail: mrose@dbc.mtview.ca.us
Michael F. Schwartz
Code On The Road, LLC
EMail: schwartz@CodeOnTheRoad.com
URI: http://www.CodeOnTheRoad.com
Eric Dixon
EMail: edixon@myrealbox.com
Huston Franklin
EMail: huston@franklin.ro
Jay Kint
EMail: d20@icosahedron.org
Klyne, et. al. Standards Track [Page 20]
RFC 3342 The Application Exchange (APEX) Party Pack July 2002
Darren New
5390 Caminito Exquisito
San Diego, CA 92130
US
Phone: +1 858 350 9733
EMail: dnew@san.rr.com
Scott Pead
EMail: spead@fiber.net
Klyne, et. al. Standards Track [Page 21]
RFC 3342 The Application Exchange (APEX) Party Pack July 2002
Full Copyright Statement
Copyright (C) The Internet Society (2002). All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this
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Klyne, et. al. Standards Track [Page 22]