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Versions: 00 01 02 03 04 05 06 07 08 09 RFC 5383
Internet Draft: Deployment Considerations for
lemonade-compliant Mobile Email R. Gellens
Document: draft-ietf-lemonade-deployments-09.txt Qualcomm
Expires: December 2007 June 2007
Deployment Considerations for lemonade-compliant Mobile Email
Status of this Memo
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Copyright Notice
Copyright (C) The IETF Trust (2007). All Rights Reserved.
Abstract
This document discusses deployment issues and describes requirements
for successful deployment of mobile email which are implicit in the
IETF lemonade documents.
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Table of Contents
1 Conventions Used in this Document . . . . . . . . . . . . . . 2
2 Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
3 Ports . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
4 TCP Connections . . . . . . . . . . . . . . . . . . . . . . 3
4.1 Lifetime . . . . . . . . . . . . . . . . . . . . . . . . 4
4.2 Maintenance during temporary transport loss . . . . . . 5
5 Dormancy . . . . . . . . . . . . . . . . . . . . . . . . . . 6
6 Firewalls . . . . . . . . . . . . . . . . . . . . . . . . . 6
6.1 Firewall Traversal . . . . . . . . . . . . . . . . . . . 7
7 NATs . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
8 Security Considerations . . . . . . . . . . . . . . . . . . . 9
9 IANA Considerations . . . . . . . . . . . . . . . . . . . . 10
10 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 10
11 Normative References . . . . . . . . . . . . . . . . . . . . 11
12 Informative References . . . . . . . . . . . . . . . . . . . 11
13 Author's Address . . . . . . . . . . . . . . . . . . . . . . 12
Appendix A: Changes from Previous Version . . . . . . . . . . 12
Intellectual Property Statement . . . . . . . . . . . . . . . 13
Full Copyright Statement . . . . . . . . . . . . . . . . . . . 14
1 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 [KEYWORDS].
2 Introduction
The IETF lemonade group has developed a set of extensions to IMAP
and Message Submission, along with a profile document which
restricts server behavior and describes client usage [PROFILE].
Successful deployment of lemonade-compliant mobile email requires
various functionality which is generally assumed and hence often not
covered in email RFCs. This document describes some of these
additional considerations, with a focus on those which have been
reported to be problematic.
3 Ports
Both IMAP and Message Submission have been assigned well-known ports
[IANA] which MUST be available. IMAP uses port 143. Message
Submission uses port 587. It is REQUIRED that the client be able to
contact the server on these ports. Hence the client and server
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systems, as well as any intermediary systems, MUST allow
communication on these ports.
Historically, MUAs have used port 25 for message submission, and
[SUBMISSION] does accommodate this. However, it has become
increasingly common for ISPs and organizations to restrict outbound
port 25. Additionally, hotels and other public accommodations
sometimes intercept port 25 connections, regardless of the
destination host, resulting in users unexpectedly submitting
potentially sensitive communications to unknown and untrusted
third-party servers. Typically, users are not aware of such
interception. (Such interception violates [FIREWALLS] and has many
negative consequences.)
Due to endemic security vulnerabilities in widely-deployed SMTP
servers, organizations often employ application-level firewalls
which intercept SMTP and permit only a limited subset of the
protocol. New extensions are therefore more difficult to deploy on
port 25. Since lemonade requires support for several [SUBMISSION]
extensions, it is extremely important that lemonade clients use, and
lemonade servers listen on, port 587 by default.
In addition to communications between the client and server systems,
lemonade requires that the Message Submission server be able to
establish a TCP connection to the IMAP server (for
forward-without-download). This uses port 143 by default.
Messaging clients sometimes use protocols to store, retrieve, and
update configuration and preference data. Functionality such as
setting a new device to use the configuration and preference data of
another device, or having a device inherit default configuration
data from a user account, an organization, or other source, is
likely to be even more useful with small mobile devices. Various
protocols can be used for configuration and preference data; most of
these protocols have designated ports. It is important that clients
be able to contact such servers on the appropriate ports. As an
example, one protocol that can be used for this purpose is [ACAP],
in which case port 674 needs to be available.
Note that systems which do not support application use of [TCP] on
arbitrary ports are not full Internet clients. As a result, such
systems use gateways to the Internet which necessarily result in
data integrity problems.
4 TCP Connections
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Both IMAP and Message Submission use [TCP]. Hence the client system
MUST be able to establish and maintain TCP connections to these
servers. The Message Submission server MUST be able to initiate a
connection to the IMAP server. Support for application use of [TCP]
is REQUIRED of both client and server systems.
The requirements and advice in [HOST-REQUIREMENTS] SHOULD be
followed.
Note that, for environments that do not support application use of
[TCP] but do so for HTTP, email can be offered by deploying webmail.
Webmail is a common term for email over the web, where a server
speaks HTTP to the client and an email protocol (often IMAP) to the
mail store. Its functionality is necessarily limited by the
capabilities of the web client, the webmail server, the protocols
used between the webmail server and the client (HTTP and a markup
language such as HTML), and between the webmail server and the mail
store. However, if HTTP is all that is available to an application,
the environment is by definition limited and thus functionality
offered to the user must also be limited, and can't be lemonade
compliant.
4.1 Lifetime
In this document, "idle" refers to the idle time, as in the
"established connection idle-timeout" of [BEHAVE-TCP], while
"duration" refers to the total time that a TCP connection has been
established.
The duration of the TCP connections between the client and server
systems for both IMAP and Message Submission can be arbitrarily
long. The client system, the server, as well as all intermediate
systems MUST NOT terminate these TCP connections simply because of
their duration (that is, just because of how long they have been
open).
Lemonade depends on idle timers being enforced only at the
application level (IMAP and Message Submission): if no data is
received within a period of time, either side MAY terminate the
connection as permitted by the protocol (see [SUBMISSION] or
[IMAP]). Since IMAP permits unsolicited notifications of state
changes, it is reasonable for clients to remain connected for
extended periods with no data being exchanged. Being forced to send
data just to keep the connection alive can prevent or hinder
optimizations such as dormancy mode (see section 5).
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Two hours is a fairly common configuration timeout at middleboxes.
That is, there are a number of sites at which TCP connections are
torn down by the network two hours after data was last sent in
either direction (for example, REQ-5 in [BEHAVE-TCP]). Thus,
lemonade clients and servers SHOULD make sure that, in the absence
of specific configuration otherwise, the TCP connection not remain
idle for two hours. This rule ensures that, by default, lemonade
clients and servers operate in environments configured with a
two-hour maximum for idle TCP connections. Network and server
operators can still permit IMAP connections to remain idle in excess
of two hours and thus increase the benefits of dormancy, by
configuring lemonade clients and servers, and network equipment, to
allow this.
It has been reported that some networks impose duration time
restrictions of their own on TCP connections other than HTTP. Such
behavior is harmful to email and all other TCP-based protocols. It
is unclear how widespread such reported behavior is, or if it is an
accidental consequence of an attempt at optimizing for HTTP traffic,
implementation limitations in firewalls, NATs or other devices, or a
deliberate choice. Either way, such a barrier to TCP connections is
a significant risk to the increasing usage of IETF protocols on such
networks. Note that TCP is designed to be more efficient when it is
used to transfer data over time. Prohibiting such connections thus
imposes hidden costs on an operator's network, forcing clients to
use TCP in inefficient ways. One way in which carriers can
inadvertently force TCP connections closed, resulting in users
wasting packets by reopening them, is described in Section 7
Note that systems remain able to terminate TCP connections at any
time based on local decisions, for example, to prevent overload
during a denial-of-service attack. These mechanisms are permitted
to take idle time into consideration and are not affected by these
requirements.
4.2 Maintenance during temporary transport loss
TCP is designed to withstand temporary loss of lower-level
connectivity. Such transient loss is not uncommon in mobile systems
(for example, due to handoffs, fade, etc.). The TCP connection
SHOULD be able to survive temporary lower-level loss when the IP
address of the client does not change (for example, short-duration
loss of the mobile device's traffic channel or periods of high
packet loss). Thus, the TCP/IP stack on the client, the server, and
all intermediate systems SHOULD maintain the TCP connection during
transient loss of connectivity.
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In general, applications can choose to enable TCP keep-alives or
not, but in many cases are unable to affect any other aspect of TCP
keep-alive operation, such as time between keep-alive packets,
number of packets sent before the connection is aborted, etc. In
some environments these are operating system tuning parameters not
under application control. In some cases, operational difficulties
have been reported with application use of the TCP keep-alive
option, which might be the result of TCP implementation differences
or defects specific to a platform. Lemonade client and server
systems SHOULD NOT set the TCP keep-alive socket option unless
operating in environments where this works correctly and such
packets will not be sent more frequently than every two hours.
Application-level keep-alives (such as IMAP NOOP) MAY be used
instead of the TCP keep-alive option.
Client, server, and intermediate systems MUST comply with the
"Destination Unreachable -- codes 0, 1, 5" text in Section 4.2.3.9
of [HOST-REQUIREMENTS], which states "Since these Unreachable
messages indicate soft error conditions, TCP MUST NOT abort the
connection."
5 Dormancy
Cellular data channels are connection-oriented (they are brought up
or down to establish or tear down connections); it costs network
resources to establish connections. Generally speaking, mobile
device battery charges last longer when the traffic channel is used
less.
Some mobile devices and networks support dormant mode, in which the
traffic channel is brought down during idle periods, yet the PPP or
equivalent level remains active, and the mobile retains its IP
address.
Maintenance of TCP connections during dormancy SHOULD be supported
by the client, server, and any intermediate systems, as described in
Sections 4.1 and 4.2.
Sending packets just to keep the session active causes unnecessary
channel establishment and timeout; with a long-idle TCP connection,
this would periodically bring up the channel and then let it idle
until it times out, again and again. However, in the absence of
specific configuration information to the contrary, it is necessary
to do this to ensure correct operation by default.
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6 Firewalls
New services must necessarily have their traffic pass through
firewalls in order to be usable by corporate employees or
organization members connecting externally, such as when using
mobile devices. Firewalls exist to block traffic, yet exceptions
must be made for services to be used. There is a body of best
practices based on long experience in this area. Numerous
techniques exist to help organizations balance protecting themselves
and providing services to their members, employees, and/or
customers. (Describing, or even enumerating, such techniques and
practices is beyond the scope of this document, but section 8 does
mention some.)
It is critical that protocol design and architecture permit such
practices, and not constrain them. One key way in which the design
of a new service can aid its secure deployment is to maintain the
one-to-one association of services and port numbers.
One or more firewalls might exist in the path between the client and
server systems, as well as between the Message Submission and IMAP
servers. Proper deployment REQUIRES that TCP connections be
possible from the client system to the IMAP and Message Submission
ports on the servers, as well as from the Message Submission server
to the IMAP server. This may require configuring firewalls to
permit such usage.
Firewalls deployed in the network path MUST NOT damage protocol
traffic. In particular, both message submission and IMAP
connections from the client MUST be permitted. Firewalls MUST NOT
partially block extensions to these protocols, such as by allowing
one side of an extension negotiation, as doing so results in the two
sides being out of synch, with later failures. See [FIREWALLS] for
more discussion.
Application proxies, which are a not uncommon mechanism, are
discussed in [PROXIES].
6.1 Firewall Traversal
An often-heard complaint from those attempting to deploy new
services within an organization is that the group responsible for
maintaining the firewall is unable or unwilling to open the required
ports. The group which owns the firewall, being charged with
organizational network security, is often reluctant to open firewall
ports without an understanding of the benefits and the security
implications of the new service.
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The group wishing to deploy a new service is often tempted to bypass
the procedure and internal politics necessary to open the firewall
ports. A tempting kludge is to tunnel the new service over an
existing service that is already permitted to pass through the
firewall, typically HTTP on port 80 or sometimes SMTP on port 25.
Some of the downsides to this are discussed in [KLUDGE].
Such bypass can appear to be immediately successful, since the new
service seems to deploy. However, assuming the network security
group is competent, when they become aware of the kludge, their
response is generally to block the violation of organizational
security policy. It is difficult to design an application-level
proxy/firewall which can provide such access control without
violating the transparency requirements of firewalls, as described
in [FIREWALLS]. Collateral damage is common in these circumstances.
The new service (which initially appeared to have been successfully
deployed) as well as those existing services which were leveraged to
tunnel the new service, becomes subject to arbitrary and
unpredictable failures. This encourages an adversarial relationship
between the two groups, which hinders attempts at resolution.
Even more serious is what happens if a vulnerability is discovered
in the new service. Until the vulnerability is corrected, the
network security group must disable both the new service and the
(typically mission-critical) existing service on which it is
layered.
An often-repeated truism is that any computer which is connected to
a network is insecure. Security and usefulness are both
considerations, with organizations making choices about achieving
acceptable measures in both areas. Deploying new services typically
requires deciding to permit access to the ports used by the service,
with appropriate protections. While the delay necessary to review
the implications of a new service may be frustrating, in the long
run it is likely to be less expensive than a kludge.
7 NATs
Any NAT boxes which are deployed between client and server systems
MUST comply with REQ-5 in [BEHAVE-TCP], which requires that 'the
value of the "established connection idle-timeout" MUST NOT be less
than 2 hours 4 minutes.'
See section 5 for additional information on connection lifetimes.
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Note that IMAP and message submission clients will automatically
re-open TCP connections as needed, but it saves time, packets, and
processing to avoid the need to do so. Re-opening IMAP and message
submission connections generally incurs costs for authentication,
TLS negotiation, and server processing, as well as resetting of TCP
behavior such as windows. It is also wasteful to force clients to
send NOOP commands just to maintain NAT state, especially since this
can defeat dormancy mode.
8 Security Considerations
There are numerous security considerations whenever an organization
chooses to make any of its services available via the Internet.
This includes email from mobile clients.
Sites concerned about email security should perform a threat
analysis, get relevant protections in place and then make a
conscious decision to open up this service. As discussed in section
6.1, piggybacking email traffic on the HTTP port in an attempt to
avoid making a firewall configuration change to explicitly permit
mobile email connections would bypass this important step and
reduces the overall security of the system.
Organizations deploying a messaging server "on the edge" (that is,
accessible from the open Internet) are encouraged to choose one that
has been designed to operate in that environment.
This document does not attempt to catalogue either the various risks
an organization might face or the numerous techniques which can be
used to protect against the risks. However, to help illustrate the
deployment considerations, a very small sample of some of the risks
and countermeasures appear below.
Some organizations are concerned that permitting direct access to
their mail servers via the Internet increases their vulnerability,
since a successful exploit against a mail server can potentially
expose all mail and authentication credentials stored on that
server, and can serve as an injection point for spam. In addition,
there are concerns over eavesdropping or modification of mail data
and authentication credentials.
A large number of approaches exist which can mitigate the risks
while allowing access to mail services via mobile clients.
Placing servers inside one or more DMZs (demilitarized zones, also
called perimeter networks) can protect the rest of the network from
a compromised server. An additional way to reduce the risk is to
store authentication credentials on a system which is not accessible
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from the Internet, and which the servers within the DMZ can access
only by sending the credentials as received from the client and
receiving an authorized/not authorized response. Such isolation
reduces the ability of a compromised server to serve as a base for
attacking other network hosts.
Many additional techniques for further isolation exist, such as
having the DMZ IMAP server have no mail store of its own. When a
client connects to such a server, the DMZ IMAP server might contact
the authentication server and receive a ticket, which it passes to
the mail store in order to access the client's mail. In this way a
compromised IMAP server cannot be used to access the mail or
credentials for other users.
It is important to realize that simply throwing an extra box in
front of the mail servers, such as a gateway which may use HTTP or
any of a number of synchronization protocols to communicate with
clients, does not itself change the security aspects. By adding
such a gateway, the overall security of the system, and the
vulnerability of the mail servers, may remain unchanged or may be
significantly worsened. Isolation and indirection can be used to
protect against specific risks, but to be effective, such steps need
to be done after a threat analysis, and with understanding of the
issues involved.
Organizations SHOULD deploy servers which support the use of TLS for
all connections and which can be optionally configured to require
TLS. When TLS is used, it SHOULD be via the STARTTLS extensions
rather than the alternate port method. TLS can be an effective
measure to protect against specific threats, including eavesdropping
and alteration, of the traffic between the end-points. However,
just because TLS is deployed does not mean the system is "secure."
Attempts at bypassing current firewall policy when deploying new
services have serious risks, as discussed in section 6.1.
It's rare for a new service to not have associated security
considerations. Making email available to an organization's members
using mobile devices can offer significant benefits.
9 IANA Considerations
None.
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10 Acknowledgments
Chris Newman and Phil Karn suggested very helpful text. Brian Ross
and Dave Cridland reviewed drafts and provided excellent
suggestions.
11 Normative References
[BEHAVE-TCP] "NAT Behavioral Requirements for TCP",
draft-ietf-behave-tcp-07.txt [in RFC Editor's Queue -- RFC Editor,
please replace with reference to RFC when published].
[HOST-REQUIREMENTS] "Requirements for Internet Hosts --
Communication Layers", R. Braden, RFC 1122, October 1989.
[KEYWORDS] "Key words for use in RFCs to Indicate Requirement
Levels", S. Bradner, RFC 2119, BCP 14, March 1997.
[IANA] IANA Port Number Registry,
<http://www.iana.org/assignments/port-numbers>
[TCP] "Transmission Control Protocol", J. Postel, RFC 793, STD 7,
September 1981.
12 Informative References
[ACAP] "ACAP -- Application Configuration Access Protocol", C.
Newman, J.G. Myers, RFC 2244, November 1997.
[FIREWALLS] "Behavior of and Requirements for Internet Firewalls",
N. Freed, RFC 2979, October 2000.
[IMAP] "Internet Message Access Protocol -- Version 4rev1", M.
Crispin, RFC 3501, March 2003.
[KLUDGE] "On the use of HTTP as a Substrate", K. Moore, BCP 56,
February 2002.
[PROFILE] "Lemonade Profile", S. Maes, A. Melnikov, RFC 4550, June
2006.
[PROXIES] "Classical versus Transparent IP Proxies", M. Chatel, RFC
1919, March 1996.
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[SUBMISSION] "Message Submission for Mail", R. Gellens, J. Klensin,
RFC 4409, April 2006.
13 Author's Address
Randall Gellens
QUALCOMM Incorporated
5775 Morehouse Drive
San Diego, CA 92121
randy@qualcomm.com
Appendix A: Changes from Previous Version
THIS SECTION TO BE REMOVED PRIOR TO PUBLICATION.
Changes made from version -08 to -09 as a result of IESG DISCUSS by
Cullen Jennings:
o Added informative reference to draft-ietf-behave-tcp-07.txt in
third paragraph of Section 4.
o Replaced some text in Section 7 with a normative reference to
draft-ietf-behave-tcp-07.txt.
o Defined "idle" and "duration" at start of Section 4.1.
o Replaced text in Section 4.2 on TCP soft errors with reference
to RFC 1122.
o Replaced repetition of requirements in Section 5 with references
to Sections 4.1 and 4.2.
o Deleted first paragraph in Section 7.
o Reworded third paragraph in Section 8 to eliminate the word
"indemnity".
o Reworded text in Section 3 regarding configuration data.
o Reworded text on the TCP keep-alive option in Section 4.2.
o Deleted word "ridiculously" in Section 7.
o Added clarification at end of last paragraph of Section 5.
o Added normative reference [BEHAVE-TCP].
Changes made from version -07 to -08 as a result of IESG DISCUSS by
Cullen Jennings:
o Added text to section 4.1 explaining that the two-hour maximum
idle time is the default, while still permitting specific
configurations to exceed this to maximize dormancy.
o Changed ICMP soft errors from SHOULD NOT to MUST NOT.
Changes made from version -06 to -07 as a result of IESG DISCUSS by
Cullen Jennings:
o Removed prohibition on NATs closing connections just because of
how long they have been open, without taking any other factor
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into account. Replaced with discussion on the issue and
recommendation that lemonade clients not allow connections to
remain idle for two hours, unless they have specific
configuration instructions to do so (e.g., in networks where
NATs and firewalls allow this, and the IMAP server permits it)
o Changed explicit note to permit all systems to terminate TCP
connections at any time based on local decisions
o Removed word "insurance"
Changes made from version -05 to -06 as a result of IESG DISCUSSes
by Jari Arkko, Lars Eggert, Cullen Jennings, and Magnus Westerlund:
o Clarified that "support" for TCP means availability of TCP to
applications, as opposed to deployment or use of TCP within a
network
o Clarified that an HTTP-only environment can offer webmail, which
may be email, but isn't lemonade email
o Added explicit note that end systems remain able to terminate
TCP connections at any time based on local decisions
o Made [FIREWALLS] an informative, not normative reference by
restating requirement
o Additional clarifications to make draft easier to read from a
non-Applications Area viewpoint
Changes made from version -04 to -05 as a result of IETF Last Call:
o Fixed some typos.
o Made first use of TCP into a reference.
Changes made from version -03 to -04 as a result of WG Last Call:
o New boilerplate text
o Wording tweaks from lemonade list (e.g., expanding contractions)
o Explcitly state that support for TCP is REQUIRED
o Correct reference in timeout text from PROFILE to IMAP
o Add RFC number to KEYWORDS reference (nit checker doesn't like
BCP number only)
o Move HOST-REQUIREMENTS reference to normative from informative
o Add TCP reference (since TCP support is REQUIRED)
o Add IMAP reference (for port number)
o Update PROFILE reference to RFC (from RFC Ed pub queue)
Intellectual Property Statement
The IETF takes no position regarding the validity or scope of any
Intellectual Property Rights or other rights that might be claimed
to pertain to the implementation or use of the technology described
in this document or the extent to which any license under such
rights might or might not be available; nor does it represent that
it has made any independent effort to identify any such rights.
Information on the procedures with respect to rights in RFC
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documents can be found in BCP 78 and BCP 79.
Copies of IPR disclosures made to the IETF Secretariat and any
assurances of licenses to be made available, or the result of an
attempt made to obtain a general license or permission for the use
of such proprietary rights by implementers or users of this
specification can be obtained from the IETF on-line IPR repository
at http://www.ietf.org/ipr.
The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights that may cover technology that may be required to implement
this standard. Please address the information to the IETF at
ietf-ipr@ietf.org.
Full Copyright Statement
Copyright (C) The IETF Trust (2007).
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
REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE
IETF TRUST AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL
WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY
WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE
ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS
FOR A PARTICULAR PURPOSE.
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