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Versions: 00 01 02 03 04 05 06 07 08 09 rfc5383    Best Current Practice
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

    By submitting this Internet-Draft, each author represents that any
    applicable patent or other IPR claims of which he or she is aware
    have been or will be disclosed, and any of which he or she becomes
    aware will be disclosed, in accordance with Section 6 of BCP 79.

    Internet-Drafts are working documents of the Internet Engineering
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    The list of current Internet-Drafts can be accessed at
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Copyright Notice

    Copyright (C) The IETF Trust (2007).  All Rights Reserved.


    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",
    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

    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

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

    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

    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

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
    of [HOST-REQUIREMENTS], which states "Since these Unreachable
    messages indicate soft error conditions, TCP MUST NOT abort the

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

    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

    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

    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


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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

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,

    [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

    [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

Appendix A:  Changes from Previous Version


    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
    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
    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
    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
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    in this document or the extent to which any license under such
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    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.

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    The IETF invites any interested party to bring to its attention any
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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

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