Internet Draft: Deployment Considerations for
                lemonade-compliant Mobile Email               R. Gellens
Document: draft-ietf-lemonade-deployments-02.txt                Qualcomm
Expires: November 2006                                          May 2006

     Deployment Considerations for lemonade-compliant Mobile Email

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Copyright Notice

    Copyright (C) The Internet Society (2006).  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  . . . . . . . . . . . . . . . . . . . . . . . .  3
       4.2  Maintenance during temporary transport loss  . . . . . .   4
     5  Dormancy  . . . . . . . . . . . . . . . . . . . . . . . . . .  5
     6  Firewalls  . . . . . . . . . . . . . . . . . . . . . . . . .   5
       6.1  Firewall Traversal  . . . . . . . . . . . . . . . . . . .  6
     7  NATs . . . . . . . . . . . . . . . . . . . . . . . . . . . .   7
     8  Security Considerations . . . . . . . . . . . . . . . . . . .  7
     9  IANA Considerations  . . . . . . . . . . . . . . . . . . . .   9
    10  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  9
    11  Normative References . . . . . . . . . . . . . . . . . . . .   9
    12  Informative References  . . . . . . . . . . . . . . . . . . .  9
    13  Author's Address . . . . . . . . . . . . . . . . . . . . . .  10
       Intellectual Property Statement  . . . . . . . . . . . . . . . 10
       Full Copyright Statement  . . . . . . . . . . . . . . . . . .  10

1 Conventions Used in this Document

    The key words "REQUIRED", "MUST", "MUST NOT", "SHOULD", "SHOULD
    NOT", and "MAY" in this document are to be interpreted as described
    in "Key words for use in RFCs to Indicate Requirement Levels"

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 difficult to deploy on port
    25.  Since lemonade requires support for several Submit 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).  Unless specially configured otherwise,
    this uses port 143.

    It should be noted that systems which don't support TCP on arbitrary
    ports aren't full Internet clients.  As a result, such systems use
    gateways to the Internet which necessarily result in data integrity

4 TCP Connections

    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.

    The requirements and advice in [HOST-REQUIREMENTS] SHOULD be

4.1 Lifetime

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    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.

    The only permissible timeouts on TCP connections occur at the IMAP
    and Message Submission application level: if no data is received
    within a period of time, either side MAY terminate the connection as
    permitted by the protocol (see [SUBMISSION] or [PROFILE]).  Such
    timeouts MUST only be enforced by the server or client, not an
    intermediary system.  Since IMAP permits unsolicited notifications
    of state changes, it is reasonable for clients to remain connected
    for extended periods with no data being exchanged.

    It has been reported that some mobile carrier network infrastructure
    elements impose time restrictions of their own on TCP connections
    other than HTTP.  Such behavior is harmful to mobile 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 or a deliberate choice.
    Either way, such a barrier to TCP connections is a significant risk
    to the increasing usage of IETF protocols on mobile 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.

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.

    To this end, client and server systems SHOULD NOT set the TCP
    keep-alive socket option, and SHOULD NOT close a connection based on
    ICMP host unreachable messages.

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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.

    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.  Thus, as
    stated in 4.2 above, client and server systems SHOULD NOT set the
    TCP keep-alive socket option, and SHOULD NOT close a connection
    based on ICMP host unreachable messages.

    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.

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.

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    Firewalls deployed in the network path MUST conform to [FIREWALLS].

    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.

    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

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    run it is likely to be less expensive than a kludge.

7 NATs

    Many NAT boxes place lifetime limits on state, which has the effect
    of aging out long-idle TCP connections.  Since memory is relatively
    cheap, there's little benefit in arbitrary timeouts.  Instead, the
    oldest unused connection can be recycled if memory or other
    resources (such as IP addresses) become exhausted, allowing
    connections to stay stay up forever when resources are available.

    Any NAT boxes which are deployed between client and server systems
    SHOULD be configured to have extremely long connection lifetimes.
    Unlimited lifetimes are RECOMMENDED.

    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 ridiculously 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 defenses and/or insurance 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 might wish to purchase a messaging server which comes
    with some indemnity and/or a messaging server which is used "on the
    edge" by the organization that sells the server.

    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.

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    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.

    There exist a large number of approaches which can mitigate the
    risks while allowing access to mail services via mobile clients.

    Placing servers inside one or more DMZs 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 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."

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    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


10 Acknowledgments

    Chris Newman and Phil Karn suggested very helpful text.  Brian Ross
    and Dave Cridland reviewed drafts and provided excellent

11 Normative References

    [KEYWORDS] "Key words for use in RFCs to Indicate Requirement
    Levels", S. Bradner, BCP 14, March 1997.

    [IANA] IANA Port Number Registry,

    [PROFILE] "LEMONADE profile bis", S. Maes, A. Melnikov, D. Cridland,
    editors, draft-ietf-lemonade-profile, work in progress.

12 Informative References

    [FIREWALLS] "Behavior of and Requirements for Internet Firewalls",
    N. Freed, RFC 2979, October 2000.

    [HOST-REQUIREMENTS] "Requirements for Internet Hosts --
    Communication Layers", R. Braden, RFC 1122, October 1989.

    [KLUDGE] "On the use of HTTP as a Substrate", K. Moore, BCP 56,
    February 2002.

    [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

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    This document and the information contained herein are provided on

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