Network Working Group R. Gellens
Request for Comments: 5383 Qualcomm
BCP: 143 October 2008
Category: Best Current Practice
Deployment Considerations for Lemonade-Compliant Mobile Email
Status of This Memo
This document specifies an Internet Best Current Practices for the
Internet Community, and requests discussion and suggestions for
improvements. Distribution of this memo is unlimited.
This document discusses deployment issues and describes requirements
for successful deployment of mobile email that are implicit in the
IETF lemonade documents.
Table of Contents
1. Introduction ....................................................2
2. Conventions Used in This Document ...............................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 .........................................8
9. Acknowledgments ................................................10
10. Normative References ..........................................10
11. Informative References ........................................10
The IETF lemonade group has developed a set of extensions to IMAP and
Message Submission, along with a profile document that restricts
server behavior and describes client usage [PROFILE].
Successful deployment of lemonade-compliant mobile email requires
various functionality that is generally assumed and hence not often
covered in email RFCs. This document describes some of these
additional considerations, with a focus on those that have been
reported to be problematic.
2. 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].
Both IMAP and Message Submission have been assigned well-known ports
[IANA] that 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
systems, as well as any intermediary systems, MUST allow
communication on these ports.
Historically, Message User Agents (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
Due to endemic security vulnerabilities in widely deployed SMTP
servers, organizations often employ application-level firewalls that
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 that 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 that necessarily result in data
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. 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
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 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).
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 a
specific configuration setting that specifies a longer maximum idle
interval, the TCP connection does 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. In any case, 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.
In general, applications can choose whether or not to enable TCP
keep-alives, 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
Client, server, and intermediate systems MUST comply with the
"Destination Unreachable -- codes 0, 1, 5" text in Section 22.214.171.124 of
[HOST-REQUIREMENTS], which states "Since these Unreachable messages
indicate soft error conditions, TCP MUST NOT abort the connection".
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.
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
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
Application proxies, which are not uncommon mechanisms, are discussed
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 that 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
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 a 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 that 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 that were leveraged to
tunnel the new service, become 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 that 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.
Any NAT boxes that 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.
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,
Transport Layer Security (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 reduce
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 that 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
A large number of approaches exist that 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 that is not accessible from
the Internet and that 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 that 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 an understanding of the issues involved.
Organizations SHOULD deploy servers that support the use of TLS for
all connections and that 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 endpoints. 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.
Chris Newman and Phil Karn suggested very helpful text. Brian Ross
and Dave Cridland reviewed drafts and provided excellent suggestions.
10. Normative References
[BEHAVE-TCP] Guha, S., Ed., Biswas, K., Ford, B., Sivakumar,
S., and P. Srisuresh, "NAT Behavioral
Requirements for TCP", BCP 142, RFC 5382, October
[HOST-REQUIREMENTS] Braden, R., Ed., "Requirements for Internet Hosts
- Communication Layers", STD 3, RFC 1122, October
[KEYWORDS] Bradner, S., "Key words for use in RFCs to
Indicate Requirement Levels", BCP 14, RFC 2119,
[IANA] IANA Port Number Registry,
[TCP] Postel, J., "Transmission Control Protocol", STD
7, RFC 793, September 1981.
11. Informative References
[ACAP] Newman, C. and J. Myers, "ACAP -- Application
Configuration Access Protocol", RFC 2244,
[FIREWALLS] Freed, N., "Behavior of and Requirements for
Internet Firewalls", RFC 2979, October 2000.
[IMAP] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL -
VERSION 4rev1", RFC 3501, March 2003.
[KLUDGE] Moore, K., "On the use of HTTP as a Substrate",
BCP 56, RFC 3205, February 2002.
[PROFILE] Maes, S. and A. Melnikov, "Internet Email to
Support Diverse Service Environments (Lemonade)
Profile", RFC 4550, June 2006.
[PROXIES] Chatel, M., "Classical versus Transparent IP
Proxies", RFC 1919, March 1996.
[SUBMISSION] Gellens, R. and J. Klensin, "Message Submission
for Mail", RFC 4409, April 2006.
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