Network Working Group J. Klensin
Request for Comments: 4084 May 2005
Category: Best Current Practice
Terminology for Describing Internet Connectivity
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.
Copyright (C) The Internet Society (2005).
As the Internet has evolved, many types of arrangements have been
advertised and sold as "Internet connectivity". Because these may
differ significantly in the capabilities they offer, the range of
options, and the lack of any standard terminology, the effort to
distinguish between these services has caused considerable consumer
confusion. This document provides a list of terms and definitions
that may be helpful to providers, consumers, and, potentially,
regulators in clarifying the type and character of services being
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 21.1. The Problem and the Requirement . . . . . . . . . . . . 21.2. Adoption and a Non-pejorative Terminology . . . . . . . 22. General Terminology . . . . . . . . . . . . . . . . . . . . . 33. Filtering or Security Issues and Terminology . . . . . . . . . 54. Additional Terminology . . . . . . . . . . . . . . . . . . . . 75. Security Considerations . . . . . . . . . . . . . . . . . . . 96. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 97. Informative References . . . . . . . . . . . . . . . . . . . . 9
1.1. The Problem and the Requirement
Different ISPs and other providers offer a wide variety of products
that are identified as "Internet" or "Internet access". These
products offer different types of functionality and, as a result,
some may be appropriate for certain users and uses and not others.
For example, a service that offers only access to the Web (in this
context, the portion of the Internet that is accessible via the HTTP
and HTTPS protocols) may be appropriate for someone who is
exclusively interested in browsing and in Web-based email services.
It will not be appropriate for someone who needs to download files or
use email more frequently. And it is likely to be even less
appropriate for someone who needs to operate servers for other users,
who needs virtual private network (VPN) capabilities or other secured
access to a remote office, or who needs to synchronize mail for
Recent and rapidly evolving changes to the Internet's email
environment have led to additional restrictions on sending and
retrieving email. These restrictions, most of them developed as part
of well intentioned attempts to prevent or fight unsolicited mail,
may be imposed independently of the service types described below and
are discussed separately in Section 3.
This document describes only the functions provided or permitted by
the service provider. It does not and cannot specify the functions
that pass through and are supported by various user-provided
The terms SHOULD, MUST, or MAY are capitalized in this document, as
defined in .
1.2. Adoption and a Non-pejorative Terminology
The definitions proposed here are of little value if service
providers and vendors are not willing to adopt them. The terms
proposed are intended not to be pejorative, despite the belief of
some members of the IETF community that some of these connectivity
models are simply "broken" or "not really an Internet service". The
mention of a particular service or model in this document does not
imply any endorsement of it, only recognition of something that
exists or might exist in the marketplace. Thus, the Best Current
Practice described in this document is about terminology and
information that should be supplied to the user and not about the
types of service that should be offered.
2. General Terminology
This section lists the primary IP service terms. It is hoped that
service providers will adopt these terms, to better define the
services to potential users or customers. The terms refer to the
intent of the provider (ISP), as expressed in either technical
measures or terms and conditions of service. It may be possible to
work around particular implementations of these characteristic
connectivity types, but that freedom is generally not the intent of
the provider and is unlikely to be supported if the workarounds stop
The service terms are listed in order of ascending capability, to
reach "full Internet connectivity".
o Web connectivity.
This service provides connectivity to the Web, i.e., to services
supported through a "Web browser" (such as Firefox, Internet
Explorer, Mozilla, Netscape, Lynx, or Opera), particularly those
services using the HTTP or HTTPS protocols. Other services are
generally not supported. In particular, there may be no access to
POP3 or IMAP4 email, encrypted tunnels or other VPN mechanisms.
The addresses used may be private and/or not globally reachable.
They are generally dynamic (see the discussion of dynamic
addresses in Section 3 for further discussion of this terminology
and its implications) and relatively short-lived (hours or days
rather than months or years). These addresses are often announced
as "dynamic" to those who keep lists of dial-up or dynamic
addresses. The provider may impose a filtering Web proxy on the
connections; that proxy may change and redirect URLs to other
sites than the one originally specified by the user or embedded
o Client connectivity only, without a public address.
This service provides access to the Internet without support for
servers or most peer-to-peer functions. The IP address assigned
to the customer is dynamic and is characteristically assigned from
non-public address space. Servers and peer-to-peer functions are
generally not supported by the network address translation (NAT)
systems that are required by the use of private addresses. (The
more precise categorization of types of NATs given in  are
somewhat orthogonal to this document, but they may be provided as
additional terms, as described in Section 4.)
Filtering Web proxies are common with this type of service, and
the provider SHOULD indicate whether or not one is present.
o Client only, public address.
This service provides access to the Internet without support for
servers or most peer-to-peer functions. The IP address assigned
to the customer is in the public address space. It is usually
nominally dynamic or otherwise subject to change, but it may not
change for months at a time. Most VPN and similar connections
will work with this service. The provider may prohibit the use of
server functions by either legal (contractual) restrictions or by
filtering incoming connection attempts.
Filtering Web proxies are uncommon with this type of service, and
the provider SHOULD indicate if one is present.
o Firewalled Internet Connectivity.
This service provides access to the Internet and supports most
servers and most peer-to-peer functions, with one or (usually)
more static public addresses. It is similar to "Full Internet
Connectivity", below, and all of the qualifications and
restrictions described there apply. However, this service places
a provider-managed "firewall" between the customer and the public
Internet, typically at customer request and at extra cost compared
to non-firewalled services. Typically by contractual arrangements
with the customer, this may result in blocking of some services.
Other services may be intercepted by proxies, content-filtering
arrangements, or application gateways. The provider SHOULD
specify which services are blocked and which are intercepted or
altered in other ways.
In most areas, this service arrangement is offered as an add-on,
extra-cost, option with what would otherwise be Full Internet
Connectivity. It is distinguished from the models above by the
fact that any filtering or blocking services are ultimately
performed at customer request, rather than being imposed as
o Full Internet Connectivity.
This service provides the user full Internet connectivity, with
one or more static public addresses. Dynamic addresses that are
long-lived enough to make operating servers practical without
highly dynamic DNS entries are possible, provided that they are
not characterized as "dynamic" to third parties.
Filtering Web proxies, interception proxies, NAT, and other
provider-imposed restrictions on inbound or outbound ports and
traffic are incompatible with this type of service. Servers on a
connected customer LAN are typically considered normal. The only
compatible restrictions are bandwidth limitations and prohibitions
against network abuse or illegal activities.
3. Filtering or Security Issues and Terminology
As mentioned in the Introduction, the effort to control or limit
objectionable network traffic has led to additional restrictions on
the behavior and capabilities of internet services. Such
objectionable traffic may include unsolicited mail of various types
(including "spam"), worms, viruses, and their impact, and in some
cases, specific content.
In general, significant restrictions are most likely to be
encountered with Web connectivity and non-public-address services,
but some current recommendations would apply restrictions at all
levels. Some of these mail restrictions may prevent sending outgoing
mail (except through servers operated by the ISP for that purpose),
may prevent use of return addresses of the user's choice, and may
even prevent access to mail repositories (other than those supplied
by the provider) by remote-access protocols such as POP3 or IMAP4.
Because users may have legitimate reasons to access remote file
services, remote mail submission servers (or, at least, to use their
preferred email addresses from multiple locations), and to access
remote mail repositories (again, a near-requirement if a single
address is to be used), it is important that providers disclose the
services they are making available and the filters and conditions
they are imposing.
Several key issues in email filtering are of particular importance.
o Dynamic Addresses.
A number of systems, including several "blacklist" systems, are
based on the assumption that most undesired email originates from
systems with dynamic addresses, especially dialup and home
broadband systems. Consequently, they attempt to prevent the
addresses from being used to send mail, or perform some other
services, except through provider systems designated for that
Different techniques are used to identify systems with dynamic
addresses, including provider advertising of such addresses to
blacklist operators, heuristics that utilize certain address
ranges, and inspection of reverse-mapping domain names to see if
they contain telltale strings such as "dsl" or "dial". In some
cases, the absence of a reverse-mapping DNS address is taken as an
indication that the address is "dynamic". (Prohibition on
connections based on the absence of a reverse-mapping DNS record
was a technique developed for FTP servers many years ago; it was
found to have fairly high rates of failure, both prohibiting
legitimate connection attempts and failing to prevent illegitimate
ones). Service providers SHOULD describe what they are doing in
this area for both incoming and outgoing message traffic, and
users should be aware that, if an address is advertised as
"dynamic", it may be impossible to use it to send mail to an
arbitrary system even if Full Internet Connectivity is otherwise
o Non-public addresses and NATs.
The NAT systems that are used to map between private and public
address spaces may support connections to distant mail systems for
outbound and inbound mail, but terms of service often prohibit the
use of systems not supplied by the connectivity provider and
prohibit the operation of "servers" (typically not precisely
defined) on the client connection.
o Outbound port filtering from the provider.
Another common technique involves blocking connections to servers
outside the provider's control by blocking TCP "ports" that are
commonly used for messaging functions. Different providers have
different theories about this. Some prohibit their customers from
accessing external SMTP servers for message submission, but they
permit the use of the mail submission protocol () with sender
authentication. Others try to block all outgoing messaging-
related protocols, including remote mail retrieval protocols;
however, this is less common with public-address services than
those that are dependent on private addresses and NATs. If this
type of filtering is present, especially with "Client only, public
address" and "Full Internet Connectivity" services, the provider
MUST indicate that fact (see also Section 4).
Still others may divert (reroute) outbound email traffic to their
own servers, on the theory that this eliminates the need for
reconfiguring portable machines as they connect from a different
network location. Again, such diversion MUST be disclosed,
especially since it can have significant security and privacy
More generally, filters that block some or all mail being sent to
(or submitted to) remote systems (other than via provider-
supported servers), or that attempt to divert that traffic to
their own servers, are, as discussed above, becoming common and
SHOULD be disclosed.
4. Additional Terminology
These additional terms, while not as basic to understanding a service
offering as the ones identified above, are listed as additional
information that a service provider might choose to provide to
complement those general definitions. A potential customer might use
those that are relevant to construct a list of specific questions to
ask, for example.
o Version support.
Does the service include IPv4 support only, both IPv4 and IPv6
support, or IPv6 support only?
o Authentication support.
Which technical mechanism(s) are used by the service to establish
and possibly authenticate connections? Examples might include
unauthenticated DHCP, PPP, RADIUS, or HTTP interception.
o VPNs and Tunnels.
Is IPSec blocked or permitted? Are other tunneling techniques at
the IP layer or below, such as L2TP, permitted? Is there any
attempt to block applications-layer tunnel mechanisms such as SSH?
o Multicast support
Does the user machine have access to multicast packets and
o DNS support.
Are users required to utilize DNS servers provided by the service
provider, or are DNS queries permitted to reach arbitrary servers?
o IP-related services.
Are ICMP messages to and from end user sites generally blocked or
permitted? Are specific functions such as ping and traceroute
blocked and, if so, at what point in the network?
o Roaming support.
Does the service intentionally include support for IP roaming and,
if so, how is this defined? For "broadband" connections, is some
dialup arrangement provided for either backup or customer travel?
If present, does that arrangement have full access to mailboxes,
o Applications services provided.
Are email services and/or Web hosting provided as part of the
service, and on what basis? An email services listing should
identify whether POP3, IMAP4, or Web access are provided and in
what combinations, and what types of authentication and privacy
services are supported or required for each.
o Use and Blocking of Outbound Applications Services.
Does the service block use of SMTP or mail submission to other
than its own servers or intercept such submissions and route them
to its servers? Do its servers restrict the user to use of its
domain names on outbound email? (For email specifically, also see
Section 3 above.) Is the FTP PASV command supported or blocked?
Are blocks or intercepts imposed on other file sharing or file
transfer mechanisms, on conferencing applications, or on private
More generally, the provider should identify any actions of the
service to block, restrict, or alter the destination of, the
outbound use (i.e., the use of services not supplied by the
provider or on the provider's network) of applications services.
o Blocking of Inbound Applications Services.
In addition to issues raised by dynamic or private address space
(when present), does the service take any other measures that
specifically restrict the connections that can be made to
equipment operated by the customer? Specifically, are inbound
SMTP, HTTP or HTTPS, FTP, or various peer-to-peer or other
connections (possibly including applications not specifically
recognized by the provider) prohibited and, if so, which ones?
o Application Content Filtering.
The service should declare whether it provides filtering or
protection against worms or denial of service attacks against its
customers, virus and spam filtering for its mail services (if
any), non-discretionary or "parental control" filtering of
content, and so on.
o Wiretapping and interception.
The service SHOULD indicate whether traffic passing through it is
subject to lawful intercept, and whether the provider will make a
proactive attempt to inform the user of such an intercept when
such notice is legal. Analogous questions can be asked for
traffic data that is stored for possible use by law enforcement.
5. Security Considerations
This document is about terminology, not protocols, so it does not
raise any particular security issues. However, if the type of
terminology that is proposed is widely adopted, it may become easier
to identify security-related expectations of particular hosts, LANs,
and types of connections.
This document was inspired by an email conversation with Vernon
Schryver, Paul Vixie, and Nathaniel Bornstein. While there have been
proposals to produce such definitions for many years, that
conversation convinced the author that it was finally time to put a
strawman on the table to see if the IETF could actually carry it
forward. Harald Alvestrand, Brian Carpenter, George Michaelson,
Vernon Schryver, and others made several suggestions on the initial
draft that resulted in clarifications to the second one and Stephane
Bortzmeyer, Brian Carpenter, Tony Finch, Susan Harris, David Kessens,
Pekka Savola, and Vernon Schryver made very useful suggestions that
were incorporated into subsequent versions. Susan Harris also gave
the penultimate version an exceptionally careful reading, which is
greatly appreciated, as are editorial suggestions by the RFC Editor.
7. Informative References
 Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
 Srisuresh, P. and M. Holdrege, "IP Network Address Translator
(NAT) Terminology and Considerations", RFC 2663, August 1999.
 Gellens, R. and J. Klensin, "Message Submission", RFC 2476,
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