Internet DRAFT - draft-ietf-tsvwg-port-use
draft-ietf-tsvwg-port-use
TSVWG J. Touch
Internet Draft USC/ISI
Intended status: Best Current Practice April 24, 2015
Expires: October 2015
Recommendations on Using Assigned Transport Port Numbers
draft-ietf-tsvwg-port-use-11.txt
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Abstract
This document provides recommendations to application and service
protocol designers on how to use the assigned transport protocol
port number space and when to request a port assignment from IANA.
It provides designer guidelines on how to interact with the IANA
processes defined in RFC6335, thus serving to complement (but not
update) that document.
Table of Contents
1. Introduction...................................................2
2. Conventions used in this document..............................3
3. History........................................................3
4. Current Port Number Use........................................5
5. What is a Port Number?.........................................5
6. Conservation...................................................7
6.1. Guiding Principles........................................7
6.2. Firewall and NAT Considerations...........................8
7. Considerations for Requesting Port Number Assignments..........9
7.1. Is a port number assignment necessary?....................9
7.2. How Many Assigned Port Numbers?..........................11
7.3. Picking an Assigned Port Number..........................12
7.4. Support for Security.....................................13
7.5. Support for Future Versions..............................14
7.6. Transport Protocols......................................15
7.7. When to Request an Assignment............................16
7.8. Squatting................................................17
7.9. Other Considerations.....................................18
8. Security Considerations.......................................18
9. IANA Considerations...........................................19
10. References...................................................19
10.1. Normative References....................................19
10.2. Informative References..................................20
11. Acknowledgments..............................................22
1. Introduction
This document provides information and advice to application and
service designers on the use of assigned transport port numbers. It
provides a detailed historical background of the evolution of
transport port numbers and their multiple meanings. It also provides
specific recommendations to designers on how to use assigned port
numbers. Note that this document provides information to potential
port number applicants that complements the IANA process described
in BCP165 [RFC6335], but it does not change any of the port number
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assignment procedures described therein. This document is intended
to address concerns typically raised during Expert Review of
assigned port number applications, but it is not intended to bind
those reviews. RFC 6335 also describes the interaction between port
experts and port requests in IETF consensus document. Authors of
IETF consensus documents should nevertheless follow the advice in
this document and can expect comment on their port requests from the
port experts during IETF last call or at other times when review is
explicitly sought.
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 RFC-2119 [RFC2119].
In this document, these words will appear with that interpretation
only when in ALL CAPS. Lower case uses of these words are not to be
interpreted as carrying RFC-2119 significance.
In this document, the characters ">>" preceding an indented line(s)
indicates a statement using the key words listed above. This
convention aids reviewers in quickly identifying or finding
requirements for registration and recommendations for use of port
numbers in this RFC.
3. History
The term 'port' was first used in [RFC33] to indicate a simplex
communication path from an individual process and originally applied
to only the Network Control Program (NCP) connection-oriented
protocol. At a meeting described in [RFC37], an idea was presented
to decouple connections between processes and links that they use as
paths, and thus to include numeric source and destination socket
identifiers in packets. [RFC38] provides further detail, describing
how processes might have more than one of these paths and that more
than one path may be active at a time. As a result, there was the
need to add a process identifier to the header of each message so
that incoming messages could be demultiplexed to the appropriate
process. [RFC38] further suggested that 32 bit numbers would be used
for these identifiers. [RFC48] discusses the current notion of
listening on a specific port number, but does not discuss the issue
of port number determination. [RFC61] notes that the challenge of
knowing the appropriate port numbers is "left to the processes" in
general, but introduces the concept of a "well-known" port number
for common services.
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[RFC76] proposed a "telephone book" by which an index would allow
port numbers to be used by name, but still assumed that both source
and destination port numbers are fixed by such a system. [RFC333]
proposed that a port number pair, rather than an individual port
number, would be used on both sides of the connection for
demultiplexing messages. This is the final view in [RFC793] (and its
predecessors, including [IEN112]), and brings us to their current
meaning. [RFC739] introduced the notion of generic reserved port
numbers for groups of protocols, such as "any private RJE server"
[RFC739]. Although the overall range of such port numbers was (and
remains) 16 bits, only the first 256 (high 8 bits cleared) in the
range were considered assigned.
[RFC758] is the first to describe port numbers as being used for TCP
(previous RFCs all refer to only NCP). It includes a list of such
well-known port numbers, as well as describing ranges used for
different purposes:
Decimal Octal
-----------------------------------------------------------
0-63 0-77 Network Wide Standard Function
64-127 100-177 Hosts Specific Functions
128-223 200-337 Reserved for Future Use
224-255 340-377 Any Experimental Function
In [RFC820] those range meanings disappeared, and a single list of
number assignments is presented. This is also the first time that
port numbers are described as applying to a connectionless transport
(UDP) rather than only connection-oriented transports.
By [RFC900] the ranges appeared as decimal numbers rather than the
octal ranges used previously. [RFC1340] increased this range from
0..255 to 0..1023, and began to list TCP and UDP port number
assignments individually (although the assumption was that once
assigned a port number applies to all transport protocols, including
TCP, UDP, recently SCTP and DCCP, as well as ISO-TP4 for a brief
period in the early 1990s). [RFC1340] also established the
Registered range of 1024-59151, though it notes that it is not
controlled by the IANA at that point. The list provided by [RFC1700]
in 1994 remained the standard until it was declared replaced by an
on-line version, as of [RFC3232] in 2002.
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4. Current Port Number Use
RFC6335 indicates three ranges of port number assignments:
Binary Hex
-----------------------------------------------------------
0-1023 0x0000-0x03FF System (also Well-Known)
1024-49151 0x0400-0xBFFF User (also Registered)
49152-65535 0xC000-0xFFFF Dynamic (also Private)
System (also Well-Known) encompasses the range 0..1023. On some
systems, use of these port numbers requires privileged access, e.g.,
that the process run as 'root' (i.e., as a privileged user), which
is why these are referred to as System port numbers. The port
numbers from 1024..49151 denotes non-privileged services, known as
User (also Registered), because these port numbers do not run with
special privileges. Dynamic (also Private) port numbers are not
assigned.
Both System and User port numbers are assigned through IANA, so both
are sometimes called 'registered port numbers'. As a result, the
term 'registered' is ambiguous, referring either to the entire range
0-49151 or to the User port numbers. Complicating matters further,
System port numbers do not always require special (i.e., 'root')
privilege. For clarity, the remainder of this document refers to the
port number ranges as System, User, and Dynamic, to be consistent
with IANA process [RFC6335].
5. What is a Port Number?
A port number is a 16-bit number used for two distinct purposes:
o Demultiplexing transport endpoint associations within an end
host
o Identifying a service
The first purpose requires that each transport endpoint association
(e.g., TCP connection or UDP pairwise association) using a given
transport between a given pair of IP addresses use a different pair
of port numbers, but does not require either coordination or
registration of port number use. It is the second purpose that
drives the need for a common registry.
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Consider a user wanting to run a web server. That service could run
on any port number, provided that all clients knew what port number
to use to access that service at that host. Such information can be
explicitly distributed - for example, by putting it in the URI:
http://www.example.com:51509/
Ultimately, the correlation of a service with a port number is an
agreement between just the two endpoints of the association. A web
server can run on port number 53, which might appear as DNS traffic
to others but will connect to browsers that know to use port number
53 rather than 80.
As a concept, a service is the combination of ISO Layers 5-7 that
represents an application protocol capability. For example www (port
number 80) is a service that uses HTTP as an application protocol
and provides access to a web server [RFC7230]. However, it is
possible to use HTTP for other purposes, such as command and
control. This is why some current services (HTTP, e.g.) are a bit
overloaded - they describe not only the application protocol, but a
particular service.
IANA assigns port numbers so that Internet endpoints do not need
pairwise, explicit coordination of the meaning of their port
numbers. This is the primary reason for requesting port number
assignment by IANA - to have a common agreement between all
endpoints on the Internet as to the default meaning of a port
number, which provides the endpoints with a default port number for
a particular protocol or service.
Port numbers are sometimes used by intermediate devices on a network
path, either to monitor available services, to monitor traffic
(e.g., to indicate the data contents), or to intercept traffic (to
block, proxy, relay, aggregate, or otherwise process it). In each
case, the intermediate device interprets traffic based on the port
number. It is important to recognize that any interpretation of port
numbers - except at the endpoints - may be incorrect, because port
numbers are meaningful only at the endpoints. Further, port numbers
may not be visible to these intermediate devices, such as when the
transport protocol is encrypted (as in network- or link-layer
tunnels), or when a packet is fragmented (in which case only the
first fragment has the port number information). Such port number
invisibility may interfere with these in-network port number-based
capabilities.
Port numbers can also be used for other purposes. Assigned port
numbers can simplify end system configuration, so that individual
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installations do not need to coordinate their use of arbitrary port
numbers. Such assignments may also have the effect of simplifying
firewall management, so that a single, fixed firewall configuration
can either permit or deny a service that uses the assigned ports.
It is useful to differentiate a port number from a service name. The
former is a numeric value that is used directly in transport
protocol headers as a demultiplexing and service identifier. The
latter is primarily a user convenience, where the default map
between the two is considered static and resolved using a cached
index. This document focuses on the former because it is the
fundamental network resource. Dynamic maps between the two, i.e.,
using DNS SRV records, are discussed further in Section 7.1.
6. Conservation
Assigned port numbers are a limited resource that is globally shared
by the entire Internet community. As of 2014, approximately 5850 TCP
and 5570 UDP port numbers have been assigned out of a total range of
49151. As a result of past conservation, current assigned port use
is small and the current rate of assignment avoids the need for
transition to larger number spaces. This conservation also helps
avoid the need for IANA to rely on assigned port number reclamation,
which is practically impossible even though procedurally permitted
[RFC6335].
IANA aims to assign only one port number per service, including
variants [RFC6335], but there are other benefits to using fewer port
numbers for a given service. Use of multiple assigned port numbers
can make applications more fragile, especially when firewalls block
a subset of those port numbers or use ports numbers to route or
prioritize traffic differently. As a result:
>> Each assigned port requested MUST be justified by the applicant
as an independently useful service.
6.1. Guiding Principles
This document provides recommendations for users that also help
conserve assigned port number space. Again, this document does not
update BCP165 [RFC6335], which describes the IANA procedures for
managing assigned transport port numbers and services. Assigned port
number conservation is based on a number of basic principles:
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o A single assigned port number can support different functions
over separate endpoint associations, determined using in-band
information. An FTP data connection can transfer binary or
text files, the latter translating line-terminators, as
indicated in-band over the control port number [RFC959].
o A single assigned port number can indicate the Dynamic port
number(s) on which different capabilities are supported, as
with passive-mode FTP [RFC959].
o Several existing services can indicate the Dynamic port
number(s) on which other services are supported, such as with
mDNS and portmapper [RFC1833] [RFC6762] [RFC6763].
o Copies of some existing services can be differentiated using
in-band information (e.g., URIs in HTTP Host field and TLS
Server Name Indication extension) [RFC7230] [RFC6066].
o Services requiring varying performance properties can already
be supported using separate endpoint associations (connections
or other associations), each configured to support the desired
properties. E.g., a high-speed and low-speed variant can be
determined within the service using the same assigned port.
Assigned port numbers are intended to differentiate services, not
variations of performance, replicas, pairwise endpoint associations,
or payload types. Assigned port numbers are also a small space
compared to other Internet number spaces; it is never appropriate to
consume assigned port numbers to conserve larger spaces such as IP
addresses, especially where copies of a service represent different
endpoints.
6.2. Firewall and NAT Considerations
Ultimately, port numbers numbers indicate services only to the
endpoints, and any intermediate device that assigns meaning to a
value can be incorrect. End systems might agree to run web services
(HTTP) over port number 53 (typically used for DNS) rather than port
number 80, at which point a firewall that blocks port number 80 but
permits port number 53 would not have the desired effect.
Nonetheless, assigned port numbers are often used to help configure
firewalls and other port-based systems for access control.
Using Dynamic port numbers, or explicitly-indicated port numbers
indicated in-band over another service (such as with FTP) often
complicates firewall and NAT interactions [RFC959]. FTP over
firewalls often requires direct support for deep-packet inspection
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(to snoop for the Dynamic port number for the NAT to correctly map)
or passive-mode FTP (in which both connections are opened from the
client side).
7. Considerations for Requesting Port Number Assignments
Port numbers are assigned by IANA by a set of documented procedures
[RFC6335]. The following section describes the steps users can take
to help assist with responsible use of assigned port numbers, and
with preparing an application for a port number assignment.
7.1. Is a port number assignment necessary?
First, it is useful to consider whether a port number assignment is
required. In many cases, a new number assignment may not be needed,
for example:
o Is this really a new service, or can an existing service
suffice?
o Is this an experimental service [RFC3692]? If so, consider
using the current experimental ports [RFC2780].
o Is this service independently useful? Some systems are
composed from collections of different service capabilities,
but not all component functions are useful as independent
services. Port numbers are typically shared among the smallest
independently-useful set of functions. Different service uses
or properties can be supported in separate pairwise endpoint
associations after an initial negotiation, e.g., to support
software decomposition.
o Can this service use a Dynamic port number that is coordinated
out-of-band, e.g.:
o By explicit configuration of both endpoints.
o By internal mechanisms within the same host (e.g., a
configuration file, indicated within a URI, or using
interprocess communication).
o Using information exchanged on a related service: FTP, SIP,
etc. [RFC959] [RFC3261].
o Using an existing port discovery service: portmapper, mDNS,
etc. [RFC1833] [RFC6762] [RFC6763].
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There are a few good examples of reasons that more directly suggest
that not only is a port number assignment not necessary, but it is
directly counter-indicated:
o Assigned port numbers are not intended to differentiate
performance variations within the same service, e.g., high-
speed vs. ordinary speed. Performance variations can be
supported within a single assigned port number in context of
separate pairwise endpoint associations.
o Additional assigned port numbers are not intended to replicate
an existing service. For example, if a device is configured to
use a typical web browser then it the port number used for
that service is a copy of the http service that is already
assigned to port number 80 and does not warrant a new
assignment. However, an automated system that happens to use
HTTP framing - but is not primarily accessed by a browser -
might be a new service. A good way to tell is "can an
unmodified client of the existing service interact with the
proposed service"? If so, that service would be a copy of an
existing service and would not merit a new assignment.
o Assigned port numbers not intended for intra-machine
communication. Such communication can already be supported by
internal mechanisms (interprocess communication, shared
memory, shared files, etc.). When Internet communication
within a host is desired, the server can bind to a Dynamic
port that is indicated to the client using these internal
mechanisms.
o Separate assigned port numbers are not intended for insecure
versions of existing (or new) secure services. A service that
already requires security would be made more vulnerable by
having the same capability accessible without security.
Note that the converse is different, i.e., it can be useful to
create a new, secure service that replicates an existing
insecure service on a new port number assignment. This can be
necessary when the existing service is not backward-compatible
with security enhancements, such as the use of TLS [RFC5246]
or DTLS [RFC6347].
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o Assigned port numbers are not intended for indicating
different service versions. Version differentiation should be
handled in-band, e.g., using a version number at the beginning
of an association (e.g., connection or other transaction).
This may not be possible with legacy assignments, but all new
services should incorporate support for version indication.
Some services may not need assigned port numbers at all, e.g., SIP
allows voice calls to use Dynamic ports [RFC3261]. Some systems can
register services in the DNS, using SRV entries. These services can
be discovered by a variety of means, including mDNS, or via direct
query [RFC6762] [RFC6763]. In such cases, users can more easily
request a SRV name, which are assigned first-come, first-served from
a much larger namespace.
IANA assigns port numbers, but this assignment is typically used
only for servers, i.e., the host that listens for incoming
connections or other associations. Clients, i.e., hosts that
initiate connections or other associations, typically refer to those
assigned port numbers but do not need port number assignments for
their endpoint.
Finally, an assigned port number is not a guarantee of exclusive
use. Traffic for any service might appear on any port number, due to
misconfiguration or deliberate misuse. Application and service
designers are encouraged to validate traffic based on its content.
7.2. How Many Assigned Port Numbers?
As noted earlier, systems might require a single port number
assignment, but rarely require multiple port numbers. There are a
variety of known ways to reduce assigned port number consumption.
Although some may be cumbersome or inefficient, they are nearly
always preferable to consuming additional port number assignments.
Such techniques include:
o Use of a discovery service, either a shared service (mDNS), or
a discovery service for a given system [RFC6762] [RFC6763].
o Multiplex packet types using in-band information, either on a
per-message or per-connection basis. Such demultiplexing can
even hand-off different messages and connections among
different processes, such as is done with FTP [RFC959].
There are some cases where NAT and firewall traversal are
significantly improved by having an assigned port number. Although
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NAT traversal protocols supporting automatic configuration have been
proposed and developed (e.g., STUN [RFC5389], TURN [RFC5766], and
ICE [RFC5245]), not all application and service designers can rely
on their presence as of yet.
In the past, some services were assigned multiple port numbers or
sometimes fairly large port ranges (e.g., X11). This occurred for a
variety of reasons: port number conservation was not as widely
appreciated, assignments were not as ardently reviewed, etc. This no
longer reflects current practice and such assignments are not
considered to constitute a precedent for future assignments.
7.3. Picking an Assigned Port Number
Given a demonstrated need for a port number assignment, the next
question is how to pick the desired port number. An application for
a port number assignment does not need to include a desired port
number; in that case, IANA will select from those currently
available.
Users should consider whether the requested port number is
important. For example, would an assignment be acceptable if IANA
picked the port number value? Would a TCP (or other transport
protocol) port number assignment be useful by itself? If so, a port
number can be assigned to a service for one transport protocol where
it is already (or can be subsequently) assigned to a different
service for other transport protocols.
The most critical issue in picking a number is selecting the desired
range, i.e., System vs. User port numbers. The distinction was
intended to indicate a difference in privilege; originally, System
port numbers required privileged ('root') access, while User port
numbers did not. That distinction has since blurred because some
current systems do not limit access control to System port numbers
and because some System services have been replicated on User
numbers (e.g., IRC). Even so, System port number assignments have
continued at an average rate of 3-4 per year over the past 7 years
(2007-2013), indicating that the desire to keep this distinction
continues.
As a result, the difference between System and User port numbers
needs to be treated with caution. Developers are advised to treat
services as if they are always run without privilege.
Even when developers seek a System port number assignment, it may be
very difficult to obtain. System port number assignment requires
IETF Review or IESG Approval and justification that both User and
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Dynamic port number ranges are insufficient [RFC6335]. Thus this
document recommends both:
>> Developers SHOULD NOT apply for System port number assignments
because the increased privilege they are intended to provide is not
always enforced.
>> System implementers SHOULD enforce the need for privilege for
processes to listen on System port numbers.
At some future date, it might be useful to deprecate the distinction
between System and User port numbers altogether. Services typically
require elevated ('root') privileges to bind to a System port
number, but many such services go to great lengths to immediately
drop those privileges just after connection or other association
establishment to reduce the impact of an attack using their
capabilities. Such services might be more securely operated on User
port numbers than on System port numbers. Further, if System port
numbers were no longer assigned, as of 2014 it would cost only 180
of the 1024 System values (17%), or 180 of the overall 49152
assigned (System and User) values (<0.04%).
7.4. Support for Security
Just as a service is a way to obtain information or processing from
a host over a network, a service can also be the opening through
which to compromise that host. Protecting a service involves
security, which includes integrity protection, source
authentication, privacy, or any combination of these capabilities.
Security can be provided in a number of ways, and thus:
>> New services SHOULD support security capabilities, either
directly or via a content protection such as TLS [RFC5246] or DTLS
[RFC6347] or transport protection such as TCP-AO [RFC5925]. Insecure
versions of new or existing secure services SHOULD be avoided
because of the new vulnerability they create.
Secure versions of legacy services that are not already security-
capable via in-band negotiations can be very useful. However, there
is no IETF consensus on when separate ports should be used for
secure and insecure variants of the same service [RFC2595] [RFC2817]
[RFC6335]. The overall preference is for use of a single port, as
noted in Section 6 of this document and Section 7.2 of [RFC6335],
but the appropriate approach depends on the specific characteristics
of the service. As a result:
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>> When requesting both secure and insecure port assignments for the
same service, justification is expected for the utility and safety
of each port as an independent service (Section 6). Precedent (e.g.,
citing other protocols that use a separate insecure port) is
inadequate justification by itself.
It's also important to recognize that port number assignment is not
itself a guarantee that traffic using that number provides the
corresponding service, or that a given service is always offered
only on its assigned port number. Port numbers are ultimately
meaningful only between endpoints and any service can be run on any
port. Thus:
>> Security SHOULD NOT rely on assigned port number distinctions
alone; every service, whether secure or not, is likely to be
attacked.
Applications for a new service that requires both a secure and
insecure port may be found, on expert review, to be unacceptable,
and may not be approved for allocation. Similarly, an application
for a new port to support an insecure variant of an existing secure
protocol may be found unacceptable. In both cases, the resulting
security of the service in practice will be a significant
consideration in the decision as to whether to assign an insecure
port.
7.5. Support for Future Versions
Requests for assigned port numbers are expected to support multiple
versions on the same assigned port number [RFC6335]. Versions are
typically indicated in-band, either at the beginning of a connection
or other association, or in each protocol message.
>> Version support SHOULD be included in new services rather than
relying on different port number assignments for different versions.
>> Version numbers SHOULD NOT be included in either the service name
or service description, to avoid the need to make additional port
number assignments for future variants of a service.
Again, the assigned port number space is far too limited to be used
as an indicator of protocol version or message type. Although this
has happened in the past (e.g., for NFS), it should be avoided in
new requests.
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7.6. Transport Protocols
IANA assigns port numbers specific to one or more transport
protocols, typically UDP [RFC768] and TCP [RFC793], but also SCTP
[RFC4960], DCCP [RFC4340], and any other standard transport
protocol. Originally, IANA port number assignments were concurrent
for both UDP and TCP, and other transports were not indicated.
However, to conserve the assigned port number space and to reflect
increasing use of other transports, assignments are now specific
only to the transport being used.
In general, a service should request assignments for multiple
transports using the same service name and description on the same
port number only when they all reflect essentially the same service.
Good examples of such use are DNS and NFS, where the difference
between the UDP and TCP services are specific to supporting each
transport. E.g., the UDP variant of a service might add sequence
numbers and the TCP variant of the same service might add in-band
message delimiters. This document does not describe the appropriate
selection of a transport protocol for a service.
>> Service names and descriptions for multiple transport port number
assignments SHOULD match only when they describe the same service,
excepting only enhancements for each supported transport.
When the services differ, it may be acceptable or preferable to use
the same port number, but the service names and descriptions should
be different for each transport/service pair, reflecting the
differences in the services. E.g., if TCP is used for the basic
control protocol and UDP for an alarm protocol, then the services
might be "name-ctl" and "name-alarm". A common example is when TCP
is used for a service and UDP is used to determine whether that
service is active (e.g., via a unicast, broadcast, or multicast test
message) [RFC1122]. IANA has, for several years, used the suffix "-
disc" in service names to distinguish discovery services, such as
are used to identify endpoints capable of a given service:
>> Names of discovery services SHOULD use an identifiable suffix;
the suggestion is "-disc".
Some services are used for discovery, either in conjunction with a
TCP service or as a stand-alone capability. Such services will be
more reliable when using multicast rather than broadcast (over IPv4)
because IP routers do not forward "all nodes" broadcasts (all 1's,
i.e., 255.255.255.255 for IPv4) and have not been required to
support subnet-directed broadcasts since 1999 [RFC1812] [RFC2644].
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This issue is relevant only for IPv4 because IPv6 does not support
broadcast.
>> UDP over IPv4 multi-host services SHOULD use multicast rather
than broadcast.
Designers should be very careful in creating services over
transports that do not support congestion control or error recovery,
notably UDP. There are several issues that should be considered in
such cases, as summarized in Table 1 in [RFC5405]. In addition, the
following recommendations apply to service design:
>> Services that use multipoint communication SHOULD be scalable,
and SHOULD NOT rely solely on the efficiency of multicast
transmission for scalability.
>> Services SHOULD NOT use UDP as a performance enhancement over
TCP, e.g., to circumnavigate TCP's congestion control.
7.7. When to Request an Assignment
Assignments are typically requested when a user has enough
information to reasonably answer the questions in the IANA
application. IANA applications typically take up to a few weeks to
process, with some complex cases taking up to a month. The process
typically involves a few exchanges between the IANA Ports Expert
Review team and the applicant.
An application needs to include a description of the service, as
well as to address key questions designed to help IANA determine
whether the assignment is justified. The application should be
complete and not refer solely to the Internet Draft, RFC, a website,
or any other external documentation.
Services that are independently developed can be requested at any
time, but are typically best requested in the last stages of design
and initial experimentation, before any deployment has occurred that
cannot easily be updated.
>> Users MUST NOT deploy implementations that use assigned port
numbers prior their assignment by IANA.
>> Users MUST NOT deploy implementations that default to using the
experimental System port numbers (1021 and 1022 [RFC4727]) outside a
controlled environment where they can be updated with a subsequent
assigned port [RFC3692].
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Deployments that use unassigned port numbers before assignment
complicate IANA management of the port number space. Keep in mind
that this recommendation protects existing assignees, users of
current services, and applicants for new assignments; it helps
ensure that a desired number and service name are available when
assigned. The list of currently unassigned numbers is just that -
*currently* unassigned. It does not reflect pending applications.
Waiting for an official IANA assignment reduces the chance that an
assignment request will conflict with another deployed service.
Applications made through Internet Draft / RFC publication (in any
stream) typically use a placeholder ("PORTNUM") in the text, and
implementations use an experimental port number until a final
assignment has been made [RFC6335]. That assignment is initially
indicated in the IANA Considerations section of the document, which
is tracked by the RFC Editor. When a document has been approved for
publication, that request is forwarded to IANA for handling. IANA
will make the new assignment accordingly. At that time, IANA may
also request that the applicant fill out the application form on
their website, e.g., when the RFC does not directly address the
information expected as per [RFC6335]. "Early" assignments can be
made when justified, e.g., for early interoperability testing,
according to existing process [RFC7120] [RFC6335].
>> Users writing specifications SHOULD use symbolic names for port
numbers and service names until an IANA assignment has been
completed. Implementations SHOULD use experimental port numbers
during this time, but those numbers MUST NOT be cited in
documentation except as interim.
7.8. Squatting
"Squatting" describes the use of a number from the assignable range
in deployed software without IANA assignment for that use,
regardless of whether the number has been assigned or remains
available for assignment. It is hazardous because IANA cannot track
such usage and thus cannot avoid making legitimate assignments that
conflict with such unauthorized usage.
Such "squatted" port numbers remain unassigned, and IANA retains the
right to assign them when requested by other applicants. Application
and service designers are reminded that is never appropriate to use
port numbers that have not been directly assigned [RFC6335]. In
particular, any unassigned code from the assigned ranges will be
assigned by IANA, and any conflict will be easily resolved as the
protocol designer's fault once that happens (because they would not
be the assignee). This may reflect in the public's judgment on the
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quality of their expertise and cooperation with the Internet
community.
Regardless, there are numerous services that have squatted on such
numbers that are in widespread use. Designers who are using such
port numbers are encouraged to apply for an assignment. Note that
even widespread de-facto use may not justify a later IANA assignment
of that value, especially if either the value has already been
assigned to a legitimate applicant or if the service would not
qualify for an assignment of its own accord.
7.9. Other Considerations
As noted earlier, System port numbers should be used sparingly, and
it is better to avoid them altogether. This avoids the potentially
incorrect assumption that the service on such port numbers run in a
privileged mode.
Assigned port numbers are not intended to be changed; this includes
the corresponding service name. Once deployed, it can be very
difficult to recall every implementation, so the assignment should
be retained. However, in cases where the current assignee of a name
or number has reasonable knowledge of the impact on such uses, and
is willing to accept that impact, the name or number of an
assignment can be changed [RFC6335]
Aliases, or multiple service names for the same assigned port
number, are no longer considered appropriate [RFC6335].
8. Security Considerations
This document focuses on the issues arising when designing services
that require new port assignments. Section 7.4 addresses the
security and security-related issues of that interaction.
When designing a secure service, the use of TLS [RFC5246], DTLS
[RFC6347], or TCP-AO [RFC5925] mechanisms that protect transport
protocols or their contents is encouraged. It may not be possible to
use IPsec [RFC4301] in similar ways because of the different
relationship between IPsec and port numbers and because applications
may not be aware of IPsec protections.
This document reminds application and service designers that port
numbers do not protect against denial of service attack or guarantee
that traffic should be trusted. Using assigned numbers for port
filtering isn't a substitute for authentication, encryption, and
integrity protection. The port number alone should not be used to
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avoid denial of service attacks or to manage firewall traffic
because the use of port numbers is not regulated or validated.
The use of assigned port numbers is the antithesis of privacy
because they are intended to explicitly indicate the desired
application or service. Strictly, port numbers are meaningful only
at the endpoints, so any interpretation elsewhere in the network can
be arbitrarily incorrect. However, those numbers can also expose
information about available services on a given host. This
information can be used by intermediate devices to monitor and
intercept traffic as well as to potentially identify key endpoint
software properties ("fingerprinting"), which can be used to direct
other attacks.
9. IANA Considerations
The entirety of this document focuses on suggestions that help
ensure the conservation of port numbers and provide useful hints for
issuing informative requests thereof.
10. References
10.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2780] Bradner, S., and V. Paxson, "IANA Allocation Guidelines
For Values In the Internet Protocol and Related Headers",
BCP 37, RFC 2780, March 2000.
[RFC3692] Narten, T., "Assigning Experimental and Testing Numbers
Considered Useful", BCP 82, RFC 3962, Jan. 2004.
[RFC4727] Fenner, B., "Experimental Values in IPv4, IPv6, ICMPv4,
ICMPv6, UDP, and TCP Headers", RFC 4727, November 2006.
[RFC5246] Dierks, T., and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008.
[RFC5405] Eggert, L., and G. Fairhurst, "Unicast UDP Usage
Guidelines for Application Designers", BCP 145, RFC 5405,
Nov. 2008.
[RFC5925] Touch, J., Mankin, A., and R. Bonica, "The TCP
Authentication Option", RFC 5925, June 2010.
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[RFC6335] Cotton, M., L. Eggert, J. Touch, M. Westerlund, and S.
Cheshire, "Internet Assigned Numbers Authority (IANA)
Procedures for the Management of the Service Name and
Transport Protocol Port Number Registry", BCP 165, RFC
6335, August 2011.
[RFC6347] Rescorla, E., and N. Modadugu, "Datagram Transport Layer
Security Version 1.2", RFC 6347, January 2012.
10.2. Informative References
[IEN112] Postel, J., "Transmission Control Protocol", IEN 112,
August 1979.
[RFC33] Crocker, S., "New Host-Host Protocol", RFC 33 February
1970.
[RFC37] Crocker, S., "Network Meeting Epilogue", RFC 37, March
1970.
[RFC38] Wolfe, S., "Comments on Network Protocol from NWG/RFC
#36", RFC 38, March 1970.
[RFC48] Postel, J., and S. Crocker, "Possible protocol plateau",
RFC 48, April 1970.
[RFC61] Walden, D., "Note on Interprocess Communication in a
Resource Sharing Computer Network", RFC 61, July 1970.
[RFC76] Bouknight, J., J. Madden, and G. Grossman, "Connection by
name: User oriented protocol", RFC 76, October 1970.
[RFC333] Bressler, R., D. Murphy, and D. Walden. "Proposed
experiment with a Message Switching Protocol", RFC 333,
May 1972.
[RFC739] Postel, J., "Assigned numbers", RFC 739, November 1977.
[RFC758] Postel, J., "Assigned numbers", RFC 758, August 1979.
[RFC768] Postel, J., "User Datagram Protocol", RFC 768, August
1980.
[RFC793] Postel, J., "Transmission Control Protocol" RFC 793,
September 1981
[RFC820] Postel, J., "Assigned numbers", RFC 820, August 1982.
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[RFC900] Reynolds, J., and J. Postel, "Assigned numbers", RFC 900,
June 1984.
[RFC959] Postel, J., and J. Reynolds, "FILE TRANSFER PROTOCOL
(FTP)", RFC 959, October 1985.
[RFC1122] Braden, B. (Ed.), "Requirements for Internet Hosts --
Communication Layers", RFC 1122, October 1989.
[RFC1340] Reynolds, J., and J. Postel, "Assigned numbers", RFC 1340,
July 1992.
[RFC1700] Reynolds, J., and J. Postel, "Assigned numbers", RFC 1700,
October 1994.
[RFC1812] Baker, F. (Ed.), "Requirements for IP Version 4 Routers",
RFC 1812, June 1995.
[RFC1833] Srinivasan, R., "Binding Protocols for ONC RPC Version 2",
RFC 1833, August 1995.
[RFC2595] Newman, C., "Using TLS with IMAP, POP3 and ACAP", RFC
2595, June 1999.
[RFC2644] Senie, D., "Changing the Default for Directed Broadcasts
in Routers", RFC 2644, August 1999.
[RFC2817] Khare, R., and S. Lawrence, "Upgrading to TLS Within
HTTP/1.1", RFC 2817, May 2000.
[RFC3232] Reynolds, J. (Ed.), "Assigned Numbers: RFC 1700 is
Replaced by an On-line Database", RFC 3232, January 2002.
[RFC3261] Rosenberg, J., H. Schulzrinne, G. Camarillo, A. Johnston,
J. Peterson, R. Sparks, M. Handley, and E. Schooler, "SIP:
Session Initiation Protocol", RFC 3261, June 2002.
[RFC4301] Kent, S., and K. Seo, "Security Architecture for the
Internet Protocol", RFC 4301, December 2005.
[RFC4340] Kohler, E., M. Handley, and S. Floyd, "Datagram Congestion
Control Protocol (DCCP)", RFC 4340, March 2006.
[RFC4960] Stewart, R. (Ed.), "Stream Control Transmission Protocol",
RFC 4960, September 2007.
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[RFC5245] Rosenberg, J., "Interactive Connectivity Establishment
(ICE): A Protocol for Network Address Translator (NAT)
Traversal for Offer/Answer Protocols", RFC 5245, April
2010.
[RFC5389] Rosenberg, J., R. Mahy, P. Matthews, and D. Wing, "Session
Traversal Utilities for NAT", RFC 5389, October 2008.
[RFC5766] Mahy, R., P. Matthews, and J. Rosenberg, "Traversal Using
Relays around NAT (TURN): Relay Extensions to Session
Traversal Utilities for NAT (STUN)", RFC 5766, April 2010.
[RFC6066] Eastlake 3rd, D., "Transport Layer Security (TLS)
Extensions: Extension Definitions", RFC 6066, January
2011.
[RFC6762] Cheshire, S., and M. Krochmal, "Multicast DNS", RFC 6762,
February 2013.
[RFC6763] Cheshire, S., and M. Krochmal, "DNS-Based Service
Discovery", RFC 6763, February 2013.
[RFC7120] Cotton, M., "Early IANA Allocation of Standards Track Code
Points", BCP 100, RFC 7120, January 2014.
[RFC7230] Fielding, R., (Ed.), and J. Reshke, (Ed.), "Hypertext
Transfer Protocol (HTTP/1.1): Message Syntax and Routing",
RFC 7230, June 2014.
11. Acknowledgments
This work benefitted from the feedback from David Black, Lars
Eggert, Gorry Fairhurst, and Eliot Lear, as well as discussions of
the IETF TSVWG WG.
This document was prepared using 2-Word-v2.0.template.dot.
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Authors' Addresses
Joe Touch
USC/ISI
4676 Admiralty Way
Marina del Rey, CA 90292-6695
U.S.A.
Phone: +1 (310) 448-9151
EMail: touch@isi.edu
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