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Internet Engineering Task Force (IETF) J. Touch Request for Comments: 7605 USC/ISI BCP: 165 August 2015 Category: Best Current Practice ISSN: 2070-1721

      Recommendations on Using Assigned Transport Port Numbers

Abstract

 This document provides recommendations to designers of application
 and service protocols on how to use the transport protocol port
 number space and when to request a port assignment from IANA.  It
 provides designer guidance to requesters or users of port numbers on
 how to interact with IANA using the processes defined in RFC 6335;
 thus, this document complements (but does not update) that document.
 It provides guidelines for designers regarding how to interact with
 the IANA processes defined in RFC 6335, thus serving to complement
 (but not update) that document.

Status of This Memo

 This memo documents an Internet Best Current Practice.
 This document is a product of the Internet Engineering Task Force
 (IETF).  It represents the consensus of the IETF community.  It has
 received public review and has been approved for publication by the
 Internet Engineering Steering Group (IESG).  Further information on
 BCPs is available in Section 2 of RFC 5741.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 http://www.rfc-editor.org/info/rfc7605.

Touch Best Current Practice [Page 1] RFC 7605 Recommendations for Transport Port Use August 2015

Copyright Notice

 Copyright (c) 2015 IETF Trust and the persons identified as the
 document authors.  All rights reserved.
 This document is subject to BCP 78 and the IETF Trust's Legal
 Provisions Relating to IETF Documents
 (http://trustee.ietf.org/license-info) in effect on the date of
 publication of this document.  Please review these documents
 carefully, as they describe your rights and restrictions with respect
 to this document.  Code Components extracted from this document must
 include Simplified BSD License text as described in Section 4.e of
 the Trust Legal Provisions and are provided without warranty as
 described in the Simplified BSD License.

Table of Contents

 1. Introduction ....................................................3
 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 are necessary? .............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 .......................................14
    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
 Acknowledgments ...................................................24
 Author's Address ..................................................24

Touch Best Current Practice [Page 2] RFC 7605 Recommendations for Transport Port Use August 2015

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
 [RFC6335] (the sole document of BCP 165 before this document), but it
 does not change any of the port number assignment procedures
 described therein.  Because they are thus so closely related, this
 document and RFC 6335 are now known together as BCP 165.  This
 document is intended to address concerns typically raised during
 Expert Review (see [RFC5226]) 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 documents.  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.  Lowercase uses of these words are not to be
 interpreted as carrying significance described in RFC 2119.
 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

Touch Best Current Practice [Page 3] RFC 7605 Recommendations for Transport Port Use August 2015

 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 suggests that 32-bit numbers 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.
 [RFC76] proposes a "telephone book" by which an index will allow port
 numbers to be used by name, but still assumes that both source and
 destination port numbers are fixed by such a system.  [RFC333]
 proposes that a port number pair, rather than an individual port
 number, 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] introduces 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 describes ranges used for
 different purposes:
    Decimal   Octal     Description
    -----------------------------------------------------------
    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 disappear, 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
 (e.g., UDP) rather than only connection-oriented transports.
 By [RFC900], the ranges appear as decimal numbers rather than the
 octal ranges used previously.  [RFC1340] increases this range from
 0-255 to 0-1023 and begins 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

Touch Best Current Practice [Page 4] RFC 7605 Recommendations for Transport Port Use August 2015

 TCP, UDP, recently Stream Control Transmission Protocol (SCTP) and
 Datagram Congestion Control Protocol (DCCP), as well as ISO-TP4 for a
 brief period in the early 1990s).  [RFC1340] also establishes 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 online version, as of [RFC3232] in 2002.

4. Current Port Number Use

 RFC 6335 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 it does not require either coordination or

Touch Best Current Practice [Page 5] RFC 7605 Recommendations for Transport Port Use August 2015

 registration of port number use.  It is the second purpose that
 drives the need for a common registry.
 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 capabilities, which are implemented inside the
 network and based on a port number.

Touch Best Current Practice [Page 6] RFC 7605 Recommendations for Transport Port Use August 2015

 Port numbers can also be used for other purposes.  Assigned port
 numbers can simplify end-system configuration, so that individual
 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 had 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 [RFC6335] (originally the sole document of BCP 165), which
 describes the IANA procedures for managing assigned transport port
 numbers and services, but rather augments it by now becoming part of
 BCP 165 (i.e., BCP 165 now refers to both documents together).
 Assigned port number conservation is based on a number of basic
 principles:

Touch Best Current Practice [Page 7] RFC 7605 Recommendations for Transport Port Use August 2015

 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 Multicast DNS
    (mDNS) and portmapper [RFC1833] [RFC6762] [RFC6763].
 o  Copies of some existing services can be differentiated using in-
    band information (e.g., URIs in the 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.  For example, 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 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
 (to snoop for the Dynamic port number for the NAT to correctly map)

Touch Best Current Practice [Page 8] RFC 7605 Recommendations for Transport Port Use August 2015

 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.
 The following questions may aid in making this determination:
 o  Is this really a new service or could 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?  For example:
    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 [RFC959],
       SIP [RFC3261], etc.
    o  Using an existing port discovery service: portmapper [RFC1833],
       mDNS [RFC6762] [RFC6763], etc.

Touch Best Current Practice [Page 9] RFC 7605 Recommendations for Transport Port Use August 2015

 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
    versus 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 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 to ask, "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].
 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.

Touch Best Current Practice [Page 10] RFC 7605 Recommendations for Transport Port Use August 2015

 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 an 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 are necessary?

 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
 NAT traversal protocols supporting automatic configuration have been
 proposed and developed (e.g., Session Traversal Utilities for NAT
 (STUN) [RFC5389], Traversal Using Relays around NAT (TURN) [RFC5766],
 and Interactive Connectivity Establishment (ICE) [RFC5245]), not all
 application and service designers can rely on their presence as of
 yet.

Touch Best Current Practice [Page 11] RFC 7605 Recommendations for Transport Port Use August 2015

 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 versus 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
 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.

Touch Best Current Practice [Page 12] RFC 7605 Recommendations for Transport Port Use August 2015

 >> 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 Datagram TLS
 (DTLS) [RFC6347], or transport protection such as the 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:
 >> 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.

Touch Best Current Practice [Page 13] RFC 7605 Recommendations for Transport Port Use August 2015

 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.

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.

Touch Best Current Practice [Page 14] RFC 7605 Recommendations for Transport Port Use August 2015

 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.  For example, 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.  For example, 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].
 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:

Touch Best Current Practice [Page 15] RFC 7605 Recommendations for Transport Port Use August 2015

 >> 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 an Internet-Draft, RFC, 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].
 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 posting or RFC publication
 (in any stream) typically use a placeholder ("PORTNUM") in the text,
 and implementations use an experimental port number until a final

Touch Best Current Practice [Page 16] RFC 7605 Recommendations for Transport Port Use August 2015

 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
 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.

Touch Best Current Practice [Page 17] RFC 7605 Recommendations for Transport Port Use August 2015

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
 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

Touch Best Current Practice [Page 18] RFC 7605 Recommendations for Transport Port Use August 2015

 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,
            DOI 10.17487/RFC2119, March 1997,
            <http://www.rfc-editor.org/info/rfc2119>.
 [RFC2780]  Bradner, S. and V. Paxson, "IANA Allocation Guidelines For
            Values In the Internet Protocol and Related Headers", BCP
            37, RFC 2780, DOI 10.17487/RFC2780, March 2000,
            <http://www.rfc-editor.org/info/rfc2780>.
 [RFC3692]  Narten, T., "Assigning Experimental and Testing Numbers
            Considered Useful", BCP 82, RFC 3692,
            DOI 10.17487/RFC3692, January 2004,
            <http://www.rfc-editor.org/info/rfc3692>.
 [RFC4727]  Fenner, B., "Experimental Values In IPv4, IPv6, ICMPv4,
            ICMPv6, UDP, and TCP Headers", RFC 4727,
            DOI 10.17487/RFC4727, November 2006,
            <http://www.rfc-editor.org/info/rfc4727>.
 [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
            (TLS) Protocol Version 1.2", RFC 5246,
            DOI 10.17487/RFC5246, August 2008,
            <http://www.rfc-editor.org/info/rfc5246>.
 [RFC5405]  Eggert, L. and G. Fairhurst, "Unicast UDP Usage Guidelines
            for Application Designers", BCP 145, RFC 5405,
            DOI 10.17487/RFC5405, November 2008,
            <http://www.rfc-editor.org/info/rfc5405>.
 [RFC5925]  Touch, J., Mankin, A., and R. Bonica, "The TCP
            Authentication Option", RFC 5925, DOI 10.17487/RFC5925,
            June 2010, <http://www.rfc-editor.org/info/rfc5925>.

Touch Best Current Practice [Page 19] RFC 7605 Recommendations for Transport Port Use August 2015

 [RFC6335]  Cotton, M., Eggert, L., Touch, J., Westerlund, M., 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, DOI 10.17487/RFC6335, August 2011,
            <http://www.rfc-editor.org/info/rfc6335>.
 [RFC6347]  Rescorla, E. and N. Modadugu, "Datagram Transport Layer
            Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
            January 2012, <http://www.rfc-editor.org/info/rfc6347>.

10.2. Informative References

 [IEN112]   Postel, J., "Transmission Control Protocol", IEN 112,
            August 1979.
 [RFC33]    Crocker, S., "New Host-Host Protocol", RFC 33,
            DOI 10.17487/RFC0033, February 1970,
            <http://www.rfc-editor.org/info/rfc33>.
 [RFC37]    Crocker, S., "Network Meeting Epilogue, etc", RFC 37,
            DOI 10.17487/RFC0037, March 1970,
            <http://www.rfc-editor.org/info/rfc37>.
 [RFC38]    Wolfe, S., "Comments on Network Protocol from NWG/RFC
            #36", RFC 38, DOI 10.17487/RFC0038, March 1970,
            <http://www.rfc-editor.org/info/rfc38>.
 [RFC48]    Postel, J. and S. Crocker, "Possible protocol plateau",
            RFC 48, DOI 10.17487/RFC0048, April 1970,
            <http://www.rfc-editor.org/info/rfc48>.
 [RFC61]    Walden, D., "Note on Interprocess Communication in a
            Resource Sharing Computer Network", RFC 61,
            DOI 10.17487/RFC0061, July 1970,
            <http://www.rfc-editor.org/info/rfc61>.
 [RFC76]    Bouknight, J., Madden, J., and G. Grossman, "Connection by
            name: User oriented protocol", RFC 76,
            DOI 10.17487/RFC0076, October 1970,
            <http://www.rfc-editor.org/info/rfc76>.

Touch Best Current Practice [Page 20] RFC 7605 Recommendations for Transport Port Use August 2015

 [RFC333]   Bressler, R., Murphy, D., and D. Walden, "Proposed
            experiment with a Message Switching Protocol", RFC 333,
            DOI 10.17487/RFC0333, May 1972,
            <http://www.rfc-editor.org/info/rfc333>.
 [RFC739]   Postel, J., "Assigned numbers", RFC 739,
            DOI 10.17487/RFC0739, November 1977,
            <http://www.rfc-editor.org/info/rfc739>.
 [RFC758]   Postel, J., "Assigned numbers", RFC 758,
            DOI 10.17487/RFC0758, August 1979,
            <http://www.rfc-editor.org/info/rfc758>.
 [RFC768]   Postel, J., "User Datagram Protocol", STD 6, RFC 768,
            DOI 10.17487/RFC0768, August 1980,
            <http://www.rfc-editor.org/info/rfc768>.
 [RFC793]   Postel, J., "Transmission Control Protocol", STD 7, RFC
            793, DOI 10.17487/RFC0793, September 1981,
            <http://www.rfc-editor.org/info/rfc793>.
 [RFC820]   Postel, J., "Assigned numbers", RFC 820,
            DOI 10.17487/RFC0820, August 1982,
            <http://www.rfc-editor.org/info/rfc820>.
 [RFC900]   Reynolds, J. and J. Postel, "Assigned Numbers", RFC 900,
            DOI 10.17487/RFC0900, June 1984,
            <http://www.rfc-editor.org/info/rfc900>.
 [RFC959]   Postel, J. and J. Reynolds, "File Transfer Protocol", STD
            9, RFC 959, DOI 10.17487/RFC0959, October 1985,
            <http://www.rfc-editor.org/info/rfc959>.
 [RFC1122]  Braden, R., Ed., "Requirements for Internet Hosts -
            Communication Layers", STD 3, RFC 1122,
            DOI 10.17487/RFC1122, October 1989,
            <http://www.rfc-editor.org/info/rfc1122>.
 [RFC1340]  Reynolds, J. and J. Postel, "Assigned Numbers", RFC 1340,
            DOI 10.17487/RFC1340, July 1992,
            <http://www.rfc-editor.org/info/rfc1340>.
 [RFC1700]  Reynolds, J. and J. Postel, "Assigned Numbers", RFC 1700,
            DOI 10.17487/RFC1700, October 1994,
            <http://www.rfc-editor.org/info/rfc1700>.

Touch Best Current Practice [Page 21] RFC 7605 Recommendations for Transport Port Use August 2015

 [RFC1812]  Baker, F., Ed., "Requirements for IP Version 4 Routers",
            RFC 1812, DOI 10.17487/RFC1812, June 1995,
            <http://www.rfc-editor.org/info/rfc1812>.
 [RFC1833]  Srinivasan, R., "Binding Protocols for ONC RPC Version 2",
            RFC 1833, DOI 10.17487/RFC1833, August 1995,
            <http://www.rfc-editor.org/info/rfc1833>.
 [RFC2595]  Newman, C., "Using TLS with IMAP, POP3 and ACAP", RFC
            2595, DOI 10.17487/RFC2595, June 1999,
            <http://www.rfc-editor.org/info/rfc2595>.
 [RFC2644]  Senie, D., "Changing the Default for Directed Broadcasts
            in Routers", BCP 34, RFC 2644, DOI 10.17487/RFC2644,
            August 1999, <http://www.rfc-editor.org/info/rfc2644>.
 [RFC2817]  Khare, R. and S. Lawrence, "Upgrading to TLS Within
            HTTP/1.1", RFC 2817, DOI 10.17487/RFC2817, May 2000,
            <http://www.rfc-editor.org/info/rfc2817>.
 [RFC3232]  Reynolds, J., Ed., "Assigned Numbers: RFC 1700 is Replaced
            by an On-line Database", RFC 3232, DOI 10.17487/RFC3232,
            January 2002, <http://www.rfc-editor.org/info/rfc3232>.
 [RFC3261]  Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
            A., Peterson, J., Sparks, R., Handley, M., and E.
            Schooler, "SIP: Session Initiation Protocol", RFC 3261,
            DOI 10.17487/RFC3261, June 2002,
            <http://www.rfc-editor.org/info/rfc3261>.
 [RFC4301]  Kent, S. and K. Seo, "Security Architecture for the
            Internet Protocol", RFC 4301, DOI 10.17487/RFC4301,
            December 2005, <http://www.rfc-editor.org/info/rfc4301>.
 [RFC4340]  Kohler, E., Handley, M., and S. Floyd, "Datagram
            Congestion Control Protocol (DCCP)", RFC 4340,
            DOI 10.17487/RFC4340, March 2006,
            <http://www.rfc-editor.org/info/rfc4340>.
 [RFC4960]  Stewart, R., Ed., "Stream Control Transmission Protocol",
            RFC 4960, DOI 10.17487/RFC4960, September 2007,
            <http://www.rfc-editor.org/info/rfc4960>.
 [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
            IANA Considerations Section in RFCs", BCP 26, RFC 5226,
            DOI 10.17487/RFC5226, May 2008,
            <http://www.rfc-editor.org/info/rfc5226>.

Touch Best Current Practice [Page 22] RFC 7605 Recommendations for Transport Port Use August 2015

 [RFC5245]  Rosenberg, J., "Interactive Connectivity Establishment
            (ICE): A Protocol for Network Address Translator (NAT)
            Traversal for Offer/Answer Protocols", RFC 5245,
            DOI 10.17487/RFC5245, April 2010,
            <http://www.rfc-editor.org/info/rfc5245>.
 [RFC5389]  Rosenberg, J., Mahy, R., Matthews, P., and D. Wing,
            "Session Traversal Utilities for NAT (STUN)", RFC 5389,
            DOI 10.17487/RFC5389, October 2008,
            <http://www.rfc-editor.org/info/rfc5389>.
 [RFC5766]  Mahy, R., Matthews, P., and J. Rosenberg, "Traversal Using
            Relays around NAT (TURN): Relay Extensions to Session
            Traversal Utilities for NAT (STUN)", RFC 5766,
            DOI 10.17487/RFC5766, April 2010,
            <http://www.rfc-editor.org/info/rfc5766>.
 [RFC6066]  Eastlake 3rd, D., "Transport Layer Security (TLS)
            Extensions: Extension Definitions", RFC 6066,
            DOI 10.17487/RFC6066, January 2011,
            <http://www.rfc-editor.org/info/rfc6066>.
 [RFC6762]  Cheshire, S. and M. Krochmal, "Multicast DNS", RFC 6762,
            DOI 10.17487/RFC6762, February 2013,
            <http://www.rfc-editor.org/info/rfc6762>.
 [RFC6763]  Cheshire, S. and M. Krochmal, "DNS-Based Service
            Discovery", RFC 6763, DOI 10.17487/RFC6763, February 2013,
            <http://www.rfc-editor.org/info/rfc6763>.
 [RFC7120]  Cotton, M., "Early IANA Allocation of Standards Track Code
            Points", BCP 100, RFC 7120, DOI 10.17487/RFC7120, January
            2014, <http://www.rfc-editor.org/info/rfc7120>.
 [RFC7230]  Fielding, R., Ed., and J. Reschke, Ed., "Hypertext
            Transfer Protocol (HTTP/1.1): Message Syntax and Routing",
            RFC 7230, DOI 10.17487/RFC7230, June 2014,
            <http://www.rfc-editor.org/info/rfc7230>.

Touch Best Current Practice [Page 23] RFC 7605 Recommendations for Transport Port Use August 2015

Acknowledgments

 This work benefited 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 initially prepared using 2-Word-v2.0.template.dot.

Author's Address

 Joe Touch
 USC/ISI
 4676 Admiralty Way
 Marina del Rey, CA 90292-6695
 United States
 Phone: +1 (310) 448-9151
 Email: touch@isi.edu

Touch Best Current Practice [Page 24]

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