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Recommendations for Transport Port Number Uses

The information below is for an old version of the document.
Document Type
This is an older version of an Internet-Draft that was ultimately published as RFC 7605.
Author Dr. Joseph D. Touch
Last updated 2015-03-05 (Latest revision 2015-01-23)
Replaces draft-touch-tsvwg-port-use
RFC stream Internet Engineering Task Force (IETF)
Additional resources Mailing list discussion
Stream WG state Submitted to IESG for Publication
Document shepherd Gorry Fairhurst
Shepherd write-up Show Last changed 2015-01-19
IESG IESG state Became RFC 7605 (Best Current Practice)
Consensus boilerplate Yes
Telechat date (None)
Needs a YES. Needs 10 more YES or NO OBJECTION positions to pass.
Responsible AD Spencer Dawkins
Send notices to,
IANA IANA review state IANA OK - No Actions Needed
TSVWG                                                          J. Touch
Internet Draft                                                 USC/ISI
Intended status: Best Current Practice                 January 23, 2015
Expires: July 2015

              Recommendations for Transport Port Number Uses

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on July 23, 2015.

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   Copyright (c) 2015 IETF Trust and the persons identified as the
   document authors. All rights reserved.

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   ( in effect on the date of
   publication of this document. Please review these documents
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   This document provides recommendations to application and service
   designers on how to use the transport protocol port number space. It
   complements (but does not update) RFC6335, which focuses on IANA

Table of Contents

   1. Introduction...................................................2
   2. Conventions used in this document..............................3
   3. History........................................................3
   4. Current Port Number Use........................................4
   5. What is a Port Number?.........................................5
   6. Conservation...................................................7
      6.1. Guiding Principles........................................7
      6.2. Firewall and NAT Considerations...........................8
   7. How to Use Assigned Port Numbers...............................9
      7.1. Is a port number assignment necessary?....................9
      7.2. How Many Port Numbers?...................................11
      7.3. Picking a 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............................15
      7.8. Squatting................................................17
      7.9. Other Considerations.....................................17
   8. Security Considerations.......................................18
   9. IANA Considerations...........................................18
   10. References...................................................18
      10.1. Normative References....................................18
      10.2. Informative References..................................19
   11. Acknowledgments..............................................21

1. Introduction

   This document provides information and advice to application and
   service designers on the use of 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 update that document.

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2. Conventions used in this document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   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 compliance requirement statement using the key words
   listed above. This convention aids reviewers in quickly identifying
   or finding the explicit compliance requirements of 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.

   [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

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   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:

      Binary    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.

4. Current Port Number Use

   RFC6335 indicates three ranges of port number assignments:

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      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

   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

      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.

   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

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   to use to access that service at that host. Such information can be
   distributed out-of-band, e.g., in the URI:

   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 service names (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 assigned port
   numbers with IANA - to have a common agreement between all endpoints
   on the Internet as to the default meaning of a port number.

   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

   Port numbers can also be useful 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 can also simplify firewall management, so
   that a single, fixed firewall configuration can either permit or
   deny a service.

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   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 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 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 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 port requested MUST be justified as independently necessary.

6.1. Guiding Principles

   This document provides recommendations for users that also help
   conserve port number space. Again, this document does not update
   BCP165 [RFC6335], which describes the IANA procedures for managing
   transport port numbers and services. Port number conservation is
   based on a number of basic principles:

      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].

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      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 an existing service can be differentiated by using
         different IP addresses, either on different hosts or as
         different real or virtual interfaces (or even operating
         systems) on the same host.

      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

   Port numbers are intended to differentiate services, not variations
   of performance, replicas, pairwise endpoint associations, or payload
   types. Port numbers are also a small space compared to other
   Internet number spaces; it is never appropriate to consume port
   numbers to conserve larger spaces such as IP addresses.

6.2. Firewall and NAT Considerations

   Assigned port numbers are useful for configuring firewalls and other
   port-based systems for access control. Ultimately, these 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. However, assigned
   port numbers often are important in helping configure firewalls.

   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)
   or passive-mode FTP (in which both connections are opened from the
   client side).

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7. How to Use Assigned Port Numbers

   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 the 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

      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].

   There are a few good examples of reasons that more directly suggest
   that not only is a port number not necessary, but it is directly

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      o  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 port number in context of separate pairwise endpoint

      o  Additional 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  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 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].

      o  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
         assignments should incorporate support for version indication.

   Some users 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

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   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 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 port number use. Although some may
   be cumbersome or inefficient, they are always preferable to
   consuming additional port numbers.

   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 it is still important to have assigned
   port numbers, largely to traverse either NATs or firewalls. Although
   automatic configuration protocols have been proposed and developed
   (e.g., STUN [RFC5389], TURN [RFC5766], and ICE [RFC5245]),
   application and service designers cannot yet rely on their presence.

   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.

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7.3. Picking a 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

   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 TCP
   (UDP) port number can be assigned whose port number is already (or
   can be subsequently) assigned to a different transport protocol.

   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

   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. As a result:

   >> Developers SHOULD NOT apply for System port numbers because the
   increased privilege they are intended to provide is not always

   Even when developers seek a System port number, 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].

   >> 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

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   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 attack that host. This vulnerability can be mitigated a
   number of ways:

   >> New services SHOULD support security, either directly or via a
   secure transport such as TLS [RFC5246].

   >> Insecure versions of new or existing secure services SHOULD be
   avoided because of the new vulnerability they create.

   >> When simultaneously requesting both a secure and an insecure
   port, strong justification MUST be provided for the insecure port.
   Precedent (citing other protocols that use an insecure port) is not
   strong justification by itself.  A strong case for utility of the
   insecure service is REQUIRED for approval of the insecure port.

   >> Security SHOULD NOT rely on port number distinctions alone; every
   service, whether secure or not, is likely to be attacked.

   There is debate as to how to secure legacy insecure services
   [RFC6335]. Some argue that secure variants should share the existing
   port number assignment, such that security is enabled on a per-
   connection or other association basis [RFC2817]. Others argue that
   security should be supported on a new port number assignment and be
   enabled by default. Either approach is currently permitted, although
   use of a single port number is consistent with port number
   conservation. A separate port number might be important for security
   coordination (e.g., firewall management), but this might further
   argue for deprecation of the insecure variant.

   Optional security can penalize performance, requiring additional
   round-trip exchanges before a connection or other association can be
   established. As discussed earlier, port numbers are a critical
   resource and it is inappropriate to consume assignments to increase
   performance. As a result, the need for separate ports for both
   secure and insecure variants is not justified merely for performance

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   - either for the connection or association establishment performance
   or differences in data performance between secure and insecure

   Note however that a new service might not be eligible for IANA
   assignment of both an insecure and a secure variant of the same
   service, and similarly applications requesting assignment for both
   an insecure port number for a secure service might not be
   appropriate. In both cases, security of the service is compromised
   by adding the insecure port number assignment.

7.5. Support for Future Versions

   Current IANA assignments are expected to support the 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.

   >> Version numbers SHOULD NOT be included in either the service name
   or service description.

   Again, the 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

7.6. Transport Protocols

   IANA assigns port numbers specific to one or more transport
   protocols, typically UDP and TCP, but also SCTP, DCCP, and any other
   standard transport protocol [RFC768] [RFC793] [RFC4340] [RFC4960].
   Originally, IANA port number assignments were concurrent for both
   UDP and TCP; other transports were not indicated. However, to
   conserve 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.

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   >> 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, their service names and descriptions
   should reflect that difference. 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]. The following convention has been used by IANA
   for several years 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" (all 1's, i.e., for IPv4) broadcasts 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

   >> 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, i.e., 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

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   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].

   Deployments that use port numbers before deployment 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 and proceeds to IESG Approval, 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]

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   >> 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 assigned range in
   deployed software without IANA 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 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

   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.

   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]

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   Aliases, or multiple service names for the same port number, are no
   longer considered appropriate [RFC6335].

8. Security Considerations

   This document discusses ways to conserve port numbers, notably
   through encouraging demultiplexing within a single port number.  As
   such, there may be cases where two variants of a protocol - insecure
   and secure (such as using optional TLS) or different versions - are
   suggested to share the same port number.

   This document reminds application and service designers that port
   numbers do not protect against denial of service overload 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 or firewall traffic because their use 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.

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   [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.

   [RFC5405] Eggert, L., and G. Fairhurst, "Unicast UDP Usage
             Guidelines for Application Designers", BCP 145, RFC 5405,
             Nov. 2008.

   [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.

10.2. Informative References

   [IEN112]  Postel, J., "Transmission Control Protocol", IEN 112,
             August 1979.

   [RFC33]   Crocker, S., "New Host-Host Protocol", RFC 33 February

   [RFC37]   Crocker, S., "Network Meeting Epilogue", RFC 37, March

   [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.

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   [RFC768]  Postel, J., "User Datagram Protocol", RFC 768, August

   [RFC793]  Postel, J., "Transmission Control Protocol" RFC 793,
             September 1981

   [RFC820]  Postel, J., "Assigned numbers", RFC 820, August 1982.

   [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.

   [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.

   [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

   [RFC5246] Dierks, T., and E. Rescorla, "The Transport Layer Security
             (TLS) Protocol Version 1.2", RFC 5246, August 2008.

   [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

   [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

   This document was prepared using

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Authors' Addresses

   Joe Touch
   4676 Admiralty Way
   Marina del Rey, CA 90292-6695

   Phone: +1 (310) 448-9151

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