Internet Engineering Task Force (IETF) M. Cotton
Request for Comments: 6335 ICANN
BCP: 165 L. Eggert
Updates: 2780, 2782, 3828, 4340, 4960, 5595 Nokia
Category: Best Current Practice J. Touch
ISSN: 2070-1721 USC/ISI
August 2011Internet Assigned Numbers Authority (IANA) Procedures for the Management
of the Service Name and Transport Protocol Port Number Registry
This document defines the procedures that the Internet Assigned
Numbers Authority (IANA) uses when handling assignment and other
requests related to the Service Name and Transport Protocol Port
Number registry. It also discusses the rationale and principles
behind these procedures and how they facilitate the long-term
sustainability of the registry.
This document updates IANA's procedures by obsoleting the previous
UDP and TCP port assignment procedures defined in Sections 8 and 9.1
of the IANA Allocation Guidelines, and it updates the IANA service
name and port assignment procedures for UDP-Lite, the Datagram
Congestion Control Protocol (DCCP), and the Stream Control
Transmission Protocol (SCTP). It also updates the DNS SRV
specification to clarify what a service name is and how it is
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
Copyright (c) 2011 IETF Trust and the persons identified as the
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it for publication as an RFC or to translate it into languages other
For many years, the assignment of new service names and port number
values for use with the Transmission Control Protocol (TCP) [RFC0793]
and the User Datagram Protocol (UDP) [RFC0768] has had less than
clear guidelines. New transport protocols have been added -- the
Stream Control Transmission Protocol (SCTP) [RFC4960] and the
Datagram Congestion Control Protocol (DCCP) [RFC4342] -- and new
mechanisms like DNS SRV records [RFC2782] have been developed, each
with separate registries and separate guidelines. The community also
recognized the need for additional procedures beyond just assignment;
notably modification, revocation, and release.
A key element of the procedural streamlining specified in this
document is to establish identical assignment procedures for all IETF
transport protocols. This document brings the IANA procedures for
TCP and UDP in line with those for SCTP and DCCP, resulting in a
single process that requesters and IANA follow for all requests for
all transport protocols, including future protocols not yet defined.
In addition to detailing the IANA procedures for the initial
assignment of service names and port numbers, this document also
specifies post-assignment procedures that until now have been handled
in an ad hoc manner. These include procedures to de-assign a port
number that is no longer in use, to take a port number assigned for
one service that is no longer in use and reuse it for another
service, and the procedure by which IANA can unilaterally revoke a
prior port number assignment. Section 8 discusses the specifics of
these procedures and processes that requesters and IANA follow for
all requests for all current and future transport protocols.
IANA is the authority for assigning service names and port numbers.
The registries that are created to store these assignments are
maintained by IANA. For protocols developed by IETF working groups,
IANA now also offers a method for the "early assignment" [RFC4020] of
service names and port numbers, as described in Section 8.1.
This document updates IANA's procedures for UDP and TCP port numbers
by obsoleting Sections 8 and 9.1 of the IANA Allocation Guidelines
[RFC2780]. (Note that other sections of the IANA Allocation
Guidelines, relating to the protocol field values in IPv4 headers,
were also updated in February 2008 [RFC5237].) This document also
updates the IANA assignment procedures for DCCP [RFC4340] [RFC5595]
and SCTP [RFC4960].
The Lightweight User Datagram Protocol (UDP-Lite) shares the port
space with UDP. The UDP-Lite specification [RFC3828] says: "UDP-Lite
uses the same set of port number values assigned by the IANA for use
by UDP". An update of the UDP procedures therefore also results in a
corresponding update of the UDP-Lite procedures.
This document also clarifies what a service name is and how it is
assigned. This will impact the DNS SRV specification [RFC2782],
because that specification merely makes a brief mention that the
symbolic names of services are defined in "Assigned Numbers"
[RFC1700], without stating to which section it refers within that
230-page document. The DNS SRV specification may have been referring
to the list of Port Assignments (known as /etc/services on Unix), or
to the "Protocol And Service Names" section, or to both, or to some
other section. Furthermore, "Assigned Numbers" [RFC1700] has been
obsoleted [RFC3232] and has been replaced by on-line registries
The development of new transport protocols is a major effort that the
IETF does not undertake very often. If a new transport protocol is
standardized in the future, it is expected to follow these guidelines
and practices around using service names and port numbers as much as
possible, for consistency.
At the time of writing of this document, the internal procedures of
"Expert Review" teams, including that of IANA's port review team, are
not documented in any RFC and this document doesn't change that.
Information about the assignment procedures for the port registry has
existed in three locations: the forms for requesting port number
assignments on the IANA web site [SYSFORM] [USRFORM], an introductory
text section in the file listing the port number assignments
themselves (known as the port numbers registry) [PORTREG], and two
brief sections of the IANA Allocation Guidelines [RFC2780].
Similarly, the procedures surrounding service names have been
historically unclear. Service names were originally created as
mnemonic identifiers for port numbers without a well-defined syntax,
apart from the 14-character limit mentioned on the IANA website
[SYSFORM] [USRFORM]. Even that length limit has not been
consistently applied, and some assigned service names are 15
characters long. When service identification via DNS SRV Resource
Records (RRs) was introduced [RFC2782], it became useful to start
assigning service names alone, and because IANA had no procedure for
assigning a service name without an associated port number, this led
to the creation of an informal temporary service name registry
outside of the control of IANA, which now contains roughly 500
service names [SRVREG].
This document aggregates all this scattered information into a single
reference that aligns and clearly defines the management procedures
for both service names and port numbers. It gives more detailed
guidance to prospective requesters of service names and ports than
the existing documentation, and it streamlines the IANA procedures
for the management of the registry, so that requests can be completed
in a timely manner.
This document defines rules for assignment of service names without
associated port numbers, for such usages as DNS SRV records
[RFC2782], which was not possible under the previous IANA procedures.
The document also merges service name assignments from the non-IANA
ad hoc registry [SRVREG] and from the IANA Protocol and Service Names
registry [PROTSERVREG] into the IANA Service Name and Transport
Protocol Port Number registry [PORTREG], which from here on is the
single authoritative registry for service names and port numbers.
An additional purpose of this document is to describe the principles
that guide the IETF and IANA in their role as the long-term joint
stewards of the service name and port number registry. TCP and UDP
have had remarkable success over the last decades. Thousands of
applications and application-level protocols have service names and
port numbers assigned for their use, and there is every reason to
believe that this trend will continue into the future. It is hence
extremely important that management of the registry follow principles
that ensure its long-term usefulness as a shared resource. Section 7
discusses these principles in detail.
The Transmission Control Protocol (TCP) [RFC0793] and the User
Datagram Protocol (UDP) [RFC0768] have enjoyed a remarkable success
over the decades as the two most widely used transport protocols on
the Internet. They have relied on the concept of "ports" as logical
entities for Internet communication. Ports serve two purposes:
first, they provide a demultiplexing identifier to differentiate
transport sessions between the same pair of endpoints, and second,
they may also identify the application protocol and associated
service to which processes connect. Newer transport protocols, such
as the Stream Control Transmission Protocol (SCTP) [RFC4960] and the
Datagram Congestion Control Protocol (DCCP) [RFC4342], have also
adopted the concept of ports for their communication sessions and use
16-bit port numbers in the same way as TCP and UDP (and UDP-Lite
[RFC3828], a variant of UDP).
Port numbers are the original and most widely used means for
application and service identification on the Internet. Ports are
16-bit numbers, and the combination of source and destination port
numbers together with the IP addresses of the communicating end
systems uniquely identifies a session of a given transport protocol.
Port numbers are also known by their associated service names such as
"telnet" for port number 23 and "http" (as well as "www" and
"www-http") for port number 80.
All involved parties -- hosts running services, hosts accessing
services on other hosts, and intermediate devices (such as firewalls
and NATs) that restrict services -- need to agree on which service
corresponds to a particular destination port. Although this is
ultimately a local decision with meaning only between the endpoints
of a connection, it is common for many services to have a default
port upon which those servers usually listen, when possible, and
these ports are recorded by the Internet Assigned Numbers Authority
(IANA) through the service name and port number registry [PORTREG].
Over time, the assumption that a particular port number necessarily
implies a particular service may become less true. For example,
multiple instances of the same service on the same host cannot
generally listen on the same port, and multiple hosts behind the same
NAT gateway cannot all have a mapping for the same port on the
external side of the NAT gateway, whether using static port mappings
configured by hand by the user, or dynamic port mappings configured
automatically using a port mapping protocol like the NAT Port Mapping
Protocol [NAT-PMP] or Internet Gateway Device [IGD].
Applications may use port numbers directly, look up port numbers
based on service names via system calls such as getservbyname() on
UNIX, look up port numbers by performing queries for DNS SRV records
[RFC2782] [DNS-SD], or determine port numbers in a variety of other
ways like the TCP Port Service Multiplexer (TCPMUX) [RFC1078].
Designers of applications and application-level protocols may apply
to IANA for an assigned service name and port number for a specific
application, and may -- after assignment -- assume that no other
application will use that service name or port number for its
communication sessions. Application designers also have the option
of requesting only an assigned service name without a corresponding
fixed port number if their application does not require one, such as
applications that use DNS SRV records to look up port numbers
dynamically at run-time. Because the port number space is finite
(and therefore conservation is an important goal), the alternative of
using service names instead of port numbers is RECOMMENDED whenever
4. Conventions Used in This Document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
"Key words for use in RFCs to Indicate Requirement Levels" [RFC2119].
This document uses the term "assignment" to refer to the procedure by
which IANA provides service names and/or port numbers to requesting
parties; other RFCs refer to this as "allocation" or "registration".
This document assumes that all these terms have the same meaning, and
will use terms other than "assignment" only when quoting from or
referring to text in these other documents.
5. Service Names
Service names are the unique key in the Service Name and Transport
Protocol Port Number registry. This unique symbolic name for a
service may also be used for other purposes, such as in DNS SRV
records [RFC2782]. Within the registry, this unique key ensures that
different services can be unambiguously distinguished, thus
preventing name collisions and avoiding confusion about who is the
Assignee for a particular entry.
There may be more than one service name associated with a particular
transport protocol and port. There are three ways that such port
number overloading can occur:
o Overloading occurs when one service is an extension of another
service, and an in-band mechanism exists for determining if the
extension is present or not. One example is port 3478, which has
the service name aliases "stun" and "turn". Traversal Using
Relays around NAT (TURN) [RFC5766] is an extension to the Session
Traversal Utilities for NAT (STUN) [RFC5389] service. TURN-
enabled clients wishing to locate TURN servers could attempt to
discover "stun" services and then check in-band if the server also
supports TURN, but this would be inefficient. Enabling them to
directly query for "turn" servers by name is a better approach.
(Note that TURN servers in this case should also be locatable via
a "stun" discovery, because every TURN server is also a STUN
o By historical accident, the service name "http" has two synonyms
"www" and "www-http". When used in SRV records [RFC2782] and
similar service discovery mechanisms, only the service name "http"
should be used, not these additional names. If a server were to
advertise "www", it would not be discovered by clients browsing
for "http". Advertising or browsing for the aliases as well as
the primary service name is inefficient, and achieves nothing that
is not already achieved by using the service name "http"
o As indicated in this document in Section 10.1, overloading has
been used to create replacement names that are consistent with the
syntax this document prescribes for legacy names that do not
conform to this syntax already. For such cases, only the new name
should be used in SRV records, to avoid the same issues as with
historical cases of multiple names, and also because the legacy
names are incompatible with SRV record use.
Assignment requests for new names for existing registered services
will be rejected, as a result. Implementers are requested to inform
IANA if they discover other cases where a single service has multiple
names, so that one name may be recorded as the primary name for
service discovery purposes.
Service names are assigned on a "first come, first served" basis, as
described in Section 8.1. Names should be brief and informative,
avoiding words or abbreviations that are redundant in the context of
the registry (e.g., "port", "service", "protocol", etc.) Names
referring to discovery services, e.g., using multicast or broadcast
to identify endpoints capable of a given service, SHOULD use an
easily identifiable suffix (e.g., "-disc").
5.1. Service Name Syntax
Valid service names are hereby normatively defined as follows:
o MUST be at least 1 character and no more than 15 characters long
o MUST contain only US-ASCII [ANSI.X3.4-1986] letters 'A' - 'Z' and
'a' - 'z', digits '0' - '9', and hyphens ('-', ASCII 0x2D or
o MUST contain at least one letter ('A' - 'Z' or 'a' - 'z')
o MUST NOT begin or end with a hyphen
o hyphens MUST NOT be adjacent to other hyphens
The reason for requiring at least one letter is to avoid service
names like "23" (could be confused with a numeric port) or "6000-
6063" (could be confused with a numeric port range). Although
service names may contain both upper-case and lower-case letters,
case is ignored for comparison purposes, so both "http" and "HTTP"
denote the same service.
Service names are purely opaque identifiers, and no semantics are
implied by any superficial structure that a given service name may
appear to have. For example, a company called "Example" may choose
to register service names "Example-Foo" and "Example-Bar" for its
"Foo" and "Bar" products, but the "Example" company cannot claim to
"own" all service names beginning with "Example-"; they cannot
prevent someone else from registering "Example-Baz" for a different
service, and they cannot prevent other developers from using the
"Example-Foo" and "Example-Bar" service types in order to
interoperate with the "Foo" and "Bar" products. Technically
speaking, in service discovery protocols, service names are merely a
series of byte values on the wire; for the mnemonic convenience of
human developers, it can be convenient to interpret those byte values
as human-readable ASCII characters, but software should treat them as
purely opaque identifiers and not attempt to parse them for any
additional embedded meaning.
As of August 5, 2009, approximately 98% of the so-called "Short
Names" [SYSFORM] [USRFORM] for existing port number assignments
[PORTREG] already met the rules for legal service names stated in
Section 8.1, and hence for these services their service name is
exactly the same as their historical "Short Name". In approximately
2% of cases, the new "service name" is derived based on the old
"Short Name" as described below in Section 10.1.
The rules for valid service names, excepting the limit of 15
characters maximum, are also expressed below (as a non-normative
convenience) using ABNF [RFC5234].
SRVNAME = *(1*DIGIT [HYPHEN]) ALPHA *([HYPHEN] ALNUM)
ALNUM = ALPHA / DIGIT ; A-Z, a-z, 0-9
HYPHEN = %x2D ; "-"
ALPHA = %x41-5A / %x61-7A ; A-Z / a-z [RFC5234]
DIGIT = %x30-39 ; 0-9 [RFC5234]
5.2. Service Name Usage in DNS SRV Records
The DNS SRV specification [RFC2782] states that the Service Label
part of the owner name of a DNS SRV record includes a "Service"
element, described as "the symbolic name of the desired service", but
as discussed above, it is not clear precisely what this means.
This document clarifies that the Service Label MUST be a service name
as defined herein with an underscore prepended. The service name
SHOULD be registered with IANA and recorded in the Service Name and
Transport Protocol Port Number registry [PORTREG].
The details of using Service Names in SRV Service Labels are
specified in the DNS SRV specification [RFC2782].
6. Port Number Ranges
TCP, UDP, UDP-Lite, SCTP, and DCCP use 16-bit namespaces for their
port number registries. The port registries for all of these
transport protocols are subdivided into three ranges of numbers
[RFC1340], and Section 8.1.2 describes the IANA procedures for each
range in detail:
o the System Ports, also known as the Well Known Ports, from 0-1023
(assigned by IANA)
o the User Ports, also known as the Registered Ports, from 1024-
49151 (assigned by IANA)
o the Dynamic Ports, also known as the Private or Ephemeral Ports,
from 49152-65535 (never assigned)
Of the assignable port ranges (System Ports and User Ports, i.e.,
port numbers 0-49151), individual port numbers are in one of three
states at any given time:
o Assigned: Assigned port numbers are currently assigned to the
service indicated in the registry.
o Unassigned: Unassigned port numbers are currently available for
assignment upon request, as per the procedures outlined in this
o Reserved: Reserved port numbers are not available for regular
assignment; they are "assigned to IANA" for special purposes.
Reserved port numbers include values at the edges of each range,
e.g., 0, 1023, 1024, etc., which may be used to extend these
ranges or the overall port number space in the future.
In order to keep the size of the registry manageable, IANA typically
only records the Assigned and Reserved service names and port numbers
in the registry. Unassigned values are typically not explicitly
listed. (There are very many Unassigned service names and
enumerating them all would not be practical.)
As a data point, when this document was written, approximately 76% of
the TCP and UDP System Ports were assigned, and approximately 9% of
the User Ports were assigned. (As noted, Dynamic Ports are never
6.1. Service Names and Port Numbers for Experimentation
Of the System Ports, two TCP and UDP port numbers (1021 and 1022),
together with their respective service names ("exp1" and "exp2"),
have been assigned for experimentation with new applications and
application-layer protocols that require a port number in the
assigned ports range [RFC4727].
Please refer to Sections 1 and 1.1 of "Assigning Experimental and
Testing Numbers Considered Useful" [RFC3692] for how these
experimental port numbers are to be used.
This document assigns the same two service names and port numbers for
experimentation with new application-layer protocols over SCTP and
DCCP in Section 10.2.
Unfortunately, it can be difficult to limit access to these ports.
Users SHOULD take measures to ensure that experimental ports are
connecting to the intended process. For example, users of these
experimental ports might include a 64-bit nonce, once on each segment
of a message-oriented channel (e.g., UDP), or once at the beginning
of a byte-stream (e.g., TCP), which is used to confirm that the port
is being used as intended. Such confirmation of intended use is
especially important when these ports are associated with privileged
(e.g., system or administrator) processes.
7. Principles for Service Name and Transport Protocol Port Number
Management procedures for the Service Name and Transport Protocol
Port Number registry include assignment of service names and port
numbers upon request, as well as management of information about
existing assignments. The latter includes maintaining contact and
description information about assignments, revoking abandoned
assignments, and redefining assignments when needed. Of these
procedures, careful port number assignment is most critical, in order
to continue to conserve the remaining port numbers.
As noted earlier, only about 9% of the User Port space is currently
assigned. The current rate of assignment is approximately 400 ports
per year, and has remained steady for the past 8 years. At that
rate, if similar conservation continues, this resource will sustain
another 85 years of assignment - without the need to resort to
reassignment of released values or revocation. The namespace
available for service names is much larger, which allows for simpler
7.1. Past Principles
The principles for service name and port number management are based
on the recommendations of the IANA "Expert Review" team. Until
recently, that team followed a set of informal guidelines developed
based on the review experience from previous assignment requests.
These original guidelines, although informal, had never been publicly
documented. They are recorded here for historical purposes only; the
current guidelines are described in Section 7.2. These guidelines
o TCP and UDP ports were simultaneously assigned when either was
o Port numbers were the primary assignment; service names were
informative only, and did not have a well-defined syntax
o Port numbers were conserved informally, and sometimes
inconsistently (e.g., some services were assigned ranges of many
port numbers even where not strictly necessary)
o SCTP and DCCP service name and port number registries were managed
separately from the TCP/UDP registries
o Service names could not be assigned in the old ports registry
without assigning an associated port number at the same time
7.2. Updated Principles
This section summarizes the current principles by which IANA both
handles the Service Name and Transport Protocol Port Number registry
and attempts to conserve the port number space. This description is
intended to inform applicants requesting service names and port
numbers. IANA has flexibility beyond these principles when handling
assignment requests; other factors may come into play, and exceptions
may be made to best serve the needs of the Internet. Applicants
should be aware that IANA decisions are not required to be bound to
these principles. These principles and general advice to users on
port use are expected to change over time.
IANA strives to assign service names that do not request an
associated port number assignment under a simple "First Come First
Served" policy [RFC5226]. IANA MAY, at its discretion, refer service
name requests to "Expert Review" in cases of mass assignment requests
or other situations where IANA believes "Expert Review" is advisable
[RFC5226]; use of the "Expert Review" helps advise IANA informally in
cases where "IETF Review" or "IESG Approval" is used, as with most
The basic principle of service name and port number registry
management is to conserve use of the port space where possible.
Extensions to support larger port number spaces would require
changing many core protocols of the current Internet in a way that
would not be backward compatible and interfere with both current and
Conservation of the port number space is required because this space
is a limited resource, so applications are expected to participate in
the traffic demultiplexing process where feasible. The port numbers
are expected to encode as little information as possible that will
still enable an application to perform further demultiplexing by
itself. In particular, the principles form a goal that IANA strives
to achieve for new applications (with exceptions as deemed
appropriate, especially as for extensions to legacy services) as
o IANA strives to assign only one assigned port number per service
Note: At the time of writing of this document, there is no IETF
consensus on when it is appropriate to use a second port for an
insecure version of a protocol.
o IANA strives to assign only one assigned port number for all
variants of a service (e.g., for updated versions of a service).
o IANA strives to encourage the deployment of secure protocols.
o IANA strives to assign only one assigned port number for all
different types of devices using or participating in the same
o IANA strives to assign port numbers only for the transport
protocol(s) explicitly named in an assignment request.
o IANA may recover unused port numbers, via the new procedures of
de-assignment, revocation, and transfer.
Where possible, a given service is expected to demultiplex messages
if necessary. For example, applications and protocols are expected
to include in-band version information, so that future versions of
the application or protocol can share the same assigned port.
Applications and protocols are also expected to be able to
efficiently use a single assigned port for multiple sessions, either
by demultiplexing multiple streams within one port or by using the
assigned port to coordinate using dynamic ports for subsequent
exchanges (e.g., in the spirit of FTP [RFC0959]).
Ports are used in various ways, notably:
o as endpoint process identifiers
o as application protocol identifiers
o for firewall-filtering purposes
Both the process-identifier and the protocol-identifier uses suggest
that anything a single process can demultiplex, or that can be
encoded into a single protocol, should be. The firewall-filtering
use suggests that some uses that could be multiplexed or encoded
could instead be separated to allow for easier firewall management.
Note that this latter use is much less sound, because port numbers
have meaning only for the two endpoints involved in a connection, and
drawing conclusions about the service that generated a given flow
based on observed port numbers is not always reliable.
Effective with the publication of this document, IANA will begin
assigning port numbers for only those transport protocols explicitly
included in an assignment request. This ends the long-standing
practice of automatically assigning a port number to an application
for both TCP and UDP, even if the request is for only one of these
transport protocols. The new assignment procedure conserves
resources by assigning a port number to an application for only those
transport protocols (TCP, UDP, SCTP, and/or DCCP) it actually uses.
The port number will be marked as Reserved -- instead of Assigned --
in the port number registries of the other transport protocols. When
applications start supporting the use of some of those additional
transport protocols, the Assignee for the assignment MUST request
that IANA convert these reserved ports into assignments. An
application MUST NOT assume that it can use a port number assigned to
it for use with one transport protocol with another transport
protocol without IANA converting the reservation into an assignment.
When the available pool of unassigned numbers has run out in a port
range, it will be necessary for IANA to consider the Reserved ports
for assignment. This is part of the motivation for not automatically
assigning ports for transport protocols other than the requested
one(s). This will allow more ports to be available for assignment at
that point. To help conserve ports, application developers SHOULD
request assignment of only those transport protocols that their
application currently uses.
Conservation of port numbers is improved by procedures that allow
previously assigned port numbers to become Unassigned, either through
de-assignment or through revocation, and by a procedure that lets
application designers transfer an assigned but unused port number to
a new application. Section 8 describes these procedures, which until
now were undocumented. Port number conservation is also improved by
recommending that applications that do not require an assigned port
should register only a service name without an associated port
8. IANA Procedures for Managing the Service Name and Transport Protocol
Port Number Registry
This section describes the process for handling requests associated
with IANA's management of the Service Name and Transport Protocol
Port Number registry. Such requests include initial assignment, de-
assignment, reuse, and updates to the contact information or
description associated with an assignment. Revocation is an
additional process, initiated by IANA.
8.1. Service Name and Port Number Assignment
Assignment refers to the process of providing service names or port
numbers to applicants. All such assignments are made from service
names or port numbers that are Unassigned or Reserved at the time of
o Unassigned names and numbers are assigned according to the rules
described in Section 8.1.2 below.
o Reserved numbers and names are generally only assigned by a
"Standards Action" or "IESG Approval", and MUST be accompanied by
a statement explaining the reason a Reserved number or name is
appropriate for this action. The only exception to this rule is
that the current Assignee of a port number MAY request the
assignment of the corresponding Reserved port number for other
transport protocols when needed. IANA will initiate an "Expert
Review" [RFC5226] for such requests.
When an assignment for one or more transport protocols is approved,
the port number for any non-requested transport protocol(s) will be
marked as Reserved. IANA SHOULD NOT assign that port number to any
other application or service until no other port numbers remain
Unassigned in the requested range. It is anticipated that at such
time a new document will be published specifying IANA procedures for
assignment of such ports.
8.1.1. General Assignment Procedure
A service name or port number assignment request contains the
following information. The service name is the unique identifier of
a given service:
Service Name (REQUIRED)
Transport Protocol(s) (REQUIRED)
Port Number (OPTIONAL)
Service Code (REQUIRED for DCCP only)
Known Unauthorized Uses (OPTIONAL)
Assignment Notes (OPTIONAL)
o Service Name: A desired unique service name for the service
associated with the assignment request MUST be provided. This
name may be used with various service selection and discovery
mechanisms (including, but not limited to, DNS SRV records
[RFC2782]). The name MUST be compliant with the syntax defined in
Section 5.1. In order to be unique, they MUST NOT be identical to
any currently assigned service name in the IANA registry
[PORTREG]. Service names are case-insensitive; they may be
provided and entered into the registry with mixed case for
clarity, but case is ignored otherwise.
o Transport Protocol(s): The transport protocol(s) for which an
assignment is requested MUST be provided. This field is currently
limited to one or more of TCP, UDP, SCTP, and DCCP. Requests
without any port assignment and only a service name are still
required to indicate which protocol the service uses.
o Assignee: Name and email address of the party to whom the
assignment is made. This is REQUIRED. The Assignee is the
organization, company or individual person responsible for the
initial assignment. For assignments done through RFCs published
via the "IETF Document Stream" [RFC4844], the Assignee will be the
o Contact: Name and email address of the Contact person for the
assignment. This is REQUIRED. The Contact person is the
responsible person for the Internet community to send questions
to. This person is also authorized to submit changes on behalf of
the Assignee; in cases of conflict between the Assignee and the
Contact, the Assignee decisions take precedence. Additional
address information MAY be provided. For assignments done through
RFCs published via the "IETF Document Stream" [RFC4844], the
Contact will be the IETF Chair <email@example.com>.
o Description: A short description of the service associated with
the assignment request is REQUIRED. It should avoid all but the
most well-known acronyms.
o Reference: A description of (or a reference to a document
describing) the protocol or application using this port. This is
REQUIRED. The description must state whether the protocol uses
IP-layer broadcast, multicast, or anycast communication.
For assignments requesting only a Service Name, or a Service Name
and User Port, a statement that the protocol is proprietary and
not publicly documented is also acceptable, provided that the
required information regarding the use of IP broadcast, multicast,
or anycast is given.
For any assignment request that includes a User Port, the
assignment request MUST explain why a port number in the Dynamic
Ports range (discovered by clients dynamically at run-time) is
unsuitable for the given application.
For any assignment request that includes a System Port, the
assignment request MUST explain why a port number in the User
Ports or Dynamic Ports ranges is unsuitable, and a reference to a
stable protocol specification document MUST be provided.
IANA MAY accept early assignment [RFC4020] requests (known as
"early allocation" therein) from IETF working groups that
reference a sufficiently stable Internet-Draft instead of a
published Standards-Track RFC.
o Port Number: If assignment of a port number is desired, either the
port number the requester suggests for assignment or indication of
port range (user or system) MUST be provided. If only a service
name is to be assigned, this field is left empty. If a specific
port number is requested, IANA is encouraged to assign the
requested number. If a range is specified, IANA will choose a
suitable number from the User or System Ports ranges. Note that
the applicant MUST NOT use the requested port in implementations
deployed for use on the public Internet prior to the completion of
the assignment, because there is no guarantee that IANA will
assign the requested port.
o Service Code: If the assignment request includes DCCP as a
transport protocol, then the request MUST include a desired unique
DCCP service code [RFC5595], and MUST NOT include a requested DCCP
service code otherwise. Section 19.8 of the DCCP specification
[RFC4340] defines requirements and rules for assignment, updated
by this document. Note that, as per the DCCP Service Codes
document [RFC5595], some service codes are not assigned; zero
(absence of a meaningful service code) and 4294967295 (0xFFFFFFFF;
invalid service code) are permanently reserved, and the Private
service codes 1056964608-1073741823 (0x3F000000-0x3FFFFFFF; i.e.,
32-bit values with the high-order byte equal to a value of 63
(0x3F), corresponding to the ASCII character '?') are not
o Known Unauthorized Uses: A list of uses by applications or
organizations who are not the Assignee. This is OPTIONAL. This
list may be augmented by IANA after assignment when unauthorized
uses are reported.
o Assignment Notes: Indications of owner/name change, or any other
assignment process issue. This is OPTIONAL. This list may be
updated by IANA after assignment to help track changes to an
assignment, e.g., de-assignment, owner/name changes, etc.
If the assignment request is for the addition of a new transport
protocol to a previously assigned service name and the requester is
not the Assignee or Contact for the previously assigned service name,
IANA needs to confirm with the Assignee for the existing assignment
whether this addition is appropriate.
If the assignment request is for a new service name sharing the same
port as a previously assigned service name (see port number
overloading in Section 5), IANA needs to confirm with the Assignee
for the existing service name and other appropriate experts whether
the overloading is appropriate.
When IANA receives an assignment request -- containing the above
information -- that is requesting a port number, IANA SHALL initiate
an "Expert Review" [RFC5226] in order to determine whether an
assignment should be made. For requests that are not seeking a port
number, IANA SHOULD assign the service name under a simple "First
Come First Served" policy [RFC5226].
8.1.2. Variances for Specific Port Number Ranges
Section 6 describes the different port number ranges. It is
important to note that IANA applies slightly different procedures
when managing the different port ranges of the service name and port
o Ports in the Dynamic Ports range (49152-65535) have been
specifically set aside for local and dynamic use and cannot be
assigned through IANA. Application software may simply use any
dynamic port that is available on the local host, without any sort
of assignment. On the other hand, application software MUST NOT
assume that a specific port number in the Dynamic Ports range will
always be available for communication at all times, and a port
number in that range hence MUST NOT be used as a service
o Ports in the User Ports range (1024-49151) are available for
assignment through IANA, and MAY be used as service identifiers
upon successful assignment. Because assigning a port number for a
specific application consumes a fraction of the shared resource
that is the port number registry, IANA will require the requester
to document the intended use of the port number. For most IETF
protocols, ports in the User Ports range will be assigned under
the "IETF Review" or "IESG Approval" procedures [RFC5226] and no
further documentation is required. Where these procedures do not
apply, then the requester must input the documentation to the
"Expert Review" procedure [RFC5226], by which IANA will have a
technical expert review the request to determine whether to grant
the assignment. Regardless of the path ("IETF Review", "IESG
Approval", or "Expert Review"), the submitted documentation is
expected to be the same, as described in this section, and MUST
explain why using a port number in the Dynamic Ports range is
unsuitable for the given application. Further, IANA MAY utilize
the "Expert Review" process informally to inform their position in
participating in "IETF Review" and "IESG Approval".
o Ports in the System Ports range (0-1023) are also available for
assignment through IANA. Because the System Ports range is both
the smallest and the most densely assigned, the requirements for
new assignments are more strict than those for the User Ports
range, and will only be granted under the "IETF Review" or "IESG
Approval" procedures [RFC5226]. A request for a System Port
number MUST document *both* why using a port number from the
Dynamic Ports range is unsuitable *and* why using a port number
from the User Ports range is unsuitable for that application.
8.2. Service Name and Port Number De-Assignment
The Assignee of a granted port number assignment can return the port
number to IANA at any time if they no longer have a need for it. The
port number will be de-assigned and will be marked as Reserved. IANA
should not reassign port numbers that have been de-assigned until all
unassigned port numbers in the specific range have been assigned.
Before proceeding with a port number de-assignment, IANA needs to
reasonably establish that the value is actually no longer in use.
Because there is much less danger of exhausting the service name
space compared to the port number space, it is RECOMMENDED that a
given service name remain assigned even after all associated port
number assignments have become de-assigned. Under this policy, it
will appear in the registry as if it had been created through a
service name assignment request that did not include any port
On rare occasions, it may still be useful to de-assign a service
name. In such cases, IANA will mark the service name as Reserved.
IANA will involve their IESG-appointed expert in such cases.
IANA will include a comment in the registry when de-assignment
happens to indicate its historic usage.
8.3. Service Name and Port Number Reuse
If the Assignee of a granted port number assignment no longer has a
need for the assigned number, but would like to reuse it for a
different application, they can submit a request to IANA to do so.
Logically, port number reuse is to be thought of as a de-assignment
(Section 8.2) followed by an immediate (re-)assignment (Section 8.1)
of the same port number for a new application. Consequently, the
information that needs to be provided about the proposed new use of
the port number is identical to what would need to be provided for a
new port number assignment for the specific ports range.
Because there is much less danger of exhausting the service name
space compared to the port number space, it is RECOMMENDED that the
original service name associated with the prior use of the port
number remains assigned, and a new service name be created and
associated with the port number. This is again consistent with
viewing a reuse request as a de-assignment followed by an immediate
(re-)assignment. Reusing an assigned service name for a different
application is NOT RECOMMENDED.
IANA needs to carefully review such requests before approving them.
In some instances, the Expert Reviewer will determine that the
application the port number was assigned to has found usage beyond
the original Assignee, or that there is a concern that it may have
such users. This determination MUST be made quickly. A community
call concerning revocation of a port number (see below) MAY be
considered, if a broader use of the port number is suspected.
8.4. Service Name and Port Number Revocation
A port number revocation can be thought of as an IANA-initiated de-
assignment (Section 8.2), and has exactly the same effect on the
Sometimes, it will be clear that a specific port number is no longer
in use and that IANA can revoke it and mark it as Reserved. At other
times, it may be unclear whether a given assigned port number is
still in use somewhere in the Internet. In those cases, IANA must
carefully consider the consequences of revoking the port number, and
SHOULD only do so if there is an overwhelming need.
With the help of their IESG-appointed Expert Reviewer, IANA SHALL
formulate a request to the IESG to issue a four-week community call
concerning the pending port number revocation. The IESG and IANA,
with the Expert Reviewer's support, SHALL determine promptly after
the end of the community call whether revocation should proceed, and
then communicate their decision to the community. This procedure
typically involves similar steps to de-assignment except that it is
initiated by IANA.
Because there is much less danger of exhausting the service name
space compared to the port number space, revoking service names is
8.5. Service Name and Port Number Transfers
The value of service names and port numbers is defined by their
careful management as a shared Internet resource, whereas enabling
transfer allows the potential for associated monetary exchanges. As
a result, the IETF does not permit service name or port number
assignments to be transferred between parties, even when they are
The appropriate alternate procedure is a coordinated de-assignment
and assignment: The new party requests the service name or port
number via an assignment and the previous party releases its
assignment via the de-assignment procedure outlined above.
With the help of their IESG-appointed Expert Reviewer, IANA SHALL
carefully determine if there is a valid technical, operational, or
managerial reason to grant the requested new assignment.
8.6. Maintenance Issues
In addition to the formal procedures described above, updates to the
Description and Contact information are coordinated by IANA in an
informal manner, and may be initiated by either the Assignee or by
IANA, e.g., by the latter requesting an update to current Contact
information. (Note that the Assignee cannot be changed as a separate
procedure; see instead Section 8.5 above.)
In the case of disagreements around any request, there is the
possibility of appeal following the normal appeals process for IANA
assignments as defined by Section 7 of "Guidelines for Writing an
IANA Considerations Section in RFCs" [RFC5226].
9. Security Considerations
The IANA guidelines described in this document do not change the
security properties of UDP, TCP, SCTP, or DCCP.
Assignment of a service name or port number does not in any way imply
an endorsement of an application or product, and the fact that
network traffic is flowing to or from an assigned port number does
not mean that it is "good" traffic, or even that it is used by the
assigned service. Firewall and system administrators should choose
how to configure their systems based on their knowledge of the
traffic in question, not based on whether or not there is an assigned
service name or port number.
Services are expected to include support for security, either as
default or dynamically negotiated in-band. The use of separate
service name or port number assignments for secure and insecure
variants of the same service is to be avoided in order to discourage
the deployment of insecure services.
10. IANA Considerations
This document obsoletes Sections 8 and 9.1 of the March 2000 IANA
Allocation Guidelines [RFC2780].
Upon approval of this document for publication as an RFC, IANA worked
with Stuart Cheshire, maintainer of the independent service name
registry [SRVREG], to merge the contents of that private registry
into the official IANA registry. The independent registry web page
has been updated with pointers to the IANA registry and to this RFC.
IANA created a new service name entry in the service name and port
number registry [PORTREG] for all entries in the Protocol and Service
Names registry [PROTSERVREG] that did not already have one assigned.
IANA also indicates in the Assignment Notes for "www" and "www-http"
that they are duplicate terms that refer to the "http" service, and
should not be used for discovery purposes. For this conceptual
service (human-readable web pages served over HTTP), the correct
service name to use for service discovery purposes is "http" (see
10.1. Service Name Consistency
Section 8.1 defines which character strings are well-formed service
names, which until now had not been clearly defined. The definition
in Section 8.1 was chosen to allow maximum compatibility of service
names with current and future service discovery mechanisms.
As of August 5, 2009, approximately 98% of the so-called "Short
Names" from existing port number assignments [PORTREG] met the rules
for legal service names stated in Section 8.1, and hence for these
services their service name is exactly the same as their "Short
The remaining approximately 2% of the existing "Short Names" are not
suitable to be used directly as well-formed service names because
they contain illegal characters such as asterisks, dots, pluses,
slashes, or underscores. All existing "Short Names" conform to the
length requirement of 15 characters or fewer. For these 96
unsuitable "Short Names", listed in the table below, the service name
is the Short Name with any illegal characters replaced by hyphens.
IANA added an entry to the registry that uses the new well-formed
primary service name for the existing service and that otherwise
duplicates the original assignment information. In the description
field of this new entry giving the primary service name, IANA
recorded that it has assigned a well-formed service name for the
previous service and references the original assignment. In the
There were four names recorded in IANA's Port Number Registry
[PORTREG] that conflicted with names previously recorded in the ad
hoc SRV name registry [SRVREG]: esp, hydra, recipe, and xmp.
The name conflicts were resolved amicably:
The IANA Port Number Registry Short Name "esp" had been registered by
Andrew Chernow, and he informed the authors that the port was no
longer in use and the registration was no longer required. The SRV
registry entry for "esp" remains in effect.
The SRV name "hydra" for SubEthaEdit had already been retired in
favor of the new SRV name "see". The IANA Port Number Registry entry
for "hydra" remains in effect.
The SRV name "recipe" was in use in an open source project that had
not yet been packaged for distribution, and the registrant Daniel
Taylor was willing to change to a different service name. Thanks to
Daniel Taylor for accommodating this change. The IANA Port Number
Registry entry for "recipe" remains in effect.
The IANA Port Number Registry Short Name "xmp" had been registered by
Bobby Krupczak, but since his registration included an assigned port
number (which is still in use and remains unaffected by this change),
he was willing to switch to a different service name. Thanks to
Bobby Krupczak for accommodating this change. The SRV registry entry
for "xmp" remains in effect.
10.2. Port Numbers for SCTP and DCCP Experimentation
Two System UDP and TCP ports, 1021 and 1022, have been reserved for
experimental use [RFC4727]. This document assigns the same port
numbers for SCTP and DCCP, updates the TCP and UDP assignments, and
also instructs IANA to automatically assign these two port numbers
for any future transport protocol with a similar 16-bit port number
Note that these port numbers are meant for temporary experimentation
and development in controlled environments. Before using these port
numbers, carefully consider the advice in Section 6.1 in this
document, as well as in Sections 1 and 1.1 of "Assigning Experimental
and Testing Numbers Considered Useful" [RFC3692]. Most importantly,
application developers must request a permanent port number
assignment from IANA as described in Section 8.1 before any kind of
| Service Name | exp1 |
| Transport Protocol | DCCP, SCTP, TCP, UDP |
| Assignee | IESG <firstname.lastname@example.org> |
| Contact | IETF Chair <email@example.com> |
| Description | RFC3692-style Experiment 1 |
| Reference | [RFC4727] [RFC6335] |
| Port Number | 1021 |
| Service Name | exp2 |
| Transport Protocol | DCCP, SCTP, TCP, UDP |
| Assignee | IESG <firstname.lastname@example.org> |
| Contact | IETF Chair <email@example.com> |
| Description | RFC3692-style Experiment 2 |
| Reference | [RFC4727] [RFC6335] |
| Port Number | 1022 |
10.3. Updates to DCCP Registries
This document updates the IANA assignment procedures for the DCCP
Port Number and DCCP Service Codes Registries [RFC4340].
10.3.1. DCCP Service Code Registry
Service codes are assigned on a "first come, first served" basis
according to Section 19.8 of the DCCP specification [RFC4340]. This
document updates that section by extending the guidelines given there
in the following ways:
o IANA MAY assign new service codes without seeking "Expert Review"
using their discretion, but SHOULD seek "Expert Review" if a
request asks for more than five service codes.
o IANA should feel free to contact the DCCP Expert Reviewer with any
questions related to requests for DCCP-related codepoints.
10.3.2. DCCP Port Numbers Registry
The DCCP ports registry is defined by Section 19.9 of the DCCP
specification [RFC4340]. Assignments in this registry require prior
assignment of a service code. Not all service codes require IANA-
assigned ports. This document updates that section by extending the
guidelines given there in the following way:
o IANA should normally assign a value in the range 1024-49151 to a
DCCP server port. IANA requests to assign port numbers in the
System Ports range (0 through 1023) require an "IETF Review"
[RFC5226] prior to assignment by IANA [RFC4340].
o IANA MUST NOT assign more than one DCCP server port to a single
service code value.
o The assignment of multiple service codes to the same DCCP port is
allowed, but subject to "Expert Review".
o The set of service code values associated with a DCCP server port
should be recorded in the service name and port number registry.
o A request for additional service codes to be associated with an
already assigned port number requires "Expert Review". These
requests will normally be accepted when they originate from the
contact associated with the port assignment. In other cases,
these applications will be expected to use an unassigned port,
when this is available.
The DCCP specification [RFC4340] notes that a short port name MUST be
associated with each DCCP server port that has been assigned. This
document clarifies that this short port name is the service name as
defined here, and this name MUST be unique.
Alfred Hoenes (firstname.lastname@example.org) and Allison Mankin (email@example.com) have
contributed text and ideas to this document.
The text in Section 10.3 is based on a suggestion originally proposed
as a part of the DCCP Service Codes document [RFC5595] by Gorry
Lars Eggert is partly funded by the Trilogy Project [TRILOGY], a
research project supported by the European Commission under its
Seventh Framework Program.
13.1. Normative References
[ANSI.X3.4-1986] American National Standards Institute, "Coded
Character Set - 7-bit American Standard Code for
Information Interchange", ANSI X3.4, 1986.
[RFC0768] Postel, J., "User Datagram Protocol", STD 6,
RFC 768, August 1980.
[RFC0793] Postel, J., "Transmission Control Protocol", STD 7,
RFC 793, September 1981.
[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.
[RFC2782] Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS
RR for specifying the location of services (DNS
SRV)", RFC 2782, February 2000.
[RFC3828] Larzon, L-A., Degermark, M., Pink, S., Jonsson,
L-E., and G. Fairhurst, "The Lightweight User
Datagram Protocol (UDP-Lite)", RFC 3828, July 2004.
[RFC4020] Kompella, K. and A. Zinin, "Early IANA Allocation
of Standards Track Code Points", BCP 100, RFC 4020,
[RFC4340] Kohler, E., Handley, M., and S. Floyd, "Datagram
Congestion Control Protocol (DCCP)", RFC 4340,
[RFC4727] Fenner, B., "Experimental Values In IPv4, IPv6,
ICMPv4, ICMPv6, UDP, and TCP Headers", RFC 4727,
[RFC4960] Stewart, R., "Stream Control Transmission
Protocol", RFC 4960, September 2007.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for
Writing an IANA Considerations Section in RFCs",
BCP 26, RFC 5226, May 2008.
[RFC3692] Narten, T., "Assigning Experimental and Testing
Numbers Considered Useful", BCP 82, RFC 3692,
[RFC4342] Floyd, S., Kohler, E., and J. Padhye, "Profile for
Datagram Congestion Control Protocol (DCCP)
Congestion Control ID 3: TCP-Friendly Rate Control
(TFRC)", RFC 4342, March 2006.
[RFC4844] Daigle, L. and Internet Architecture Board, "The
RFC Series and RFC Editor", RFC 4844, July 2007.
[RFC5237] Arkko, J. and S. Bradner, "IANA Allocation
Guidelines for the Protocol Field", BCP 37,
RFC 5237, February 2008.
[RFC5389] Rosenberg, J., Mahy, R., Matthews, P., and D. Wing,
"Session Traversal Utilities for NAT (STUN)",
RFC 5389, October 2008.
[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, April 2010.
[SRVREG] "DNS SRV Service Types Registry",
[SYSFORM] Internet Assigned Numbers Authority (IANA),
"Application for System (Well Known) Port Number",
[TRILOGY] "Trilogy Project",
[USRFORM] Internet Assigned Numbers Authority (IANA),
"Application for User (Registered) Port Number",
Internet Corporation for Assigned Names and Numbers
4676 Admiralty Way, Suite 330
Marina del Rey, CA 90292
Phone: +1 310 823 9358
Nokia Research Center
P.O. Box 407
Nokia Group 00045
Phone: +358 50 48 24461
4676 Admiralty Way
Marina del Rey, CA 90292
Phone: +1 310 448 9151
Stockholm 164 80
Phone: +46 8 719 0000