[Note that this file is a concatenation of more than one RFC.]
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
M. Westerlund Ericsson S. Cheshire Apple August 2011
Internet 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 registered.
Status of This Memo
This memo documents an Internet Best Current Practice.
This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on BCPs is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at http://www.rfc-editor.org/info/rfc6335.
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Copyright (c) 2011 IETF Trust and the persons identified as the document authors. All rights reserved.
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This document may contain material from IETF Documents or IETF Contributions published or made publicly available before November 10, 2008. The person(s) controlling the copyright in some of this material may not have granted the IETF Trust the right to allow modifications of such material outside the IETF Standards Process. Without obtaining an adequate license from the person(s) controlling the copyright in such materials, this document may not be modified outside the IETF Standards Process, and derivative works of it may not be created outside the IETF Standards Process, except to format it for publication as an RFC or to translate it into languages other than English.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4 2. Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . 5 3. Background . . . . . . . . . . . . . . . . . . . . . . . . . . 6 4. Conventions Used in This Document . . . . . . . . . . . . . . 8 5. Service Names . . . . . . . . . . . . . . . . . . . . . . . . 8 5.1. Service Name Syntax . . . . . . . . . . . . . . . . . . . 9 5.2. Service Name Usage in DNS SRV Records . . . . . . . . . . 10 6. Port Number Ranges . . . . . . . . . . . . . . . . . . . . . . 11 6.1. Service Names and Port Numbers for Experimentation . . . . 12 7. Principles for Service Name and Transport Protocol Port Number Registry Management . . . . . . . . . . . . . . . . . . 12 7.1. Past Principles . . . . . . . . . . . . . . . . . . . . . 13 7.2. Updated Principles . . . . . . . . . . . . . . . . . . . . 13 8. IANA Procedures for Managing the Service Name and Transport Protocol Port Number Registry . . . . . . . . . . . 16 8.1. Service Name and Port Number Assignment . . . . . . . . . 16 8.2. Service Name and Port Number De-Assignment . . . . . . . . 21 8.3. Service Name and Port Number Reuse . . . . . . . . . . . . 21 8.4. Service Name and Port Number Revocation . . . . . . . . . 22 8.5. Service Name and Port Number Transfers . . . . . . . . . . 22 8.6. Maintenance Issues . . . . . . . . . . . . . . . . . . . . 23 8.7. Disagreements . . . . . . . . . . . . . . . . . . . . . . 23 9. Security Considerations . . . . . . . . . . . . . . . . . . . 23 10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 24 10.1. Service Name Consistency . . . . . . . . . . . . . . . . . 24 10.2. Port Numbers for SCTP and DCCP Experimentation . . . . . . 26 10.3. Updates to DCCP Registries . . . . . . . . . . . . . . . . 27 11. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 28 12. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 28 13. References . . . . . . . . . . . . . . . . . . . . . . . . . . 29 13.1. Normative References . . . . . . . . . . . . . . . . . . . 29 13.2. Informative References . . . . . . . . . . . . . . . . . . 30
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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].
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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 [PORTREG] [PROTSERVREG].
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
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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).
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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 possible.
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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 server.)
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
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the primary service name is inefficient, and achieves nothing that is not already achieved by using the service name "http" exclusively.
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 decimal 45)
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.
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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].
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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 document.
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 assigned.)
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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 management procedures.
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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 previously were:
o TCP and UDP ports were simultaneously assigned when either was requested
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 IETF protocols.
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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 legacy applications.
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 follows:
o IANA strives to assign only one assigned port number per service or application.
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 service.
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]).
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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
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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 number.
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 the assignment.
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.
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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) Assignee (REQUIRED) Contact (REQUIRED) Description (REQUIRED) Reference (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 IESG <firstname.lastname@example.org>.
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
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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.
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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 centrally assigned.
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].
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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 number registry:
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 identifier.
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.
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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 numbers.
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.
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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 registry.
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 NOT RECOMMENDED.
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 mutually consenting.
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.
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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.
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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 Section 5).
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 Name".
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
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Assignment Notes field of the original assignment, IANA added a note that this entry is an alias to the new well-formed service name, and that the old service name is historic, not usable for use with many common service discovery mechanisms.
96 names containing illegal characters to be replaced by hyphens:
+----------------+-----------------+-----------------+ | 914c/g | acmaint_dbd | acmaint_transd | | atex_elmd | avanti_cdp | badm_priv | | badm_pub | bdir_priv | bdir_pub | | bmc_ctd_ldap | bmc_patroldb | boks_clntd | | boks_servc | boks_servm | broker_service | | bues_service | canit_store | cedros_fds | | cl/1 | contamac_icm | corel_vncadmin | | csc_proxy | cvc_hostd | dbcontrol_agent | | dec_dlm | dl_agent | documentum_s | | dsmeter_iatc | dsx_monitor | elpro_tunnel | | elvin_client | elvin_server | encrypted_admin | | erunbook_agent | erunbook_server | esri_sde | | EtherNet/IP-1 | EtherNet/IP-2 | event_listener | | flr_agent | gds_db | ibm_wrless_lan | | iceedcp_rx | iceedcp_tx | iclcnet_svinfo | | idig_mux | ife_icorp | instl_bootc | | instl_boots | intel_rci | interhdl_elmd | | lan900_remote | LiebDevMgmt_A | LiebDevMgmt_C | | LiebDevMgmt_DM | mapper-ws_ethd | matrix_vnet | | mdbs_daemon | menandmice_noh | msl_lmd | | nburn_id | ncr_ccl | nds_sso | | netmap_lm | nms_topo_serv | notify_srvr | | novell-lu6.2 | nuts_bootp | nuts_dem | | ocs_amu | ocs_cmu | pipe_server | | pra_elmd | printer_agent | redstorm_diag | | redstorm_find | redstorm_info | redstorm_join | | resource_mgr | rmonitor_secure | rsvp_tunnel | | sai_sentlm | sge_execd | sge_qmaster | | shiva_confsrvr | sql*net | srvc_registry | | stm_pproc | subntbcst_tftp | udt_os | | universe_suite | veritas_pbx | vision_elmd | | vision_server | wrs_registry | z39.50 | +----------------+-----------------+-----------------+
In addition to the 96 names listed above, the service name for "whois++" is "whoispp", following the example set by the "application/whoispp-query" MIME Content-Type [RFC2957].
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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 namespace.
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 non-experimental deployment.
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+--------------------+-----------------------------+ | 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.
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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 Fairhurst.
Lars Eggert is partly funded by the Trilogy Project [TRILOGY], a research project supported by the European Commission under its Seventh Framework Program.
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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, February 2005.
[RFC4340] Kohler, E., Handley, M., and S. Floyd, "Datagram Congestion Control Protocol (DCCP)", RFC 4340, March 2006.
[RFC4727] Fenner, B., "Experimental Values In IPv4, IPv6, ICMPv4, ICMPv6, UDP, and TCP Headers", RFC 4727, November 2006.
[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.
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[RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", STD 68, RFC 5234, January 2008.
[RFC5595] Fairhurst, G., "The Datagram Congestion Control Protocol (DCCP) Service Codes", RFC 5595, September 2009.
13.2. Informative References
[DNS-SD] Cheshire, S. and M. Krochmal, "DNS-Based Service Discovery", Work in Progress, February 2011.
[IGD] UPnP Forum, "Internet Gateway Device (IGD) V 1.0", November 2001.
[NAT-PMP] Cheshire, S., "NAT Port Mapping Protocol (NAT- PMP)", Work in Progress, April 2008.
[PORTREG] Internet Assigned Numbers Authority (IANA), "Service Name and Transport Protocol Port Number Registry", <http://www.iana.org/assignments/port-numbers>.
[PROTSERVREG] Internet Assigned Numbers Authority (IANA), "Protocol and Service Names Registry", <http://www.iana.org/assignments/service-names>.
[RFC0959] Postel, J. and J. Reynolds, "File Transfer Protocol", STD 9, RFC 959, October 1985.
[RFC1078] Lottor, M., "TCP port service Multiplexer (TCPMUX)", RFC 1078, November 1988.
[RFC1340] Reynolds, J. and J. Postel, "Assigned Numbers", RFC 1340, July 1992.
[RFC1700] Reynolds, J. and J. Postel, "Assigned Numbers", RFC 1700, October 1994.
[RFC2957] Daigle, L. and P. Faltstrom, "The application/ whoispp-query Content-Type", RFC 2957, October 2000.
[RFC3232] Reynolds, J., "Assigned Numbers: RFC 1700 is Replaced by an On-line Database", RFC 3232, January 2002.
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[RFC3692] Narten, T., "Assigning Experimental and Testing Numbers Considered Useful", BCP 82, RFC 3692, January 2004.
[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", <http://www.dns-sd.org/ServiceTypes.html>.
[SYSFORM] Internet Assigned Numbers Authority (IANA), "Application for System (Well Known) Port Number", <http://www.iana.org/>.
[TRILOGY] "Trilogy Project", <http://www.trilogy-project.org/>.
[USRFORM] Internet Assigned Numbers Authority (IANA), "Application for User (Registered) Port Number", <http://www.iana.org/>.
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Michelle Cotton Internet Corporation for Assigned Names and Numbers 4676 Admiralty Way, Suite 330 Marina del Rey, CA 90292 USA
Phone: +1 310 823 9358 EMail: firstname.lastname@example.org URI: http://www.iana.org/
Lars Eggert Nokia Research Center P.O. Box 407 Nokia Group 00045 Finland
Phone: +358 50 48 24461 EMail: email@example.com URI: http://research.nokia.com/people/lars_eggert/
Joe Touch USC/ISI 4676 Admiralty Way Marina del Rey, CA 90292 USA
Phone: +1 310 448 9151 EMail: firstname.lastname@example.org URI: http://www.isi.edu/touch
Magnus Westerlund Ericsson Farogatan 6 Stockholm 164 80 Sweden
Phone: +46 8 719 0000 EMail: email@example.com
Cotton, et al. Best Current Practice [Page 32] RFC 6335 Service Name and Port Number Procedures August 2011
Stuart Cheshire Apple Inc. 1 Infinite Loop Cupertino, CA 95014 USA
Phone: +1 408 974 3207 EMail: firstname.lastname@example.org URI: http://stuartcheshire.org/
Cotton, et al. Best Current Practice [Page 33]
Internet Engineering Task Force (IETF) J. Touch Request for Comments: 7605 USC/ISI BCP: 165 August 2015 Category: Best Current Practice ISSN: 2070-1721
Recommendations on Using Assigned Transport Port Numbers
This document provides recommendations to designers of application and service protocols on how to use the transport protocol port number space and when to request a port assignment from IANA. It provides designer guidance to requesters or users of port numbers on how to interact with IANA using the processes defined in RFC 6335; thus, this document complements (but does not update) that document. It provides guidelines for designers regarding how to interact with the IANA processes defined in RFC 6335, thus serving to complement (but not update) that document.
Status of This Memo
This memo documents an Internet Best Current Practice.
This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on BCPs is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at http://www.rfc-editor.org/info/rfc7605.
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Copyright (c) 2015 IETF Trust and the persons identified as the document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.
Table of Contents
1. Introduction ....................................................3 2. Conventions Used in This Document ...............................3 3. History .........................................................3 4. Current Port Number Use .........................................5 5. What is a Port Number? ..........................................5 6. Conservation ....................................................7 6.1. Guiding Principles .........................................7 6.2. Firewall and NAT Considerations ............................8 7. Considerations for Requesting Port Number Assignments ...........9 7.1. Is a port number assignment necessary? .....................9 7.2. How many assigned port numbers are necessary? .............11 7.3. Picking an Assigned Port Number ...........................12 7.4. Support for Security ......................................13 7.5. Support for Future Versions ...............................14 7.6. Transport Protocols .......................................14 7.7. When to Request an Assignment .............................16 7.8. Squatting .................................................17 7.9. Other Considerations ......................................18 8. Security Considerations ........................................18 9. IANA Considerations ............................................19 10. References ....................................................19 10.1. Normative References .....................................19 10.2. Informative References ...................................20 Acknowledgments ...................................................24 Author's Address ..................................................24
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This document provides information and advice to application and service designers on the use of assigned transport port numbers. It provides a detailed historical background of the evolution of transport port numbers and their multiple meanings. It also provides specific recommendations to designers on how to use assigned port
numbers. Note that this document provides information to potential port number applicants that complements the IANA process described in [RFC6335] (the sole document of BCP 165 before this document), but it does not change any of the port number assignment procedures described therein. Because they are thus so closely related, this document and RFC 6335 are now known together as BCP 165. This document is intended to address concerns typically raised during Expert Review (see [RFC5226]) of assigned port number applications, but it is not intended to bind those reviews. RFC 6335 also describes the interaction between port experts and port requests in IETF consensus documents. Authors of IETF consensus documents should nevertheless follow the advice in this document and can expect comment on their port requests from the port experts during IETF Last Call or at other times when review is explicitly sought.
2. Conventions Used in This Document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 [RFC2119].
In this document, these words will appear with that interpretation only when in ALL CAPS. Lowercase uses of these words are not to be interpreted as carrying significance described in RFC 2119.
In this document, the characters ">>" preceding an indented line(s) indicates a statement using the key words listed above. This convention aids reviewers in quickly identifying or finding requirements for registration and recommendations for use of port numbers in this RFC.
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
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identifiers in packets. [RFC38] provides further detail, describing how processes might have more than one of these paths and that more than one path may be active at a time. As a result, there was the need to add a process identifier to the header of each message so that incoming messages could be demultiplexed to the appropriate process. [RFC38] further suggests that 32-bit numbers be used for these identifiers. [RFC48] discusses the current notion of listening on a specific port number, but does not discuss the issue of port number determination. [RFC61] notes that the challenge of knowing the appropriate port numbers is "left to the processes" in general, but introduces the concept of a "well-known" port number for common services.
[RFC76] proposes a "telephone book" by which an index will allow port numbers to be used by name, but still assumes that both source and destination port numbers are fixed by such a system. [RFC333] proposes that a port number pair, rather than an individual port number, be used on both sides of the connection for demultiplexing messages. This is the final view in [RFC793] (and its predecessors, including [IEN112]), and brings us to their current meaning. [RFC739] introduces the notion of generic reserved port numbers for groups of protocols, such as "any private RJE server" [RFC739]. Although the overall range of such port numbers was (and remains) 16 bits, only the first 256 (high 8 bits cleared) in the range were considered assigned.
[RFC758] is the first to describe port numbers as being used for TCP (previous RFCs all refer to only NCP). It includes a list of such well-known port numbers, as well as describes ranges used for different purposes:
Decimal Octal Description ----------------------------------------------------------- 0-63 0-77 Network Wide Standard Function 64-127 100-177 Hosts Specific Functions 128-223 200-337 Reserved for Future Use 224-255 340-377 Any Experimental Function
In [RFC820], those range meanings disappear, and a single list of number assignments is presented. This is also the first time that port numbers are described as applying to a connectionless transport (e.g., UDP) rather than only connection-oriented transports.
By [RFC900], the ranges appear as decimal numbers rather than the octal ranges used previously. [RFC1340] increases this range from 0-255 to 0-1023 and begins to list TCP and UDP port number assignments individually (although the assumption was that once assigned a port number applies to all transport protocols, including
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TCP, UDP, recently Stream Control Transmission Protocol (SCTP) and Datagram Congestion Control Protocol (DCCP), as well as ISO-TP4 for a brief period in the early 1990s). [RFC1340] also establishes the Registered range of 1024-59151, though it notes that it is not controlled by the IANA (at that point). The list provided by [RFC1700] in 1994 remained the standard until it was declared replaced by an online version, as of [RFC3232] in 2002.
4. Current Port Number Use
RFC 6335 indicates three ranges of port number assignments:
Binary Hex ----------------------------------------------------------- 0-1023 0x0000-0x03FF System (also Well-Known) 1024-49151 0x0400-0xBFFF User (also Registered) 49152-65535 0xC000-0xFFFF Dynamic (also Private)
System (also Well-Known) encompasses the range 0-1023. On some systems, use of these port numbers requires privileged access, e.g., that the process run as 'root' (i.e., as a privileged user), which is why these are referred to as System port numbers. The port numbers from 1024-49151 denotes non-privileged services, known as User (also Registered), because these port numbers do not run with special privileges. Dynamic (also Private) port numbers are not assigned.
Both System and User port numbers are assigned through IANA, so both are sometimes called 'registered port numbers'. As a result, the term 'registered' is ambiguous, referring either to the entire range 0-49151 or to the User port numbers. Complicating matters further, System port numbers do not always require special (i.e., 'root') privilege. For clarity, the remainder of this document refers to the port number ranges as System, User, and Dynamic, to be consistent with IANA process [RFC6335].
5. What is a Port Number?
A port number is a 16-bit number used for two distinct purposes:
o Demultiplexing transport endpoint associations within an end host
o Identifying a service
The first purpose requires that each transport endpoint association (e.g., TCP connection or UDP pairwise association) using a given transport between a given pair of IP addresses use a different pair of port numbers, but it does not require either coordination or
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registration of port number use. It is the second purpose that drives the need for a common registry.
Consider a user wanting to run a web server. That service could run on any port number, provided that all clients knew what port number to use to access that service at that host. Such information can be explicitly distributed -- for example, by putting it in the URI:
Ultimately, the correlation of a service with a port number is an agreement between just the two endpoints of the association. A web server can run on port number 53, which might appear as DNS traffic to others but will connect to browsers that know to use port number 53 rather than 80.
As a concept, a service is the combination of ISO Layers 5-7 that represents an application-protocol capability. For example, www (port number 80) is a service that uses HTTP as an application protocol and provides access to a web server [RFC7230]. However, it is possible to use HTTP for other purposes, such as command and control. This is why some current services (HTTP, e.g.) are a bit overloaded -- they describe not only the application protocol, but a particular service.
IANA assigns port numbers so that Internet endpoints do not need pairwise, explicit coordination of the meaning of their port numbers. This is the primary reason for requesting port number assignment by IANA -- to have a common agreement between all endpoints on the Internet as to the default meaning of a port number, which provides the endpoints with a default port number for a particular protocol or service.
Port numbers are sometimes used by intermediate devices on a network path, either to monitor available services, to monitor traffic (e.g., to indicate the data contents), or to intercept traffic (to block, proxy, relay, aggregate, or otherwise process it). In each case, the intermediate device interprets traffic based on the port number. It is important to recognize that any interpretation of port numbers -- except at the endpoints -- may be incorrect, because port numbers are meaningful only at the endpoints. Further, port numbers may not be visible to these intermediate devices, such as when the transport protocol is encrypted (as in network- or link-layer tunnels) or when a packet is fragmented (in which case only the first fragment has the port number information). Such port number invisibility may interfere with these capabilities, which are implemented inside the network and based on a port number.
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Port numbers can also be used for other purposes. Assigned port numbers can simplify end-system configuration, so that individual installations do not need to coordinate their use of arbitrary port numbers. Such assignments may also have the effect of simplifying firewall management, so that a single, fixed firewall configuration can either permit or deny a service that uses the assigned ports.
It is useful to differentiate a port number from a service name. The former is a numeric value that is used directly in transport protocol headers as a demultiplexing and service identifier. The latter is primarily a user convenience, where the default map between the two is considered static and resolved using a cached index. This document focuses on the former because it is the fundamental network resource. Dynamic maps between the two, i.e., using DNS SRV records, are discussed further in Section 7.1.
Assigned port numbers are a limited resource that is globally shared by the entire Internet community. As of 2014, approximately 5850 TCP and 5570 UDP port numbers had been assigned out of a total range of 49151. As a result of past conservation, current assigned port use is small and the current rate of assignment avoids the need for transition to larger number spaces. This conservation also helps avoid the need for IANA to rely on assigned port number reclamation, which is practically impossible even though procedurally permitted [RFC6335].
IANA aims to assign only one port number per service, including variants [RFC6335], but there are other benefits to using fewer port numbers for a given service. Use of multiple assigned port numbers can make applications more fragile, especially when firewalls block a subset of those port numbers or use ports numbers to route or prioritize traffic differently. As a result:
>> Each assigned port requested MUST be justified by the applicant as an independently useful service.
6.1. Guiding Principles
This document provides recommendations for users that also help conserve assigned port number space. Again, this document does not update [RFC6335] (originally the sole document of BCP 165), which describes the IANA procedures for managing assigned transport port numbers and services, but rather augments it by now becoming part of BCP 165 (i.e., BCP 165 now refers to both documents together). Assigned port number conservation is based on a number of basic principles:
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o A single assigned port number can support different functions over separate endpoint associations, determined using in-band information. An FTP data connection can transfer binary or text files, the latter translating line-terminators, as indicated in- band over the control port number [RFC959].
o A single assigned port number can indicate the Dynamic port number(s) on which different capabilities are supported, as with passive-mode FTP [RFC959].
o Several existing services can indicate the Dynamic port number(s) on which other services are supported, such as with Multicast DNS (mDNS) and portmapper [RFC1833] [RFC6762] [RFC6763].
o Copies of some existing services can be differentiated using in- band information (e.g., URIs in the HTTP Host field and TLS Server Name Indication extension) [RFC7230] [RFC6066].
o Services requiring varying performance properties can already be supported using separate endpoint associations (connections or other associations), each configured to support the desired properties. For example, a high-speed and low-speed variant can be determined within the service using the same assigned port.
Assigned port numbers are intended to differentiate services, not variations of performance, replicas, pairwise endpoint associations, or payload types. Assigned port numbers are also a small space compared to other Internet number spaces; it is never appropriate to consume assigned port numbers to conserve larger spaces such as IP addresses, especially where copies of a service represent different endpoints.
6.2. Firewall and NAT Considerations
Ultimately, port numbers indicate services only to the endpoints, and any intermediate device that assigns meaning to a value can be incorrect. End systems might agree to run web services (HTTP) over port number 53 (typically used for DNS) rather than port number 80, at which point a firewall that blocks port number 80 but permits port number 53 would not have the desired effect. Nonetheless, assigned port numbers are often used to help configure firewalls and other port-based systems for access control.
Using Dynamic port numbers, or explicitly indicated port numbers indicated in-band over another service (such as with FTP) often complicates firewall and NAT interactions [RFC959]. FTP over firewalls often requires direct support for deep-packet inspection (to snoop for the Dynamic port number for the NAT to correctly map)
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or passive-mode FTP (in which both connections are opened from the client side).
7. Considerations for Requesting Port Number Assignments
Port numbers are assigned by IANA by a set of documented procedures [RFC6335]. The following section describes the steps users can take to help assist with responsible use of assigned port numbers and with preparing an application for a port number assignment.
7.1. Is a port number assignment necessary?
First, it is useful to consider whether a port number assignment is required. In many cases, a new number assignment may not be needed. The following questions may aid in making this determination:
o Is this really a new service or could an existing service suffice?
o Is this an experimental service [RFC3692]? If so, consider using the current experimental ports [RFC2780].
o Is this service independently useful? Some systems are composed from collections of different service capabilities, but not all component functions are useful as independent services. Port numbers are typically shared among the smallest independently useful set of functions. Different service uses or properties can be supported in separate pairwise endpoint associations after an initial negotiation, e.g., to support software decomposition.
o Can this service use a Dynamic port number that is coordinated out-of-band? For example:
o By explicit configuration of both endpoints.
o By internal mechanisms within the same host (e.g., a configuration file, indicated within a URI or using interprocess communication).
o Using information exchanged on a related service: FTP [RFC959], SIP [RFC3261], etc.
o Using an existing port discovery service: portmapper [RFC1833], mDNS [RFC6762] [RFC6763], etc.
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There are a few good examples of reasons that more directly suggest that not only is a port number assignment not necessary, but it is directly counter-indicated:
o Assigned port numbers are not intended to differentiate performance variations within the same service, e.g., high-speed versus ordinary speed. Performance variations can be supported within a single assigned port number in context of separate pairwise endpoint associations.
o Additional assigned port numbers are not intended to replicate an existing service. For example, if a device is configured to use a typical web browser, then the port number used for that service is a copy of the http service that is already assigned to port number 80 and does not warrant a new assignment. However, an automated system that happens to use HTTP framing -- but is not primarily accessed by a browser -- might be a new service. A good way to tell is to ask, "Can an unmodified client of the existing service interact with the proposed service?". If so, that service would be a copy of an existing service and would not merit a new assignment.
o Assigned port numbers not intended for intra-machine communication. Such communication can already be supported by internal mechanisms (interprocess communication, shared memory, shared files, etc.). When Internet communication within a host is desired, the server can bind to a Dynamic port that is indicated to the client using these internal mechanisms.
o Separate assigned port numbers are not intended for insecure versions of existing (or new) secure services. A service that already requires security would be made more vulnerable by having the same capability accessible without security.
Note that the converse is different, i.e., it can be useful to create a new, secure service that replicates an existing insecure service on a new port number assignment. This can be necessary when the existing service is not backward-compatible with security enhancements, such as the use of TLS [RFC5246] or DTLS [RFC6347].
o Assigned port numbers are not intended for indicating different service versions. Version differentiation should be handled in- band, e.g., using a version number at the beginning of an association (e.g., connection or other transaction). This may not be possible with legacy assignments, but all new services should incorporate support for version indication.
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Some services may not need assigned port numbers at all, e.g., SIP allows voice calls to use Dynamic ports [RFC3261]. Some systems can register services in the DNS, using SRV entries. These services can be discovered by a variety of means, including mDNS, or via direct query [RFC6762] [RFC6763]. In such cases, users can more easily request an SRV name, which are assigned first-come, first-served from a much larger namespace.
IANA assigns port numbers, but this assignment is typically used only for servers, i.e., the host that listens for incoming connections or other associations. Clients, i.e., hosts that initiate connections or other associations, typically refer to those assigned port numbers but do not need port number assignments for their endpoint.
Finally, an assigned port number is not a guarantee of exclusive use. Traffic for any service might appear on any port number, due to misconfiguration or deliberate misuse. Application and service designers are encouraged to validate traffic based on its content.
7.2. How many assigned port numbers are necessary?
As noted earlier, systems might require a single port number assignment, but rarely require multiple port numbers. There are a variety of known ways to reduce assigned port number consumption. Although some may be cumbersome or inefficient, they are nearly always preferable to consuming additional port number assignments.
Such techniques include:
o Use of a discovery service, either a shared service (mDNS) or a discovery service for a given system [RFC6762] [RFC6763].
o Multiplex packet types using in-band information, either on a per- message or per-connection basis. Such demultiplexing can even hand off different messages and connections among different processes, such as is done with FTP [RFC959].
There are some cases where NAT and firewall traversal are significantly improved by having an assigned port number. Although NAT traversal protocols supporting automatic configuration have been proposed and developed (e.g., Session Traversal Utilities for NAT (STUN) [RFC5389], Traversal Using Relays around NAT (TURN) [RFC5766], and Interactive Connectivity Establishment (ICE) [RFC5245]), not all application and service designers can rely on their presence as of yet.
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In the past, some services were assigned multiple port numbers or sometimes fairly large port ranges (e.g., X11). This occurred for a variety of reasons: port number conservation was not as widely appreciated, assignments were not as ardently reviewed, etc. This no longer reflects current practice and such assignments are not considered to constitute a precedent for future assignments.
7.3. Picking an Assigned Port Number
Given a demonstrated need for a port number assignment, the next question is how to pick the desired port number. An application for a port number assignment does not need to include a desired port number; in that case, IANA will select from those currently available.
Users should consider whether the requested port number is important. For example, would an assignment be acceptable if IANA picked the port number value? Would a TCP (or other transport protocol) port number assignment be useful by itself? If so, a port number can be assigned to a service for one transport protocol where it is already (or can be subsequently) assigned to a different service for other transport protocols.
The most critical issue in picking a number is selecting the desired range, i.e., System versus User port numbers. The distinction was intended to indicate a difference in privilege; originally, System port numbers required privileged ('root') access, while User port numbers did not. That distinction has since blurred because some current systems do not limit access control to System port numbers and because some System services have been replicated on User numbers (e.g., IRC). Even so, System port number assignments have continued at an average rate of 3-4 per year over the past 7 years (2007-2013), indicating that the desire to keep this distinction continues.
As a result, the difference between System and User port numbers needs to be treated with caution. Developers are advised to treat services as if they are always run without privilege.
Even when developers seek a System port number assignment, it may be very difficult to obtain. System port number assignment requires IETF Review or IESG Approval and justification that both User and Dynamic port number ranges are insufficient [RFC6335]. Thus, this document recommends both:
>> Developers SHOULD NOT apply for System port number assignments because the increased privilege they are intended to provide is not always enforced.
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>> System implementers SHOULD enforce the need for privilege for processes to listen on System port numbers.
At some future date, it might be useful to deprecate the distinction between System and User port numbers altogether. Services typically require elevated ('root') privileges to bind to a System port number, but many such services go to great lengths to immediately drop those privileges just after connection or other association establishment to reduce the impact of an attack using their capabilities. Such services might be more securely operated on User port numbers than on System port numbers. Further, if System port numbers were no longer assigned, as of 2014 it would cost only 180 of the 1024 System values (17%), or 180 of the overall 49152 assigned (System and User) values (<0.04%).
7.4. Support for Security
Just as a service is a way to obtain information or processing from a host over a network, a service can also be the opening through which to compromise that host. Protecting a service involves security, which includes integrity protection, source authentication, privacy, or any combination of these capabilities. Security can be provided in a number of ways, and thus:
>> New services SHOULD support security capabilities, either directly or via a content protection such as TLS [RFC5246] or Datagram TLS (DTLS) [RFC6347], or transport protection such as the TCP-AO [RFC5925]. Insecure versions of new or existing secure services SHOULD be avoided because of the new vulnerability they create.
Secure versions of legacy services that are not already security- capable via in-band negotiations can be very useful. However, there is no IETF consensus on when separate ports should be used for secure and insecure variants of the same service [RFC2595] [RFC2817] [RFC6335]. The overall preference is for use of a single port, as noted in Section 6 of this document and Section 7.2 of [RFC6335], but the appropriate approach depends on the specific characteristics of the service. As a result:
>> When requesting both secure and insecure port assignments for the same service, justification is expected for the utility and safety of each port as an independent service (Section 6). Precedent (e.g., citing other protocols that use a separate insecure port) is inadequate justification by itself.
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It's also important to recognize that port number assignment is not itself a guarantee that traffic using that number provides the corresponding service or that a given service is always offered only on its assigned port number. Port numbers are ultimately meaningful only between endpoints and any service can be run on any port. Thus:
>> Security SHOULD NOT rely on assigned port number distinctions alone; every service, whether secure or not, is likely to be attacked.
Applications for a new service that requires both a secure and insecure port may be found, on Expert Review, to be unacceptable, and may not be approved for allocation. Similarly, an application for a new port to support an insecure variant of an existing secure protocol may be found unacceptable. In both cases, the resulting security of the service in practice will be a significant consideration in the decision as to whether to assign an insecure port.
7.5. Support for Future Versions
Requests for assigned port numbers are expected to support multiple versions on the same assigned port number [RFC6335]. Versions are typically indicated in-band, either at the beginning of a connection or other association or in each protocol message.
>> Version support SHOULD be included in new services rather than relying on different port number assignments for different versions.
>> Version numbers SHOULD NOT be included in either the service name or service description, to avoid the need to make additional port number assignments for future variants of a service.
Again, the assigned port number space is far too limited to be used as an indicator of protocol version or message type. Although this has happened in the past (e.g., for NFS), it should be avoided in new requests.
7.6. Transport Protocols
IANA assigns port numbers specific to one or more transport protocols, typically UDP [RFC768] and TCP [RFC793], but also SCTP [RFC4960], DCCP [RFC4340], and any other standard transport protocol. Originally, IANA port number assignments were concurrent for both UDP and TCP, and other transports were not indicated. However, to conserve the assigned port number space and to reflect increasing use of other transports, assignments are now specific only to the transport being used.
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In general, a service should request assignments for multiple transports using the same service name and description on the same port number only when they all reflect essentially the same service. Good examples of such use are DNS and NFS, where the difference between the UDP and TCP services are specific to supporting each transport. For example, the UDP variant of a service might add sequence numbers and the TCP variant of the same service might add in-band message delimiters. This document does not describe the appropriate selection of a transport protocol for a service.
>> Service names and descriptions for multiple transport port number assignments SHOULD match only when they describe the same service, excepting only enhancements for each supported transport.
When the services differ, it may be acceptable or preferable to use the same port number, but the service names and descriptions should be different for each transport/service pair, reflecting the differences in the services. For example, if TCP is used for the basic control protocol and UDP for an alarm protocol, then the services might be "name-ctl" and "name-alarm". A common example is when TCP is used for a service and UDP is used to determine whether that service is active (e.g., via a unicast, broadcast, or multicast test message) [RFC1122]. IANA has, for several years, used the suffix "-disc" in service names to distinguish discovery services, such as are used to identify endpoints capable of a given service.
>> Names of discovery services SHOULD use an identifiable suffix; the suggestion is "-disc".
Some services are used for discovery, either in conjunction with a TCP service or as a stand-alone capability. Such services will be more reliable when using multicast rather than broadcast (over IPv4) because IP routers do not forward "all nodes" broadcasts (all 1's, i.e., 255.255.255.255 for IPv4) and have not been required to support subnet-directed broadcasts since 1999 [RFC1812] [RFC2644].
This issue is relevant only for IPv4 because IPv6 does not support broadcast.
>> UDP over IPv4 multi-host services SHOULD use multicast rather than broadcast.
Designers should be very careful in creating services over transports that do not support congestion control or error recovery, notably UDP. There are several issues that should be considered in such cases, as summarized in Table 1 in [RFC5405]. In addition, the following recommendations apply to service design:
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>> Services that use multipoint communication SHOULD be scalable and SHOULD NOT rely solely on the efficiency of multicast transmission for scalability.
>> Services SHOULD NOT use UDP as a performance enhancement over TCP, e.g., to circumnavigate TCP's congestion control.
7.7. When to Request an Assignment
Assignments are typically requested when a user has enough information to reasonably answer the questions in the IANA application. IANA applications typically take up to a few weeks to process, with some complex cases taking up to a month. The process typically involves a few exchanges between the IANA Ports Expert Review team and the applicant.
An application needs to include a description of the service, as well as to address key questions designed to help IANA determine whether the assignment is justified. The application should be complete and not refer solely to an Internet-Draft, RFC, website, or any other external documentation.
Services that are independently developed can be requested at any time, but are typically best requested in the last stages of design and initial experimentation, before any deployment has occurred that cannot easily be updated.
>> Users MUST NOT deploy implementations that use assigned port numbers prior their assignment by IANA.
>> Users MUST NOT deploy implementations that default to using the experimental System port numbers (1021 and 1022 [RFC4727]) outside a controlled environment where they can be updated with a subsequent assigned port [RFC3692].
Deployments that use unassigned port numbers before assignment complicate IANA management of the port number space. Keep in mind that this recommendation protects existing assignees, users of current services, and applicants for new assignments; it helps ensure that a desired number and service name are available when assigned. The list of currently unassigned numbers is just that -- *currently* unassigned. It does not reflect pending applications. Waiting for an official IANA assignment reduces the chance that an assignment request will conflict with another deployed service.
Applications made through Internet-Draft posting or RFC publication (in any stream) typically use a placeholder ("PORTNUM") in the text, and implementations use an experimental port number until a final
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assignment has been made [RFC6335]. That assignment is initially indicated in the IANA Considerations section of the document, which is tracked by the RFC Editor. When a document has been approved for publication, that request is forwarded to IANA for handling. IANA will make the new assignment accordingly. At that time, IANA may also request that the applicant fill out the application form on their website, e.g., when the RFC does not directly address the information expected as per [RFC6335]. "Early" assignments can be made when justified, e.g., for early interoperability testing, according to existing process [RFC7120] [RFC6335].
>> Users writing specifications SHOULD use symbolic names for port numbers and service names until an IANA assignment has been completed. Implementations SHOULD use experimental port numbers during this time, but those numbers MUST NOT be cited in documentation except as interim.
"Squatting" describes the use of a number from the assignable range in deployed software without IANA assignment for that use, regardless of whether the number has been assigned or remains available for assignment. It is hazardous because IANA cannot track such usage and thus cannot avoid making legitimate assignments that conflict with such unauthorized usage.
Such "squatted" port numbers remain unassigned, and IANA retains the right to assign them when requested by other applicants. Application and service designers are reminded that is never appropriate to use port numbers that have not been directly assigned [RFC6335]. In particular, any unassigned code from the assigned ranges will be assigned by IANA, and any conflict will be easily resolved as the protocol designer's fault once that happens (because they would not be the assignee). This may reflect in the public's judgment on the quality of their expertise and cooperation with the Internet community.
Regardless, there are numerous services that have squatted on such numbers that are in widespread use. Designers who are using such port numbers are encouraged to apply for an assignment. Note that even widespread de facto use may not justify a later IANA assignment of that value, especially if either the value has already been assigned to a legitimate applicant or if the service would not qualify for an assignment of its own accord.
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7.9. Other Considerations
As noted earlier, System port numbers should be used sparingly, and it is better to avoid them altogether. This avoids the potentially incorrect assumption that the service on such port numbers run in a privileged mode.
Assigned port numbers are not intended to be changed; this includes the corresponding service name. Once deployed, it can be very difficult to recall every implementation, so the assignment should be retained. However, in cases where the current assignee of a name or number has reasonable knowledge of the impact on such uses, and is willing to accept that impact, the name or number of an assignment can be changed [RFC6335]
Aliases, or multiple service names for the same assigned port number, are no longer considered appropriate [RFC6335].
8. Security Considerations
This document focuses on the issues arising when designing services that require new port assignments. Section 7.4 addresses the security and security-related issues of that interaction.
When designing a secure service, the use of TLS [RFC5246], DTLS [RFC6347], or TCP-AO [RFC5925] mechanisms that protect transport protocols or their contents is encouraged. It may not be possible to use IPsec [RFC4301] in similar ways because of the different relationship between IPsec and port numbers and because applications may not be aware of IPsec protections.
This document reminds application and service designers that port numbers do not protect against denial-of-service attack or guarantee that traffic should be trusted. Using assigned numbers for port filtering isn't a substitute for authentication, encryption, and integrity protection. The port number alone should not be used to avoid denial-of-service attacks or to manage firewall traffic because the use of port numbers is not regulated or validated.
The use of assigned port numbers is the antithesis of privacy because they are intended to explicitly indicate the desired application or service. Strictly, port numbers are meaningful only at the endpoints, so any interpretation elsewhere in the network can be arbitrarily incorrect. However, those numbers can also expose information about available services on a given host. This information can be used by intermediate devices to monitor and
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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.
10.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, <http://www.rfc-editor.org/info/rfc2119>.
[RFC2780] Bradner, S. and V. Paxson, "IANA Allocation Guidelines For Values In the Internet Protocol and Related Headers", BCP 37, RFC 2780, DOI 10.17487/RFC2780, March 2000, <http://www.rfc-editor.org/info/rfc2780>.
[RFC3692] Narten, T., "Assigning Experimental and Testing Numbers Considered Useful", BCP 82, RFC 3692, DOI 10.17487/RFC3692, January 2004, <http://www.rfc-editor.org/info/rfc3692>.
[RFC4727] Fenner, B., "Experimental Values In IPv4, IPv6, ICMPv4, ICMPv6, UDP, and TCP Headers", RFC 4727, DOI 10.17487/RFC4727, November 2006, <http://www.rfc-editor.org/info/rfc4727>.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security (TLS) Protocol Version 1.2", RFC 5246, DOI 10.17487/RFC5246, August 2008, <http://www.rfc-editor.org/info/rfc5246>.
[RFC5405] Eggert, L. and G. Fairhurst, "Unicast UDP Usage Guidelines for Application Designers", BCP 145, RFC 5405, DOI 10.17487/RFC5405, November 2008, <http://www.rfc-editor.org/info/rfc5405>.
[RFC5925] Touch, J., Mankin, A., and R. Bonica, "The TCP Authentication Option", RFC 5925, DOI 10.17487/RFC5925, June 2010, <http://www.rfc-editor.org/info/rfc5925>.
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[RFC6335] Cotton, M., Eggert, L., Touch, J., Westerlund, M., and S. Cheshire, "Internet Assigned Numbers Authority (IANA) Procedures for the Management of the Service Name and Transport Protocol Port Number Registry", BCP 165, RFC 6335, DOI 10.17487/RFC6335, August 2011, <http://www.rfc-editor.org/info/rfc6335>.
[RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347, January 2012, <http://www.rfc-editor.org/info/rfc6347>.
10.2. Informative References
[IEN112] Postel, J., "Transmission Control Protocol", IEN 112, August 1979.
[RFC33] Crocker, S., "New Host-Host Protocol", RFC 33, DOI 10.17487/RFC0033, February 1970, <http://www.rfc-editor.org/info/rfc33>.
[RFC37] Crocker, S., "Network Meeting Epilogue, etc", RFC 37, DOI 10.17487/RFC0037, March 1970, <http://www.rfc-editor.org/info/rfc37>.
[RFC38] Wolfe, S., "Comments on Network Protocol from NWG/RFC #36", RFC 38, DOI 10.17487/RFC0038, March 1970, <http://www.rfc-editor.org/info/rfc38>.
[RFC48] Postel, J. and S. Crocker, "Possible protocol plateau", RFC 48, DOI 10.17487/RFC0048, April 1970, <http://www.rfc-editor.org/info/rfc48>.
[RFC61] Walden, D., "Note on Interprocess Communication in a Resource Sharing Computer Network", RFC 61, DOI 10.17487/RFC0061, July 1970, <http://www.rfc-editor.org/info/rfc61>.
[RFC76] Bouknight, J., Madden, J., and G. Grossman, "Connection by name: User oriented protocol", RFC 76, DOI 10.17487/RFC0076, October 1970, <http://www.rfc-editor.org/info/rfc76>.
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[RFC333] Bressler, R., Murphy, D., and D. Walden, "Proposed experiment with a Message Switching Protocol", RFC 333, DOI 10.17487/RFC0333, May 1972, <http://www.rfc-editor.org/info/rfc333>.
[RFC739] Postel, J., "Assigned numbers", RFC 739, DOI 10.17487/RFC0739, November 1977, <http://www.rfc-editor.org/info/rfc739>.
[RFC758] Postel, J., "Assigned numbers", RFC 758, DOI 10.17487/RFC0758, August 1979, <http://www.rfc-editor.org/info/rfc758>.
[RFC768] Postel, J., "User Datagram Protocol", STD 6, RFC 768, DOI 10.17487/RFC0768, August 1980, <http://www.rfc-editor.org/info/rfc768>.
[RFC793] Postel, J., "Transmission Control Protocol", STD 7, RFC 793, DOI 10.17487/RFC0793, September 1981, <http://www.rfc-editor.org/info/rfc793>.
[RFC820] Postel, J., "Assigned numbers", RFC 820, DOI 10.17487/RFC0820, August 1982, <http://www.rfc-editor.org/info/rfc820>.
[RFC900] Reynolds, J. and J. Postel, "Assigned Numbers", RFC 900, DOI 10.17487/RFC0900, June 1984, <http://www.rfc-editor.org/info/rfc900>.
[RFC959] Postel, J. and J. Reynolds, "File Transfer Protocol", STD 9, RFC 959, DOI 10.17487/RFC0959, October 1985, <http://www.rfc-editor.org/info/rfc959>.
[RFC1122] Braden, R., Ed., "Requirements for Internet Hosts - Communication Layers", STD 3, RFC 1122, DOI 10.17487/RFC1122, October 1989, <http://www.rfc-editor.org/info/rfc1122>.
[RFC1340] Reynolds, J. and J. Postel, "Assigned Numbers", RFC 1340, DOI 10.17487/RFC1340, July 1992, <http://www.rfc-editor.org/info/rfc1340>.
[RFC1700] Reynolds, J. and J. Postel, "Assigned Numbers", RFC 1700, DOI 10.17487/RFC1700, October 1994, <http://www.rfc-editor.org/info/rfc1700>.
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[RFC1812] Baker, F., Ed., "Requirements for IP Version 4 Routers", RFC 1812, DOI 10.17487/RFC1812, June 1995, <http://www.rfc-editor.org/info/rfc1812>.
[RFC1833] Srinivasan, R., "Binding Protocols for ONC RPC Version 2", RFC 1833, DOI 10.17487/RFC1833, August 1995, <http://www.rfc-editor.org/info/rfc1833>.
[RFC2595] Newman, C., "Using TLS with IMAP, POP3 and ACAP", RFC 2595, DOI 10.17487/RFC2595, June 1999, <http://www.rfc-editor.org/info/rfc2595>.
[RFC2644] Senie, D., "Changing the Default for Directed Broadcasts in Routers", BCP 34, RFC 2644, DOI 10.17487/RFC2644, August 1999, <http://www.rfc-editor.org/info/rfc2644>.
[RFC2817] Khare, R. and S. Lawrence, "Upgrading to TLS Within HTTP/1.1", RFC 2817, DOI 10.17487/RFC2817, May 2000, <http://www.rfc-editor.org/info/rfc2817>.
[RFC3232] Reynolds, J., Ed., "Assigned Numbers: RFC 1700 is Replaced by an On-line Database", RFC 3232, DOI 10.17487/RFC3232, January 2002, <http://www.rfc-editor.org/info/rfc3232>.
[RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A., Peterson, J., Sparks, R., Handley, M., and E. Schooler, "SIP: Session Initiation Protocol", RFC 3261, DOI 10.17487/RFC3261, June 2002, <http://www.rfc-editor.org/info/rfc3261>.
[RFC4301] Kent, S. and K. Seo, "Security Architecture for the Internet Protocol", RFC 4301, DOI 10.17487/RFC4301, December 2005, <http://www.rfc-editor.org/info/rfc4301>.
[RFC4340] Kohler, E., Handley, M., and S. Floyd, "Datagram Congestion Control Protocol (DCCP)", RFC 4340, DOI 10.17487/RFC4340, March 2006, <http://www.rfc-editor.org/info/rfc4340>.
[RFC4960] Stewart, R., Ed., "Stream Control Transmission Protocol", RFC 4960, DOI 10.17487/RFC4960, September 2007, <http://www.rfc-editor.org/info/rfc4960>.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26, RFC 5226, DOI 10.17487/RFC5226, May 2008, <http://www.rfc-editor.org/info/rfc5226>.
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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, DOI 10.17487/RFC5245, April 2010, <http://www.rfc-editor.org/info/rfc5245>.
[RFC5389] Rosenberg, J., Mahy, R., Matthews, P., and D. Wing, "Session Traversal Utilities for NAT (STUN)", RFC 5389, DOI 10.17487/RFC5389, October 2008, <http://www.rfc-editor.org/info/rfc5389>.
[RFC5766] Mahy, R., Matthews, P., and J. Rosenberg, "Traversal Using Relays around NAT (TURN): Relay Extensions to Session Traversal Utilities for NAT (STUN)", RFC 5766, DOI 10.17487/RFC5766, April 2010, <http://www.rfc-editor.org/info/rfc5766>.
[RFC6066] Eastlake 3rd, D., "Transport Layer Security (TLS) Extensions: Extension Definitions", RFC 6066, DOI 10.17487/RFC6066, January 2011, <http://www.rfc-editor.org/info/rfc6066>.
[RFC6762] Cheshire, S. and M. Krochmal, "Multicast DNS", RFC 6762, DOI 10.17487/RFC6762, February 2013, <http://www.rfc-editor.org/info/rfc6762>.
[RFC6763] Cheshire, S. and M. Krochmal, "DNS-Based Service Discovery", RFC 6763, DOI 10.17487/RFC6763, February 2013, <http://www.rfc-editor.org/info/rfc6763>.
[RFC7120] Cotton, M., "Early IANA Allocation of Standards Track Code Points", BCP 100, RFC 7120, DOI 10.17487/RFC7120, January 2014, <http://www.rfc-editor.org/info/rfc7120>.
[RFC7230] Fielding, R., Ed., and J. Reschke, Ed., "Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing", RFC 7230, DOI 10.17487/RFC7230, June 2014, <http://www.rfc-editor.org/info/rfc7230>.
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This work benefited from the feedback from David Black, Lars Eggert, Gorry Fairhurst, and Eliot Lear, as well as discussions of the IETF TSVWG WG.
This document was initially prepared using 2-Word-v2.0.template.dot.
Joe Touch USC/ISI 4676 Admiralty Way Marina del Rey, CA 90292-6695 United States
Phone: +1 (310) 448-9151 Email: email@example.com
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