The Multicast Application Port
draft-ietf-intarea-multicast-application-port-07

Network Working Group                                        N. Karstens
Internet-Draft                                                    Garmin
Intended status: Standards Track                             S. Cheshire
Expires: 15 December 2026                                     Apple Inc.
                                                              M. McBride
                                                               Futurewei
                                                            13 June 2026

                     The Multicast Application Port
            draft-ietf-intarea-multicast-application-port-07

Abstract

   This document discusses the drawbacks of the current practice of
   assigning a UDP port to each multicast application.  Such assignments
   are redundant because the multicast address already uniquely
   identifies the data.  The document assigns a UDP port specifically
   for use with multicast applications and lists requirements for using
   this port.  This approach provides immediate compatibility with
   existing protocol stacks, while also requiring improvements to make
   the port easier to use.

Status of This Memo

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   This Internet-Draft will expire on 15 December 2026.

Copyright Notice

   Copyright (c) 2026 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 (https://trustee.ietf.org/
   license-info) in effect on the date of publication of this document.

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   Please review these documents carefully, as they describe your rights
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   extracted from this document must include Revised BSD License text as
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   3
   2.  Assignment  . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Host Requirements . . . . . . . . . . . . . . . . . . . . . .   4
   4.  Application Requirements  . . . . . . . . . . . . . . . . . .   4
   5.  Firewall Considerations . . . . . . . . . . . . . . . . . . .   4
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
   8.  Acknowledgement . . . . . . . . . . . . . . . . . . . . . . .   6
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .   7
     9.2.  Informative References  . . . . . . . . . . . . . . . . .   7
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   8

1.  Introduction

   The Internet community has recognized the need to be judicious when
   assigning port numbers (see [RFC7605], Section 6).  With unicast
   applications, the need for explicit port assignment has been reduced
   by techniques such as locally assigning a dynamic port, combined with
   some mechanism for advertising that port (see [RFC7605],
   Section 7.1).  Dynamic assignment does not work with multicast
   applications because it is impossible to guarantee that the port
   remains unused by all hosts that may want to join a given multicast
   group.  The result is that each multicast application-layer protocol
   has had to have its own dedicated port assignment.  Even worse, each
   different use of that multicast application-layer protocol has had to
   have a different unique port assigned.

   In the TCP/IP model, the port number in the transport layer
   multiplexes applications within a host (see [RFC1122], Section 4.1.1
   and [RFC7605], Section 5).  With Any-Source Multicast (ASM; see
   [RFC1112] and [RFC4607]), the use of a port number to multiplex
   applications is unnecessary because the destination multicast address
   already provides a unique identifier for the application.  The same
   applies to Source-Specific Multicast (SSM; see [RFC4607]) if both
   source unicast address and destination multicast address are
   considered.

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   Because of the desire to conserve port numbers and the fact that a
   port is not necessary to multiplex multicast applications, this
   document assigns a UDP port that may be used with multicast
   applications: the Multicast Application Port.

   Assigning a UDP port for multicast applications (as opposed to other
   methods) provides immediate compatibility with existing network
   protocol stacks.  Section 3 contains requirements that facilitate use
   of the port on a given platform, but incorporating these requirements
   into existing platforms is expected to be a gradual process.

   The Multicast Application Port can be used with both IPv6 and IPv4.

   Use of this port is optional because there may be circumstances where
   assigning a port is preferred, such as when participants cannot meet
   the requirements in Section 3 and Section 4, or when a firewall
   blocks a traffic pattern used by the application (see Section 5).

   An application sending multicast messages using the Multicast
   Application Port as the destination port MAY also send unicast
   messages using a different destination port.  One possible benefit of
   doing so is to balance out deficiencies related to multicast
   distribution (see [RFC9119], for example).

1.1.  Requirements Language

   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 BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

2.  Assignment

   This document _requests_ assignment of UDP port 8738 (0x2222) and
   gives it the service name "mcast-app-port".

   Applications using the Multicast Application Port are identified by
   the destination multicast address (ASM) or by the combination of
   source unicast address and destination multicast address (SSM).  As
   such, the Multicast Application Port MUST NOT be used as either a
   source port or destination port for any non-multicast messages.

   The Multicast Application Port MAY be used as a source port by an
   application that exclusively uses multicast messages.

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   If any application messages are unicast, then it is RECOMMENDED that
   the source port used for multicast messages be set to the port that
   receivers can send unicast replies to.  Note that such arrangements
   would likely require receivers to use multiple sockets, as the
   application would need to bind to multiple ports.

3.  Host Requirements

   Hosts SHALL require applications using this port to use it non-
   exclusively.  In practice, this means hosts using POSIX-like socket
   APIs would require applications to set the SO_REUSEADDR and/or
   SO_REUSEPORT socket options before binding the socket [POSIX].  This
   ensures that applications developed on a conformant host will also
   work on a non-conformant host.

   Hosts SHALL prevent use of the port with the wildcard address (see
   [RFC3493], Section 3.8) by having the socket bind operation return an
   error code.

   Hosts SHALL prevent applications from sending non-multicast packets
   using the Multicast Application Port as either a source or
   destination port by having the send operation return an error code.

   Hosts SHALL discard all incoming, non-multicast packets that use the
   Multicast Application Port as either a source or destination port.

4.  Application Requirements

   Applications running on non-conformant hosts can ensure compatibility
   with conformant hosts by meeting the requirements in this section.

   Applications running on a non-conformant host SHALL NOT prevent other
   applications from using this port.  In practice, this means that
   applications using POSIX-like socket APIs would enable the
   SO_REUSEADDR and/or SO_REUSEPORT socket options before binding the
   socket [POSIX].

   Applications running on a non-conformant host SHALL discard all
   datagrams received on the Multicast Application Port that do not have
   the multicast address used by the application.

5.  Firewall Considerations

   Many network firewalls (which operate on network infrastructure) and
   host firewalls (which operate on individual hosts) are configured to
   accept incoming messages as long as there was first an outgoing
   message using the same set of ports (see [RFC7288] for more
   discussion).  Consider the following sequence of messages:

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   1.  (Multicast) Host A to group containing Host B
       Source Port: 50000 (Dynamic)
       Dest Port: 8738

   2.  (Unicast) Host B to Host A
       Source Port: 60000 (Dynamic)
       Dest Port: 50000

   3.  (Unicast) Host A to Host B
       Source Port: 50000
       Dest Port: 60000

   A host firewall running on Host A (or a network firewall between Host
   A and Host B) may block Message 2 because it uses a different set of
   ports than Message 1 uses.

   While a firewall could be configured to accept Message 2, this
   configuration would have to be automated because the use of dynamic
   ports makes manual configuration too costly even in small
   deployments.  However, this automation leads to unacceptable security
   risks.  For example, if a firewall were to respond to Message 1 by
   opening up the source port used in the message, a malicious
   application running on the host could open large holes in the
   firewall by repeatedly sending variations of Message 1 using
   different source ports.

   Host firewalls could be configured to allow messages associated with
   a given application (instead of specific ports), but this approach
   would not work with a network firewall.

   Ultimately, the current practice of assigning a multicast application
   its own port is more practical for any application that uses both
   multicast and unicast messages and could be deployed in an
   environment using firewalls.  Firewall configuration with an assigned
   port is straightforward: allow all messages using that port.

6.  Security Considerations

   Firewall rules referencing ports are typically intended to match
   specific applications.  Because applications using the Multicast
   Application Port are identified by both port and destination
   multicast address, rules referencing the Multicast Application Port
   SHOULD also consider the destination multicast address.

   Applications running on non-conformant hosts are vulnerable to a
   denial of service attack if another application claims exclusive
   access to the port.

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   Systems that use POSIX-like socket APIs typically have restrictions
   on binding multiple sockets to the same port.  This can serve as a
   rudimentary security mechanism in that other local applications
   cannot eavesdrop on the multicast stream.  A necessary side-effect of
   using the Multicast Application Port is that applications can no
   longer rely on these security mechanisms.  These applications may
   want to incorporate additional security measures into their protocol.
   Note that the problem of local eavesdropping is typically no worse
   than eavesdropping in-flight, so it is likely that both attack
   vectors can be resolved by the same security measure.

7.  IANA Considerations

   IANA is _requested_ to assign the following port:

   Service Name        mcast-app-port
   Transport Protocol  UDP
   Assignee            IESG <iesg@ietf.org>
   Contact             IETF Chair <chair@ietf.org>
   Description         Multicast Application Port
   Reference           This document
   Port Number         8738

   IANA is requested to update its "Application for Service Names and
   User Port Numbers" [IANA-APP] to reference this document and ask the
   applicant if the Multicast Application Port may be used.

8.  Acknowledgement

   Special thanks to the National Marine Electronics Association for
   their contributions in developing marine industry standards and their
   support for this research.

   The authors are grateful to the members of the PIM and INT-AREA
   working groups for their review of this draft, and to the following
   individuals specifically:

   *  Dr. Joe Touch for consulting on port assignment

   *  Lorenzo Colitti for his suggestions for host requirements

   *  David Schinazi for pointing out likely port conflicts with several
      major OSes

   *  Juliusz Chroboczek for suggestions on the source port used for
      unicast

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   *  Dave Thaler for insightful discussion on how the Multicast
      Application Port would operate in the presence of a firewall

9.  References

9.1.  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,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

9.2.  Informative References

   [IANA-APP] Internet Assigned Numbers Authority, "Application for
              Service Names and User Port Numbers",
              <https://www.iana.org/form/ports-services>.

   [POSIX]    The Open Group, ""The Open Group Base Specifications",
              Issue 7, 2018 edition", December 2001,
              <https://pubs.opengroup.org/onlinepubs/9699919799/>.

   [RFC1112]  Deering, S., "Host extensions for IP multicasting", STD 5,
              RFC 1112, DOI 10.17487/RFC1112, August 1989,
              <https://www.rfc-editor.org/info/rfc1112>.

   [RFC1122]  Braden, R., Ed., "Requirements for Internet Hosts -
              Communication Layers", STD 3, RFC 1122,
              DOI 10.17487/RFC1122, October 1989,
              <https://www.rfc-editor.org/info/rfc1122>.

   [RFC3493]  Gilligan, R., Thomson, S., Bound, J., McCann, J., and W.
              Stevens, "Basic Socket Interface Extensions for IPv6",
              RFC 3493, DOI 10.17487/RFC3493, March 2003,
              <https://www.rfc-editor.org/info/rfc3493>.

   [RFC4607]  Holbrook, H. and B. Cain, "Source-Specific Multicast for
              IP", RFC 4607, DOI 10.17487/RFC4607, August 2006,
              <https://www.rfc-editor.org/info/rfc4607>.

   [RFC7288]  Thaler, D., "Reflections on Host Firewalls", RFC 7288,
              DOI 10.17487/RFC7288, June 2014,
              <https://www.rfc-editor.org/info/rfc7288>.

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   [RFC7605]  Touch, J., "Recommendations on Using Assigned Transport
              Port Numbers", BCP 165, RFC 7605, DOI 10.17487/RFC7605,
              August 2015, <https://www.rfc-editor.org/info/rfc7605>.

   [RFC9119]  Perkins, C., McBride, M., Stanley, D., Kumari, W., and JC.
              Zúñiga, "Multicast Considerations over IEEE 802 Wireless
              Media", RFC 9119, DOI 10.17487/RFC9119, October 2021,
              <https://www.rfc-editor.org/info/rfc9119>.

Authors' Addresses

   Nate Karstens
   Garmin International, Inc.
   1200 E. 151st St.
   Olathe, KS 66062-3426
   United States of America
   Email: nate.karstens@gmail.com

   Stuart Cheshire
   Apple Inc.
   Email: cheshire@apple.com

   Mike McBride
   Futurewei
   United States of America
   Email: michael.mcbride@futurewei.com

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