The Multicast Application Port

The Internet community has recognized the need to be judicious when assigning port numbers (see ). 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 ). 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 and ). With Any-Source Multicast, 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 if both source address and destination multicast address are considered. 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. 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 and .

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 when, and only when, they appear in all capitals, as shown here.

This document REQUESTS assignment of UDP port 49149 and gives it the service name "Multicast Application Port". This port is near the end of the range of User ports to differentiate it from other port assignments. The REQUESTED port may be used as a source port if the application exclusively uses multicast messages. If any application messages are unicast, then a dynamic port should be used as the source port. This allows receivers to know which port to send replies to.

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 . This ensures that applications developed on a conformant host will also work on a non-conformant host. Hosts shall discard all incoming, non-multicast packets that use this destination port.

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 . Applications running on a non-conformant host shall discard all datagrams that do not have the multicast address used by the application.

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

IANA is REQUESTED to assign the following port: Contact IETF Chair Description Multicast data where the application is identified by the multicast address Reference This document Port Number 49149]]> IANA is requested to update its "Application for Service Names and User Port Numbers" to reference this document, ask if a Multicast Application Port may be used, and require an explanation if not.

Special thanks to the National Marine Electronics Association for their contributions in developing marine industry standards and their support for this research. Thanks also to the members of the PIM and INT-AREA working groups for their review of this draft and to Dr. Joe Touch for consulting on port assignment.