Multicast Requirements for a Network Address Port Translator (NAPT)

For users to accept and enjoy multicast, multicast UDP must work as seamlessly as unicast UDP. However, NATs have little consistency in multicast operation which results in inconsistant user experiences and failed multicast operation.

This document describes the behavior of a device providing multicast proxy functions as described in and that additionally functions as a Network Address and Port Translator (NAPT), as described in section 4.1.2 of . Specifically out of scope of this document are PIM-SM, IPv6, and IGMPv1. PIM is used only between routers and the IGMP Proxy devices that are scoped in this document do not function as routers. IPv6 is out of scope because NAPT is not considered necessary with IPv6. IGMPv1 is not significantly deployed on the Internet. This document does not describe how to implement multicast, IGMPv2, or IGMPv3 in an IGMP Proxy device. Rather, it provides requirements for an IGMP Proxy device so that hosts behind the NAPT can receive multicast traffic without any knowledge of the IGMP Proxy.

The primary functions of an IGMP proxy device are to collect IGMP traffic from one interface and relay it to another interface, and accept multicast traffic from that interface and route -- or replicate it -- to other interface(s).

When a NAPT isn't used, a host might be connected to the Internet in a configuration such as this:

When an IGMP Proxy device is added to such a network, its behavior is identical towards the upstream (WAN) router. Specifically, when dealing with multicast, the IGMP Proxy has the same behavior towards the WAN as if it was a host. This document is a companion document to "NAT Behavioral Requirements for Unicast UDP".

The NAPTed hosts will periodically send IGMP Report messages to indicate continued interest in receiving the multicast traffic. Per IGMPv3, the default transmission interval for the periodic Membership Report is one second. Per IGMPv2, the default transmission interval for the periodic Unsolicited Report Interval is 10 seconds. If a NAPTed host no longer sends its periodic messages within those timeframes, the NAPT device MAY consider the host no longer wants to receive the multicast traffic and can inform the upstream WAN router and close the NAPT mapping. However, it is RECOMMENDED that the NAPT wait until 3 missing unsolicited reports (to account for packet loss on the LAN, especially wireless LANs), or that the NAPT first query the host using IGMPv2 or IGMPv3. In addition to the above requirements, the NAPT device MUST also: follow the RFC2119 requirements of , and; have "NAT Inbound Refresh Behavior", as described in (see ), and; extend the Mapping Refresh Timer, as described in , to 60 minutes for UDP packets originating from hosts receiving any multicast stream that are sent to UDP ports above 1023 (see ).

Multicast traffic arrives only outside-to-inside. Thus, a NAPT needs to also meet REQ-6a of NAT UDP requirements because multicast traffic typically only flows ouside-to-inside.

RTP uses the source transport address (source IP address and source UDP port), in addition to the the RTP/RTCP SSRC value, to identify session members. If a session member sees the same SSRC arrive from a different transport address, that session member will perform RTP collision detection (section 8.2 of ). If a NAPT followed the requirements of (and timed out a UDP session after 2 minutes of inactivity) and the multicast group is sufficiently large (approximately 300 members with a normal 50kbps audio RTP stream), the elapsed time between a NAPTed host sending its RTCP Receiver Reports would exceed 2 minutes, causing an unnecessary RTP collision detection to be performed by other session members. To prevent this unnecessary RTP collision detection by other session members, the other session members need to see the same source transport address for the RTP and RTCP traffic from the NAPTed host. This requires the NAPT not assign a new UDP source port for that traffic. A NAPT is unable to associate a received multicast session with its unicasted RTCP Receiver Reports. Thus, this document requires the NAPT to extend its UDP mapping refresh timer. This requirement also facilitates other, non-RTP multicast applications. This requirement applies to ports above 1023 because RTP and RTCP are only used on ports above 1023. Other, non-RTP multicast feedback protocols are also expected to use ports above 1023. If a NAPT has exhausted its resources, the NAPT MAY time out a mapping before 60 minutes have elapsed. However, a NAPT is still required to follow the minimum mapping duration of in order to comply with that specification.

Compliance with this specification does not increase security risks beyond those already discussed in the Security Considerations section of IGMPv3 and IGMP/MLD Proxying.

This document does not require any IANA registrations.

Thanks to Bryan McLaughlin and Yiqun Cai for their assistance in writing this document.