Multicast Requirements for a Network Address Port Translator (NAPT)

In order for multicast applications to function well over NATs, multicast UDP must work as seamlessly as unicast UDP. However, NATs have little consistency in multicast operation which results in inconsistent user experiences and failed multicast operation.

This document describes how an IGMP/MLD proxy device can NAPT multicast traffic so that existing any-source multicast applications and single-source multicast applications can function without awareness of NAPT device. Such a NAPT device will send and receive IGMP membership reports and queries. When it does so, it functions exactly like a non-NAPTted IGMP/MLD Proxy . This is because IGMP packets are link local, and for IGMPv3 are not forwarded (but rather aggregated) by the device's IGMP/MLD Proxying function. For example, when the IGMPv3 proxying device receives an IGMP membership on an inside interface, it creates its own IGMP proxying membership state and its own IGMP/MLD forwarding table. It then creates an independent IGMP membership report on its outside interface reporting the multicast groups/channels -- but there is no direct relationship or "relaying" of IGMP membership reports or queries across the interfaces. The NAPT device will subsequently receive a multicast data packet on the outside ('public') interface and forward the multicast packet to inside ('internal') interfaces based on its IGMP/MLD forwarding table. As with normal NAPT operation for unicast flows, multicast packets received from the outside interface and forwarded to the inside interface do not have their source IP addresses changed. Such multicast packets do not need to have their destination IP address changed (unless the NAT device wishes to establish septate multicast domains, but this is not the typical operation). As with normal NAPT operation for unicast flows, multicast packets received from an inside interface and forwarded to the public interface do have their source IP address changed to the public IP interface of the NAPT and their source UDP port changed to a port selected by the NAPT. This NAPT is commonly called a 'mapping'.

This document describes the behavior of a device providing any-source multicast or source-specific multicast proxy functions as described in using IGMPv1, or IGMPv2, or IGMPv3 and that additionally functions as a Network Address and Port Translator (NAPT), as described in section 4.1.2 of . Out of scope of this document are PIM-SM and IPv6. The IGMP Proxy devices that are scoped in this document do not forward PIM-SM. IPv6 is out of scope because NAPT is not considered necessary with IPv6. This document is a companion document to "NAT Behavioral Requirements for Unicast UDP".

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

The primary functions of an IGMP proxy device are to collect IGMP traffic from the 'inside' interface, aggregate them, and transmit the aggregated IGMP membership information to the 'outside' interface. Additionally, multicast traffic is accepted from the 'inside' and (more typically) from the 'outside' interface and routed to the other interface(s) based on forwarding state established by the IGMP/MLP proxy routing. Packets with a multicast destination IP address do not have their destination IP address changed by a NAPT. However, their source IP address and source UDP port is changed if the packet goes from an 'inside' interface of a NAPT to the 'outside' interface of a NAPT -- similar to the behavior of a unicast packet across those same interfaces.

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.

A device compliant with this specification MUST implement .

If a NAPT supports only IGMPv1 or IGMPv2 (but not IGMPv2), it MAY simply receive IGMP membership reports on the inside interface, NAT them, and relay the IGMP membership report. That is, the NAPT does not need to do any aggregation of IGMP messages.

If a NAPT supports IGMPv3, the NAPT MUST aggregate the IGMPv3 membership reports, and report the aggregated information upstream. Failure to do this aggregation will cause undesired temporary blackholing of multicast traffic. For example, consider two hosts behind the same NAPT. If one host is joining a session at the same time another is lesaving the session, and the NAPT merely relays the join and leave upstream, the session will be terminated and the join and leave announcements do not comply with section 5 of .

Any-sosurce multicast (ASM) uses the IP addresses in the 224.0.0.0 - 239.255.255.255 range . If a host on the inside interface of a NAPT belongs to an any-source multicast host group and sends a UDP packet to the same group, the NAPT MUST have a UDP mapping timer of 60 minutes for that mapping. This UDP mapping SHOULD be destroyed when the host leaves that host group. If a NAPT has exhausted its resources, the NAPT MAY time out that mapping before 60 minutes have elapsed, but this is discouraged.Note that even in a situation with resource exhaustion, a NAPT is still required to follow the minimum mapping duration of 2 minutes (REQ-5 of ). Discussion: If a NAPT merely followed the requirements of [RFC4787] it could cause problems for RTP. RTP uses the source transport address (source IP address and source UDP port) and 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 merely followed the requirements of and timed out a UDP session after 2 minutes of inactivity and RTCP receiver reports are sent less often than every 2 minutes, RTP collision detection would be performed by other session members sharing the same SSRC, complicating diagnostic tools and potentially interfering with jitter buffer algorithms. This situation can occur, for example, with a multicast group of approximately 300 members with a normal 50kbps audio RTP stream. 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 to assign the same UDP source port for that RTCP traffic. This requirement also facilitates other, non-RTP multicast applications which may function similarly.

Source-specific multicast uses the IP addresses in the 232/8 (232.0.0.0 to 232.255.255.255) range . When implementing a NAPT that operates with Source-Specific Multicast, the NAPT MUST: implement IGMPv3 according to section 4.2 of , and; follow the requirements in section of this document. Primarily due to NAPTs functioning as IGMP/MLD proxies with multiple receivers behind the NAT, multicast applications are encouraged to use identifiers, rather than IP addresses and UDP ports, to identify specific multicast receivers (e.g., encourages SSM applications to not rely exclusively on transport addresses for collision detection). As compared to any-source multicast, the use of such receiver identifiers removes the need for the NAT to have long mapping timers; instead, the timers in are used when a host transmits to a single-source multicast address.

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 Yiqun Cai, Stephen Casner, Toerless Eckert, Alfred Hines, Marcus Maranhao, Bryan McLaughlin, and Magnus Westerlund for their assistance in writing this document.