An IPv4 - IPv6 multicast translator
draft-venaas-behave-mcast46-00.txt

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Abstract

This document describes an IPv4 - IPv6 translator device that embeds all IPv4 multicast group addresses into IPv6, and allows IPv6 hosts to receive from and send to IPv4 multicast groups.

Table of Contents

1.  Introduction
2.  Embedding IPv4 multicast group addresses into IPv6
3.  Architecture
4.  Address rewriting
5.  Examples
    5.1.  IPv6 host joining a group inside the /96 prefix
    5.2.  IPv6 host sending to group inside the /96 prefix
6.  Acknowledgments
7.  Security Considerations
8.  Normative References
§  Author's Address
§  Intellectual Property and Copyright Statements

1.  Introduction

IPv4 and IPv6 will co-exist for many years, possibly decades. There are several solutions for how IPv4 and IPv6 hosts and networks can inter-operate. This is usually easy if a host is dual stack. If however an IPv6-only host needs to communicate with an IPv4-only host, then somewhere along the data path there must be some form of translation. There are several ways of doing this for unicast, but not much work has been done on multicast.

Here we describe a multicast translator solution. This translator could be placed at the border between IPv6-only and IPv4-only networks to allow multicast access between them, or it may also be placed in a dual-stack network, where it can support hosts or other networks that are IPv6-only or IPv4-only. The goal is to give an IPv6 host full access to send to and receive from any IPv4 multicast group by using the usual IPv6 multicast protocols and applications which will then operate on the respective IPv6 groups. It should also allow this for multiple hosts. Multiple IPv4 hosts should be able to use a single IPv4 group, multiple IPv6 hosts a corresponding IPv6 group, and all hosts should be able to send to and receive from all the others. Similar to hosts using the same group from the same address family. The translator solution should work with no changes to other infrastructure.

We will define a one-to-one mapping of multicast IPv4 addresses onto a subset of the IPv6 multicast addresses. An IPv6 host will then be able to receive data from any IPv4 multicast group by joining the corresponding IPv6 group. An IPv6 host can also send, without necessarily joining, to any IPv4 multicast group by sending to the corresponding IPv6 group. Some way of translating unicast addresses is also needed to translate addresses of multicast sources.

2.  Embedding IPv4 multicast group addresses into IPv6

We need a way of referring to an IPv4 multicast group using an IPv6 address. One could embed IPv4 multicast addresses into IPv6 by simply prepending them with a specific /96 IPv6 prefix such that for each IPv4 multicast address we have a respective IPv6 multicast address. However, both IPv4 and IPv6 have special ranges for SSM usage, and one might want to take scoping into account. We suggest using one specific /96 IPv6 SSM prefix for all IPv4 SSM addresses, and one specific /96 IPv6 ASM (non-SSM) prefix for all IPv4 ASM (non-SSM) addresses.

An administrator may choose the exact prefixes used, and depending on the prefix, also which IPv6 scope. The prefix must be in accordance with the IPv6 multicast address format defined in section 2.7 of [1] (Hinden, R. and S. Deering, “IP Version 6 Addressing Architecture,” February 2006.). The addresses used will then be of the form FFxx:<blah>:<IPv4> where flags, scope and the value of "blah" are chosen by the administrator. "IPv4" is the last 32 bits specifying the IPv4 address of the IPv4 multicast group. For ASM it may be useful to use an Embedded-RP [2] (Savola, P. and B. Haberman, “Embedding the Rendezvous Point (RP) Address in an IPv6 Multicast Address,” November 2004.) prefix based on an IPv6 unicast address of the translator.

The unicast addresses of multicast sources also need to be translated. We recommend embedding all IPv4 unicast addresses into a /96 IPv6 prefix. This allows different IPv4 unicast addresses to be mapped to different IPv6 unicast addresses, and for IPv6 SSM joins to address specific IPv4 SSM sources. Note that for ASM use, it may be sufficient to map all IPv4 sources to one single IPv6 address. For translating IPv6 sources into IPv4 sources, one may use a single address, or a pool of IPv4 addresses. The same IPv4 address may need to be re-used for different IPv6 sources. If the translator also translates unicast packets, then it should use the same unicast translation mechanism for source addresses in multicast packets. Due to multicast RPF checks, the IPv4 and IPv6 unicast addresses used need to be routed towards the translator.

3.  Architecture

We propose that the translator makes use of PIM-SM (Sparse Mode) [3] (Fenner, B., Handley, M., Holbrook, H., and I. Kouvelas, “Protocol Independent Multicast - Sparse Mode (PIM-SM): Protocol Specification (Revised),” August 2006.) for IPv6. For ASM it should then be the RP for the /96 IPv6 prefix used for ASM. This allows the translator to know which IPv4 groups the IPv6 hosts join, and also to learn of IPv6 sources for those groups. It is sufficient to support MLD if there are no IPv6 PIM neighbors (e.g. a single link or MLD proxies).

With respect to the IPv4 network, it may be sufficient to behave as an IPv4 multicast host. When it receives a PIM or MLD join for a new IPv6 group corresponding to some IPv4 multicast group, x, it simply joins the IPv4 multicast group. If it learns of an IPv6 source for IPv6 group corresponding to some IPv4 multicast group, it will send the IPv6 packets to the IPv4 group. As an RP, it may receive IPv6 PIM registers, it may then as a regular IPv6 RP, join towards the source to receive packets natively. If it is an IPv4 host, it will not know whether there are IPv4 receivers, and hence it must alway do this.

One can improve on this by making the translator behave as an IPv4 RP, or be an IPv4 PIM router running MSDP to exchange information about active IPv4 sources. The translator can then use MSDP to signal its active IPv4 sources (that may be translated IPv6 sources) so that it will receive PIM joins if there are IPv4 receivers for the groups. It can also use MSDP to see if there are IPv4 sources for IPv4 groups that IPv6 hosts have joined.

Note that for SSM this is much simpler with no RP nor MSDP involved. It may still be an advantage to act as an IPv4 PIM router, in order to only do translation from IPv6 to IPv4 when there are IPv4 listeners.

4.  Address rewriting

When IPv4 packets are resent as IPv6 we will need to replace the source and destination addresses with suitable IPv6 addresses. And similar replacement going from IPv6 to IPv4.

The destination address is easy. That is the multicast address. As described above, we map IPv4 multicast addresses into IPv6 by prepending them with a /96 prefix, using different prefixes for SSM and ASM. Going the other direction, we simply extract the last 32 bits.

For the source addresses we propose a similar mapping from IPv4 to IPv6, using some /96 unicast prefix. In the other direction we suggest having a pool of IPv4 addresses (possibly just a single address) that is used for all IPv6 multicast translated to IPv4. If unicast traffic is translated, then similar translation should be used for the multicast source addresses. Note that for RTP the application can know the real source and tell streams apart, even if they are translated into the same multicast source address.

One could consider using just a single IPv6 unicast address for all IPv4 multicast translated into IPv6. For ASM it has the same issues as using a single IPv4 unicast address for translating into IPv4. However, for SSM one would like an IPv6 SSM join to uniquely specify a corresponding IPv4 SSM join. In order to do this, the simplest is what we propose above with a /96 prefix used for all IPv4 unicast addresses.

5.  Examples

To illustrate how the translator works, we will look at two examples. In both examples we assume that there is no previous state in the translator.

5.1.  IPv6 host joining a group inside the /96 prefix

An IPv6 host joins the group FFxx:<blah>:a.b.c.d. If the translator is the DR for the host, it will receive an MLD membership report. If not, it will receive a PIM join since it is the RP for the group. The translator will then get (*, G) state for the group. So far this is normal PIM behaviour. The translator checks whether the address is inside the /96 prefix, and whether the last 32 bits (a.b.c.d) is an IPv4 multicast address. If it is, it joins a.b.c.d using IGMP, and stays joined as long as it has state for the group.

For SSM the translator would in addition check if the source in the join is inside the /96 unicast prefix used. If this is the case, it then uses the last 32 bits as the IPv4 source. It can then do a source-specific IPv4 join.

When the translator receives a multicast packet for a.b.c.d it prepends the /96 prefix to form the IPv6 address FFxx:<blah>:a.b.c.d. If the translator has outgoing interfaces for this group, it will send an IPv6 packet to the same interfaces to which it would have forwarded an IPv6 packet for the group. The destination address will be FFxx:<blah>:a.b.c.d, and the source address will be computed using the /96 unicast prefix. For SSM, the translator would also check that it got an outgoing interface for the specific source.

5.2.  IPv6 host sending to group inside the /96 prefix

An IPv6 host sends to the group FFxx:<blah>:a.b.c.d. If the translator is the DR for the host, it will receive the data natively. If not, it will receive PIM register messages containing the data since it is the RP. For each packet received, either natively or inside register messages, it will first check that the destination address is inside the /96 prefix and that the last 32 bits (a.b.c.d) is an IPv4 multicast address. If this is okay, it will resend the packet to the IPv4 address a.b.c.d. The source address would be chosen from a given pool of IPv4 unicast addresses (this may just be a single fixed address).

If the translator is also an IPv4 PIM router, then we do some further steps. For ASM, if the translator is an RP and uses MSDP, it should announce the translated source in MSDP, and only forward translated packets if it has a join for the group. For SSM, it should only forward translated packets if it has a join for the specific source and group.

6.  Acknowledgments

The author wishes to thank Michal Przybylski and Pekka Savola for valuable comments, and also people from the M6Bone community for testing a prototype implementation.

7.  Security Considerations

When using such a translator one needs to take some care of scoping and TTL values. Due to differences in IPv4 and IPv6 scoping, a narrow scope might be translated into a wider one.

One may wish to limit who can access the translator. If for instance one wishes to restrict it to a site, one can use a /96 prefix of site-local scope, and then filter at the site border, just like one would for multicast in general. A translator implementation could also offer a way of restricting which groups and sources should be accepted.

8. Normative References

Author's Address

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