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Versions: 00 01 02 03

Network Working Group                                          S. Venaas
Internet-Draft                                             cisco Systems
Intended status: Informational                              July 2, 2009
Expires: January 3, 2010


             Framework for IPv4/IPv6 Multicast Translation
              draft-venaas-behave-v4v6mc-framework-00.txt

Status of this Memo

   This Internet-Draft is submitted to IETF in full conformance with the
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   This Internet-Draft will expire on January 3, 2010.

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Abstract

   This draft describes how IPv4/IPv6 multicast translation may be used
   in various scenarios and attempts to be a framework for possible
   solutions.  This can be seen as a companion document to the draft
   "Framework for IPv4/IPv6 translation" by Baker et al.  When
   considering scenarios and solutions for unicast translation, one
   should also see how they may be extended to provide multicast
   translation.


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Translation scenarios  . . . . . . . . . . . . . . . . . . . .  4
     2.1.  An IPv6 network receiving multicast from IPv4 Internet . .  5
     2.2.  IPv6 Internet receiving multicast from an IPv4 network . .  5
     2.3.  An IPv4 network receiving multicast from IPv6 Internet . .  6
     2.4.  IPv4 Internet receiving multicast from an IPv6 network . .  6
     2.5.  An IPv6 network receiving multicast from an IPv4
           network  . . . . . . . . . . . . . . . . . . . . . . . . .  7
     2.6.  An IPv4 network receiving multicast from an IPv6
           network  . . . . . . . . . . . . . . . . . . . . . . . . .  8
   3.  Framework  . . . . . . . . . . . . . . . . . . . . . . . . . .  9
     3.1.  Routing  . . . . . . . . . . . . . . . . . . . . . . . . .  9
       3.1.1.  Translation with PIM and SSM . . . . . . . . . . . . .  9
       3.1.2.  Translation with PIM and ASM . . . . . . . . . . . . .  9
       3.1.3.  Translation with IGMP/MLD  . . . . . . . . . . . . . . 10
     3.2.  Application layer issues . . . . . . . . . . . . . . . . . 10
   4.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 12
   5.  Security Considerations  . . . . . . . . . . . . . . . . . . . 13
   6.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 14
   7.  Informative References . . . . . . . . . . . . . . . . . . . . 15
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 16

















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1.  Introduction

   There will be a long period of time where IPv4 and IPv6 systems and
   networks need to coexist.  There are various solutions for how this
   can be done for unicast, some of which are based on translation.  The
   document [I-D.baker-behave-v4v6-framework] discusses the needs and
   provides a framework for unicast translation for various scenarios.
   Here we discuss the need for multicast translation for those
   scenarios.

   For unicast the problem is basically how two hosts can communicate
   when they are not able to use the same IP protocol.  For multicast we
   can restrict ourselves to looking at how a single source can
   efficiently send to multiple receivers.  When using a single IP
   protocol one builds a multicast distribution tree from the source to
   the receivers, and independent of the number of receivers, one sends
   each data packet only once on each link.  We wish to maintain the
   same characteristics when there are different IP protocols used.
   That is, when the nodes of the tree (source, receivers and routers)
   cannot all use the same IP protocol.  In general there may be
   multiple sources sending to a multicast group, but that can be
   thought of as separate trees, one per source.  We will focus on the
   case where the source and the receivers cannot all use the same IP
   protocol.  If the issue is the network in between, encapsulation may
   be a better alternative.  Note that if the source supports both IPv4
   and IPv6, then one alternative could be for the source to send two
   streams.  This need not be the same host.  There could be two
   different hosts, and in different locations/networks, sending the
   same content.






















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2.  Translation scenarios

   We will consider six different translation scenarios.  For each of
   the scenarios we will look at how host in one network can receive
   multicast from a source in another network.  For unicast one might
   consider the following six scenarios:

   (1) An IPv6 network to IPv4 Internet

   (2) IPv6 Internet to an IPv4 network

   (3) An IPv4 network to IPv6 Internet

   (4) IPv4 Internet to an IPv6 network

   (5) An IPv6 network to an IPv4 network

   (6) An IPv4 network to an IPv6 network

   We have intentionally left out how one might connect the entire IPv4
   Internet with the entire IPv6 Internet.  In these scenarios one would
   look at how a host in one network initiates a uni- or bi-directional
   flow to another network.  The initiator needs to somehow know which
   address to send the initial packet to, and the initial packet gets
   translated before reaching its destination.

   For multicast one generally need a receiver to signal the group (and
   sometimes also the source) it wants to receive from.  The signalling
   generally goes hop-by-hop towards the source to build multicast
   forwarding state that later is used to forward multicast in the
   reverse direction.  This means that for the receiving host to receive
   multicast, it must first somehow know which group (and possibly
   source) it should signal that it wants to receive.  These signals
   would then probably go hop-by-hop to a translator, and then the
   translated signalling would go hop-by-hop from the translator to the
   source.  Note that this description si correct for SSM (source-
   specific multicast), but is in reality more complex for ASM (any-
   source multicast).  An anology to unicast might perhaps be TCP
   streaming where a SYN is sent from the host that wants to receive the
   stream to the source of the stream.  Then the application data flows
   in the reverse direction of the initial signal.  Hence we argue that
   the above unicast scenarios correspond to the following multicast
   scenarios, respectively:

   (1) An IPv6 network receiving multicast from IPv4 Internet

   (2) IPv6 Internet receiving multicast from an IPv4 network




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   (3) An IPv4 network receiving multicast from IPv6 Internet

   (4) IPv4 Internet receiving multicast from an IPv6 network

   (5) An IPv6 network receiving multicast from an IPv4 network

   (6) An IPv4 network receiving multicast from an IPv6 network

2.1.  An IPv6 network receiving multicast from IPv4 Internet

   Here we have a network, say ISP or enterprise, that for some reason
   is IPv6-only, but the hosts in the IPv6-only network should be able
   to receive multicast from sources in the IPv4 internet.

   This is simple because the global IPv4 address space can be embedded
   into IPv6.  Unicast addresses according to the unicast translation in
   use.  For multicast one may embed all IPv4 multicast addresses inside
   a single IPv6 multicast prefix.  Or one may have multiple embeddings
   to allow for appropriate mapping of scopes and ASM versus SSM.  Using
   this embedding, the IPv6 host (or an application running on the host)
   can send IPv6 MLD reports for IPv6 groups (and if SSM, also sources)
   that specify which IPv4 source and groups that it wants to receive.
   The usual IPv6 state (including MLD and possibly PIM) needs to be
   created.  If PIM is involved we need to use RPF to set up the tree
   and accept data, so the source addresses must be routed towards some
   translation device.  This is likely to be the same device that would
   do the unicast translation.  The translation device can in this case
   be completely stateless.  There is some multicast state, but that is
   similar to the state in a multicast router when translation is not
   performed.  Basically if the translator receives MLD or PIM messages
   asking for a specific group (or source and group), it uses these
   mappings to find out which IPv4 group (or source and group) it needs
   to send IGMP or PIM messages for.  This is no different than
   multicast in general, except for the translation.  Whenever the
   translator receives data from the IPv4 source, it checks if it has
   anyone interested in the respective IPv6 group (or source and group),
   and if so, translates and forwards the data packets.

   IPv6 applications need to somehow learn which IPv6 group (or source
   and group) to join.  This is further discussed in Section 3.2.

2.2.  IPv6 Internet receiving multicast from an IPv4 network

   We here consider the case where the Internet is IPv6, but there is
   some network of perhaps legacy IPv4 hosts that is IPv4-only.  We want
   any IPv6 host on the Internet to be able to receive multicast from an
   IPv4 source.  This scenario can be solved using the same techniques
   as in scenario (1), Section 2.1.  There may however be differences



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   regarding exactly which mappings are used and how applications may
   become aware of them.  To obtain full benefit of multicast, all IPv6
   hosts need to use the same mappings.

2.3.  An IPv4 network receiving multicast from IPv6 Internet

   Here we consider how an IPv4-only host in an IPv4 network may receive
   from an IPv6 multicast sender on the Internet.  For dual-stack hosts
   in an IPv4 network one should consider tunneling.  This is difficult
   since we cannot embed the entire IPv6 space into IPv4.  For unicast
   one might use a DNS-ALG for this, where the ALG would instantiate
   translator mappings as needed.  However, for multicast one generally
   does not use DNS.  One could consider doing the same with an ALG for
   some other protocol.  E.g. translate addresses in SDP files when they
   pass the translator, or in any other protocol that might transfer
   multicast addresses.  This would be very complicated and not
   recommended.

   Rather than using an ALG that translates addresses in application
   protocol payload, one could consider new signalling mechanisms for
   more explicit signalling.  The additional signalling could be either
   on the IPv6 or the IPv4 side.  It may however not be a good idea to
   require additional behavior by host and applications on the IPv6
   Internet to accomodate legacy IPv4 networks.  Also, since one may not
   be able to provide unique IPv4 multicast addresses for all the IPv6
   multicast groups that are in use, it makes more sense that the
   mappings are done locally in each of the IPv4 networks, where IPv4
   multicast addresses might be assigned on-demand.  An IPv4 receiver
   might somehow request an IPv4 mapping for an IPv6 group (and possibly
   source).  This creates a mapping in the translator so that when the
   IPv4 receiver joins the IPv4 group, the translator knows which IPv6
   group (and possibly source) to translate it into.  Of course the
   signalling could also be done manually by adding a static mapping to
   the translator and somehow specifying the right IPv4 address to the
   application.

2.4.  IPv4 Internet receiving multicast from an IPv6 network

   Here we will consider an IPv6 network connected to the IPv4 internet,
   and how any IPv4 host may receive multicast from a source in the IPv6
   network.  This is difficult since the IPv6 multicast address space
   cannot be embedded into IPv4.  Indeed this case has many similarities
   with how IPv4 networks can receive from the IPv6 Internet.  However,
   in this case, all IPv4 hosts on the Internet should use the same
   mapping, and it might make sense to have additional requirements on
   the IPv6 network, rather than to add requirements for the IPv4
   Internet.




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   One solution here might be for the IPv6 source application to somehow
   register with the translator to set up a mapping and receive an IPv4
   address.  The application could then possibly send SDP that includes
   both its IPv6 source and group, and the IPv4 source and group it got
   from the translator.  Of course the signalling could also be done by
   manually adding a static mapping to the translator and specifying
   that address to the application.  If instead we were to do signalling
   on the IPv4 side, then an IPv4 receiver would probably need a
   mechanism for finding an IPv4 address of the translator for a given
   IPv6 group.  The IPv4 address could perhaps be embedded in the IPv6
   group address?  Or with say SDP there could be a way of specifying
   the IPv4 translator address.  The IPv4 host could then communicate
   with the translator to establish a mapping (unless one exists) and
   learn which IPv4 group to join.

   The best alternative might be to restrict the IPv6 multicast groups
   that should be accessible on the IPv4 internet to a certain IPv6
   prefix.  This may allow stateless translation.  This could also be
   used in the reverse direction, for an IPv6 host to receive from an
   IPv4 source.  Or in other words, the same mapping can be used in both
   directions.  This has similarities with IVI [I-D.baker-behave-ivi]
   and also [I-D.venaas-behave-mcast46].  By using IVI source addresses
   and a similar technique for multicast addresses, the correct IPv4
   source and group addresses can be derived from those.  This method
   has many benefits, the main issue is that it cannot work for
   arbitrary IPv6 multicast addresses.

2.5.  An IPv6 network receiving multicast from an IPv4 network

   In this scenario we consider IPv4 and IPv6 networks belonging to the
   same organization.  We would like any IPv6 host to receive from any
   IPv4 sources.  Here one can use the same techniques as for an IPv6
   network receiving from IPv4 internet.  It is really a special case of
   scenario (1), Section 2.1.

   The fact that the number of hosts are limited and that there is
   common management might simplify things.  Due to the limited scale,
   one could perhaps just manually configure all the static mappings
   needed in the translator and manually create the necessary
   announcements or in some cases have the applications create the
   necessary announcements.  But it might be better to use a stateless
   approach where IPv4 unicast and multicast addresses are embedded into
   IPv6.  Like IVI [I-D.baker-behave-ivi] or
   [I-D.venaas-behave-mcast46].







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2.6.  An IPv4 network receiving multicast from an IPv6 network

   In this scenario we consider IPv4 and IPv6 networks belonging to the
   same organization.  We would like any IPv4 host to receive from any
   IPv6 source.  This can be scene as special cases of either scenario
   (3), Section 2.3 or scenario (4), Section 2.4, where any of those
   techniques might apply.  However, as discussed in scenario (5)
   Section 2.5 where we looked at how to do multicast in the reverse
   direction; the limited number of hosts and common managment might
   allow us to just use static mappings or a stateless approach by
   restricting which IPv6 addresses are used.  By using these techniques
   one may be able to create mappings that can be used for multicast in
   both directions, combining this scenario with scenario (5).






































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3.  Framework

   Having considered some possible scenarios for where and how we may
   use multicast translation, we will now consider some general issues
   and the different components of such solutions.

3.1.  Routing

   The actual translation of multicast packets may not be very
   complicated, in particular if it can be stateless.  For the multicast
   to actually go through the translator we need to have routes for the
   multicast source addresses involved, so that multicast packets both
   on their way to and from the translator satisfy RPF checks.  These
   routes are also needed for protocols like PIM-SM to establish a
   multicast tree, since RPF is used to determine where to send join
   messages.  To go into more detail we need to look at different
   scenarios like SSM (Source-Specific Multicast) and ASM (Any-Source
   Multicast), and PIM versus IGMP/MLD.

3.1.1.  Translation with PIM and SSM

   When doing SSM, a receiver specifies both source and group addresses.
   If the receiver is to receive translated packets, it must do an IGMP/
   MLD join for the source and group address that the data packets will
   have after translation.  We will later look at how it may learn those
   addresses.  For the source address it joins, the unicast routing (or
   it may be an alternate topology specific to multicast), must point
   towards the translator.  With this in place, PIM should build a tree
   hop-by-hop from the last-hop router to the translator.  The
   translator then maps the source and group addresses in the PIM join
   to the source and group the data packets have before translation.
   The translator then does a PIM join for that source and group.
   Provided the routing is correct, this will then build a tree all the
   way to the source.  Finally when these joins reach the source, any
   data sent by the source will follow this path to the translator, get
   translated, and then continue to the receiver.

3.1.2.  Translation with PIM and ASM

   Let us first consider PIM Sparse Mode.  In this case a receiver just
   joins a group.  If this group is to be received via the translator we
   need to send joins towards the translator, but initially PIM will
   send joins towards the RP (Rendezvous-Point) for the group.  The most
   efficient solution is probably to make sure that the translator is
   configured as an RP for all groups that one may receive through it.
   That is, for the groups it translates to.  E.g. if IPv4 groups are
   embedded into an IPv6 multicast prefix, then the translator could be
   an RP for that specific prefix.  The translator may then translate



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   the group and join towards the group address that is used before
   translation.  Note that if the translator also is an RP for the
   addresses used before translation, it should know which sources exist
   and join each of these.  If it is not an RP, it needs to join towards
   the RP.  If the translator did not know the sources, it may join each
   of the sources as soon as it receives from them (that is, switching
   to Shortest Path Trees).  When the translator receives data, it
   translates it and then sends the translated data.  This then follows
   the joins for the translated groups to the receivers.  When the last-
   hop routers start receiving, they will probably (this is usually the
   default behavior) switch to SPTs (Shortest Path Trees).  These trees
   also need to go to the translator and would probably follow the same
   path as the previously built shared tree.  One might argue here that
   switching to the SPT has no benefit if it is the same path anyway.
   Also with shared trees, RPF is not an issue, so the translated source
   addresses don't need to be routed towards the translator.

   At the end of the previous paragraph we pointed out that there is no
   benefit in switching to shortest path trees if they have to go via
   the translator anyway.  A possibility here could be to use
   Bidirectional PIM where there is no source specific state and data
   always go through the RP.  It is possible to use Bidir just for those
   groups that are translated, and then make the translator the RP.

3.1.3.  Translation with IGMP/MLD

   For translation taking place close to the edge, e.g. a home gateway,
   one may consider just using IGMP and MLD, and no PIM.  In that case
   the translator should for any received MLD reports for IPv6 groups
   that correspond to translated IPv4 groups, map those into IGMP
   reports that it sends out on the IPv4 side.  And vice versa for data
   in the other direction.  Note that a translator implementation could
   also choose to do this in just one direction.  For SSM it would also
   need to translate the source addresses.

3.2.  Application layer issues

   The main application layer issue is perhaps how the applications
   learn what groups (or sources and groups) to join.  For unicast,
   applications may often obtain addresses via DNS and a DNS-ALG.  For
   multicast, DNS is usually not used, and there are a wide range of
   different ways applications learn addresses.  It can be through
   configuration or user input, it can be URLs on a web page, it can be
   SDP files (via SAP or from web page or mail etc), or also via
   protocols like RTSP/SIP.  It is no easy task to handle all of these
   possible methods using ALGs.

   An alternative to rewriting addresses in the network is to make the



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   applications aware of the translation and mappings in use.  One
   approach could be for the source to create say SDP that includes both
   the original and the translated addresses.  This may require use of
   ICE [I-D.ietf-mmusic-ice] for specifying both IPv4 and IPv6 multicast
   addresses, allowing the receiver to choose which one to use.  The
   other alternative would be for the receiving application to be aware
   of the translation and the mapping in use.  This means that the
   receiving application can receive the original SDP, but then apply
   the mapping to those addresses.

   As we just discussed, it may be useful for applications to perform
   the mappings.  The next question is how they may learn those
   mappings.  The easiest would be if there was a standard way used for
   all mappings, e.g. a well-known IPv6 prefix for embedding IPv4
   addresses.  But that does not work in all scenarios.  There could be
   a way for applications to learn which prefix to use, see
   [I-D.wing-behave-learn-prefix].  But note that there may be different
   multicast prefixes depending on whether we are doing SSM or ASM and
   scope.  In addition we need the unicast prefix for the multicast
   source addresses.  Alternatively one could imagine applications
   requesting mappings for specific addresses on demand from the
   translator.  The translator could have static mappings, or install
   mappings as requested by applications.

   An alternative to making applications aware of the translation and
   rewriting addresses as needed, could be to do translation in the API
   or stack, so that e.g. an application joins an IPv4 group, the API or
   stack rewrites that into IPv6 and sends the necessary MLD reports.
   When IPv6 packets arrive, the API/stack can rewrite those packets
   back to IPv4.  This could allow legacy IPv4 applications to run on a
   dual-stack node (or IPv6-only with translation in the API) to receive
   IPv4 packets through an IPv6-only network.  But in this case it might
   be better to just use tunneling.


















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4.  IANA Considerations

   This document requires no IANA assignments.
















































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5.  Security Considerations

   This requires more thought, but the author is not aware of any
   obvious security issues specific to multicast translation.















































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6.  Acknowledgements

   Dan Wing provided early feedback that helped shape this document.
















































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7.  Informative References

   [I-D.baker-behave-ivi]
              Li, X., Bao, C., Baker, F., and K. Yin, "IVI Update to
              SIIT and NAT-PT", draft-baker-behave-ivi-01 (work in
              progress), September 2008.

   [I-D.baker-behave-v4v6-framework]
              Baker, F., Li, X., and C. Bao, "Framework for IPv4/IPv6
              Translation", draft-baker-behave-v4v6-framework-02 (work
              in progress), February 2009.

   [I-D.ietf-mmusic-ice]
              Rosenberg, J., "Interactive Connectivity Establishment
              (ICE): A Protocol for Network Address  Translator (NAT)
              Traversal for Offer/Answer Protocols",
              draft-ietf-mmusic-ice-19 (work in progress), October 2007.

   [I-D.venaas-behave-mcast46]
              Venaas, S., "An IPv4 - IPv6 multicast translator",
              draft-venaas-behave-mcast46-00 (work in progress),
              December 2008.

   [I-D.wing-behave-learn-prefix]
              Wing, D., Wang, X., and X. Xu, "Learning the IPv6 Prefix
              of an IPv6/IPv4 Translator",
              draft-wing-behave-learn-prefix-02 (work in progress),
              May 2009.























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Author's Address

   Stig Venaas
   cisco Systems
   Tasman Drive
   San Jose, CA  95134
   USA

   Email: stig@cisco.com










































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