Network Working Group S. Venaas
Internet-Draft cisco Systems
Intended status: Informational X. Li
Expires: June 25, 2011 C. Bao
CERNET Center/Tsinghua University
December 22, 2010
Framework for IPv4/IPv6 Multicast Translation
draft-venaas-behave-v4v6mc-framework-02.txt
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.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on June 25, 2011.
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document authors. All rights reserved.
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to this document. Code Components extracted from this document must
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Translation scenarios . . . . . . . . . . . . . . . . . . . . 4
2.1. Scenario 1: An IPv6 network receiving multicast from
the IPv4 Internet . . . . . . . . . . . . . . . . . . . . 5
2.2. Scenario 2: The IPv4 Internet receiving multicast from
an IPv6 network . . . . . . . . . . . . . . . . . . . . . 6
2.3. Scenario 3: The IPv6 Internet receiving multicast from
an IPv4 network . . . . . . . . . . . . . . . . . . . . . 6
2.4. Scenario 4: An IPv4 network receiving multicast from
the IPv6 Internet . . . . . . . . . . . . . . . . . . . . 7
2.5. Scenario 5: An IPv6 network receiving multicast from
an IPv4 network . . . . . . . . . . . . . . . . . . . . . 8
2.6. Scenario 6: An IPv4 network receiving multicast from
an IPv6 network . . . . . . . . . . . . . . . . . . . . . 8
3. Framework . . . . . . . . . . . . . . . . . . . . . . . . . . 9
3.1. Addressing . . . . . . . . . . . . . . . . . . . . . . . . 9
3.1.1. Source addressing . . . . . . . . . . . . . . . . . . 9
3.1.2. Group addressing . . . . . . . . . . . . . . . . . . . 9
3.2. Routing . . . . . . . . . . . . . . . . . . . . . . . . . 9
3.2.1. Translation with PIM and SSM . . . . . . . . . . . . . 10
3.2.2. Translation with PIM and ASM . . . . . . . . . . . . . 10
3.2.3. Translation with IGMP/MLD . . . . . . . . . . . . . . 11
3.3. Translation in operation . . . . . . . . . . . . . . . . . 11
3.3.1. Stateless Translation . . . . . . . . . . . . . . . . 11
3.3.2. Stateful Translation . . . . . . . . . . . . . . . . . 12
3.4. Application layer issues . . . . . . . . . . . . . . . . . 14
3.5. Further Work . . . . . . . . . . . . . . . . . . . . . . . 16
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 17
5. Security Considerations . . . . . . . . . . . . . . . . . . . 18
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 19
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 20
7.1. Normative References . . . . . . . . . . . . . . . . . . . 20
7.2. Informative References . . . . . . . . . . . . . . . . . . 21
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 22
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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.ietf-behave-v6v4-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
[I-D.ietf-behave-v6v4-framework]:
Scenario 1: An IPv6 network to the IPv4 Internet
Scenario 2: The IPv4 Internet to an IPv6 network
Scenario 3: The IPv6 Internet to an IPv4 network
Scenario 4: An IPv4 network to the IPv6 Internet
Scneario 5: An IPv6 network to an IPv4 network
Scenario 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 is 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:
Scenario 1: An IPv6 network receiving multicast from the IPv4
Internet
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Scenario 2: The IPv4 Internet receiving multicast from an IPv6
network
Scneario 3: The IPv6 Internet receiving multicast from an IPv4
network
Scenario 4: An IPv4 network receiving multicast from the IPv6
Internet
Scenario 5: An IPv6 network receiving multicast from an IPv4 network
Scenario 6: An IPv4 network receiving multicast from an IPv6 network
2.1. Scenario 1: An IPv6 network receiving multicast from the 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. The unicast
equivalent is "IPv6 network to the IPv4 Internet".
This is simple because the global IPv4 address space can be embedded
into IPv6 [RFC6052]. 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 3.4.
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2.2. Scenario 2: The 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. The unicast equivalent is "the IPv4 Internet to an 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. See scenario (4),
Section 2.4. 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.
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.xli-behave-ivi],
[I-D.ietf-behave-v6v4-xlate], [RFC6052] and also
[I-D.venaas-behave-mcast46]. By using IVI source addresses (IPv4-
translatable 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.
2.3. Scenario 3: The IPv6 Internet receiving multicast from an IPv4
network
We here consider the case where the Internet is IPv6, but there is
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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. The unicast equivalent is "the IPv6 Internet to an IPv4
network".
This scenario can be solved using the same techniques as in Scenario
1, Section 2.1. There may however be differences 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.
2.4. Scenario 4: An IPv4 network receiving multicast from the IPv6
Internet
Here we consider how an IPv4-only host in an IPv4 network may receive
from an IPv6 multicast sender on the Internet. The unicast
equivalent is "an IPv4 network to the IPv6 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. One might consider some of the techniques from
scenario (2), Section 2.2. That scenario is however much easier
since one may restrict which IPv6 groups are used and there is a
limited number of sources.
For unicast one might use a DNS-ALG for this, where the ALG would
instantiate translator mappings as needed. This is the technique
used in NAT-PT [RFC2766], which was deprecated by [RFC4966].
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
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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.
2.5. Scenario 5: An IPv6 network receiving multicast from an IPv4
network
In this scenario we consider IPv4 and IPv6 networks belonging to the
same organization. The unicast equivalent is "an IPv6 network to an
IPv4 network".
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
the 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.xli-behave-ivi], or [I-D.venaas-behave-mcast46].
2.6. Scenario 6: An IPv4 network receiving multicast from an IPv6
network
In this scenario we consider IPv4 and IPv6 networks belonging to the
same organization. The unicast equialent is "an IPv4 network to an
IPv6 network".
We would like any IPv4 host to receive from any IPv6 source. This
can be seen as special cases of either scenario (2), Section 2.2 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. Addressing
When doing stateless translation, one need to somehow encode IPv4
addresses inside IPv6 addresses so that there is a well defined way
for the translator to transform an IPv6 address into IPv4. This can
be done with techniques like IVI [I-D.xli-behave-ivi] and
[I-D.venaas-behave-mcast46].
There are two types of addressing schemes related to the IPv4/IPv6
multicast translation. The source addressing and the group
addressing.
3.1.1. Source addressing
Source addressing issues is the same as in the unicast IPv4/IPv6
translation defined in [RFC6052]. The IPv4-mapped address is used
for representing IPv4 in IPv6 and the IPv4-translatable address is
used for representing IPv6 in IPv4 when the stateless translator is
used. The multicast RPF relies on the source address to build the
distribution tree. Therefore, depending on the operation mode of the
IPv4/IPv6 translator and receiving directions, the IPv4-mapped or the
IPv4-translatable addresses will be used.
3.1.2. Group addressing
Group addressing issue is unique to the IPv4/IPv6 multicast
translation. The entire IPv4 group addresses can be uniquely
represented by the IPv6 group addresses, while the entire IPv6 group
addresses cannot be uniquely represented by the IPv6 group addresses.
Therefore, special group address mapping rule between IPv4 group
addresses and IPv6 group addresses should be defined for the IPv4/
IPv6 multicast translation.
3.2. 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
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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.
3.2.1. 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.
3.2.2. 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
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
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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.
3.2.3. 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.
3.3. Translation in operation
Currently, the proposed solutions for IPv6/IPv4 translation are
classified into stateless translation and stateful translation.
3.3.1. Stateless Translation
For stateless translation, the translation information is carried in
the address itself, permitting both IPv4->IPv6 and IPv6-<IPv4
sessions establishment. The stateless translation supports end-to-
end address transparency and has better scalability compared with the
stateful translation [I-D.ietf-behave-v6v4-xlate],
[I-D.xli-behave-ivi].
Stateless translation can be used for Scenarios 1, 2, 5 and 6, i.e.
it supports "An IPv6 network receiving multicast from the IPv4
Internet", "the IPv4 Internet receiving multicast from an IPv6
network", "An IPv6 network receiving multicast from an IPv4 network"
and "An IPv4 network receiving multicast from an IPv6 network".
In the stateless translation, an IPv6 network uses the IPv4-
translatable addresses, while the IPv4 Internet or an IPv4 network
can be represented by IPv4-mapped addresses.
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--------
// \\ -----------
/ \ // \\
/ +----+ \
| |XLAT| |
| The IPv4 +----+ An IPv6 |
| Internet +----+ Network | XLAT: Stateless v4/v6
| |DNS | (address | Translator
\ +----+ subset) / DNS: DNS64/DNS46
\ / \\ //
\\ // ----------
--------
sending ----> receiving
receiving <---- sending
Figure 1: Stateless translation for Scenarios 1 and 2
-------- ---------
// \\ // \\
/ +----+ \
| |XLAT| |
| An IPv4 +----+ An IPv6 |
| Network +----+ Network | XLAT: v4/v6
| |DNS | | Translator
\ +----+ / DNS: DNS64/DNS46
\\ // \\ //
-------- ---------
sending ----> receiving
receiving <---- sending
Figure 2: Stateless translator for Scenarios 5 and 6
3.3.2. Stateful Translation
For stateful translation, the translation state is maintained between
IPv4 address/port pairs and IPv6 address/port pairs, enabling IPv6
systems to open sessions with IPv4 systems
[I-D.ietf-behave-v6v4-xlate], [I-D.ietf-behave-v6v4-xlate-stateful].
Stateful translator can be used for Scenarios 1, 3 and 5, i.e. it
supports "An IPv6 network receiving multicast from the IPv4
Internet", "The IPv6 Internet receiving multicast from an IPv4
network" and "An IPv6 network receiving multicast from an IPv4
network".
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In the stateful translation, an IPv6 network or the IPv6 Internet use
any IPv6 addresses, while the IPv4 Internet or an IPv4 network can be
represented by IPv4-mapped addresses.
--------
// \\ -----------
/ \ // \\
/ +----+ \
| |XLAT| |
| The IPv4 +----+ An IPv6 |
| Internet +----+ Network | XLAT: Stateful v4/v6
| |DNS | | Translator
\ +----+ / DNS: DNS64
\ / \\ //
\\ // -----------
--------
sending ----> receiving
Figure 3: Stateful translator for Scenario 1
-----------
---------- // \\
// \\ / \
/ +----+ \
| |XLAT| |
| An IPv4 +----+ The IPv6 |
| Network +----+ Internet | XLAT: v4/v6
| |DNS | | Translator
\ +----+ / DNS: DNS64
\\ // \ /
--------- \\ //
-----------
sending ----> receiving
Figure 4: Stateful translator for Scenario 3
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-------- ---------
// \\ // \\
/ +----+ \
| |XLAT| |
| An IPv4 +----+ An IPv6 |
| Network +----+ Network | XLAT: v4/v6
| |DNS | | Translator
\ +----+ / DNS: DNS64
\\ // \\ //
-------- ---------
sending ----> receiving
Figure 5: Stateful translator for Scenario 5
3.4. 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.
SDP is maybe the most common way for applications to learn which
multicast addresses (and other parameters) to use in order to receive
a multicast session. Inside the SDP files it is common to use
literal IP addresses, but it is also possible to specify domain
names. Applications would then query the DNS for the addresses, and
a DNS-ALG could perform the necessary translation. There is however
a problem with this.
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Here is a typical SDP taken from RFC 2327:
v=0
o=mhandley 2890844526 2890842807 IN IP4 126.16.64.4
s=SDP Seminar
i=A Seminar on the session description protocol
u=http://www.cs.ucl.ac.uk/staff/M.Handley/sdp.03.ps
e=mjh@isi.edu (Mark Handley)
c=IN IP4 224.2.17.12/127
t=2873397496 2873404696
a=recvonly
m=audio 49170 RTP/AVP 0
m=video 51372 RTP/AVP 31
m=application 32416 udp wb
a=orient:portrait
The line of interest here is "c=IN IP4 224.2.17.12/127". It is legal
to use a domain name, this line would then become e.g. "c=IN IP4
mcast.example.com/127". The problem here is that the application is
told to use IPv4. It will expect the name to resolve to an IPv4
address, and may ignore any IPv6 replies. One could argue that it
would be incorrect to use IPv6, since IPv4 is specified. For DNS to
solve our problem, we would need a new IP neutral SDP syntax, and
applications would need to be updated to support it.
An alternative to rewriting addresses in the network is to make the
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
techniques like CCAP [I-D.boucadair-mmusic-ccap] 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
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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.
3.5. Further Work
There are some special cases and scenarios that should be added to
this document. One is addressing. Are there certain types of IPv6
multicast addresses that could make translation easier? What happens
if there are multiple translators? And also more details on
translation in the host, e.g. bump-in-the-stack or bump-in-the-API.
The document layout of the IPv4/IPv6 multicast translation should be
presented in this document.
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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.
Dave Thaler also provided good feedback that unfortunately still has
not been addressed in this document. See Section 3.5.
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7. References
7.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2766] Tsirtsis, G. and P. Srisuresh, "Network Address
Translation - Protocol Translation (NAT-PT)", RFC 2766,
February 2000.
[RFC4966] Aoun, C. and E. Davies, "Reasons to Move the Network
Address Translator - Protocol Translator (NAT-PT) to
Historic Status", RFC 4966, July 2007.
[RFC5245] Rosenberg, J., "Interactive Connectivity Establishment
(ICE): A Protocol for Network Address Translator (NAT)
Traversal for Offer/Answer Protocols", RFC 5245,
April 2010.
[RFC6052] Bao, C., Huitema, C., Bagnulo, M., Boucadair, M., and X.
Li, "IPv6 Addressing of IPv4/IPv6 Translators", RFC 6052,
October 2010.
[I-D.ietf-behave-v6v4-framework]
Baker, F., Li, X., Bao, C., and K. Yin, "Framework for
IPv4/IPv6 Translation",
draft-ietf-behave-v6v4-framework-10 (work in progress),
August 2010.
[I-D.ietf-behave-v6v4-xlate]
Li, X., Bao, C., and F. Baker, "IP/ICMP Translation
Algorithm", draft-ietf-behave-v6v4-xlate-23 (work in
progress), September 2010.
[I-D.ietf-behave-v6v4-xlate-stateful]
Bagnulo, M., Matthews, P., and I. Beijnum, "Stateful
NAT64: Network Address and Protocol Translation from IPv6
Clients to IPv4 Servers",
draft-ietf-behave-v6v4-xlate-stateful-12 (work in
progress), July 2010.
[I-D.ietf-behave-dns64]
Bagnulo, M., Sullivan, A., Matthews, P., and I. Beijnum,
"DNS64: DNS extensions for Network Address Translation
from IPv6 Clients to IPv4 Servers",
draft-ietf-behave-dns64-11 (work in progress),
October 2010.
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7.2. Informative References
[I-D.xli-behave-ivi]
Li, X., Bao, C., Chen, M., Zhang, H., and J. Wu, "The
CERNET IVI Translation Design and Deployment for the IPv4/
IPv6 Coexistence and Transition", draft-xli-behave-ivi-07
(work in progress), January 2010.
[I-D.venaas-behave-mcast46]
Venaas, S., Asaeda, H., SUZUKI, S., and T. Fujisaki, "An
IPv4 - IPv6 multicast translator",
draft-venaas-behave-mcast46-01 (work in progress),
July 2009.
[I-D.wing-behave-learn-prefix]
Wing, D., "Learning the IPv6 Prefix of a Network's IPv6/
IPv4 Translator", draft-wing-behave-learn-prefix-04 (work
in progress), October 2009.
[I-D.boucadair-mmusic-ccap]
Boucadair, M. and H. Kaplan, "Session Description Protocol
(SDP) Connectivity Capability (CCAP) Attribute",
draft-boucadair-mmusic-ccap-00 (work in progress),
July 2009.
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Authors' Addresses
Stig Venaas
cisco Systems
Tasman Drive
San Jose, CA 95134
USA
Email: stig@cisco.com
Xing Li
CERNET Center/Tsinghua University
Room 225, Main Building, Tsinghua University
Beijing 100084
CN
Phone: +86 10-62785983
Email: xing@cernet.edu.cn
Congxiao Bao
CERNET Center/Tsinghua University
Room 225, Main Building, Tsinghua University
Beijing 100084
CN
Phone: +86 10-62785983
Email: congxiao@cernet.edu.cn
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