Delivery of IPv4 Multicast
Services to IPv4 Clients over an IPv6 Multicast NetworkCiscoShanghaiChinajacni@jacni.comOrangeRennes35000Francemohamed.boucadair@orange.comOrangeRennes35000Francechristian.jacquenet@orange.comComcastU.S.A.yiu_lee@cable.comcast.comhttp://www.comcast.comChina TelecomChina+86 10 58502462wangqian@chinatelecom.cn
Internet
Softwire WGMulticast, DS-Lite, IPv4-IPv6 Interconnection, PREFIX64, SSM,
ASM, IPv4 service continuity, Multicast service continuity, IPv6-only,
IPv6-only multicast, PIM, MLD, IGMPThis document specifies a solution for the delivery of IPv4 multicast
services to IPv4 clients over an IPv6 multicast network. The solution
relies upon a stateless IPv4-in-IPv6 encapsulation scheme and uses the
IPv6 multicast distribution tree to deliver IPv4 multicast traffic. The
solution is particularly useful for the delivery of multicast service
offerings to DS-Lite serviced customers.DS-Lite is a technique that
rationalizes the usage of the remaining global IPv4 addresses during the
transition period by sharing a single IPv4 address with multiple users.
A typical DS-Lite scenario is the delivery of an IPv4 service to an IPv4
user over an IPv6 network (denoted as a 4-6-4 scenario). covers unicast services exclusively.This document specifies a generic solution for the delivery of IPv4
multicast services to IPv4 clients over an IPv6 multicast network. The
solution was developed with DS-Lite in mind (see more discussion below).
The solution is however not limited to DS-Lite; it can be applied in
other deployment contexts such as .If customers have to access IPv4 multicast-based services through a
DS-Lite environment, Address Family Transition Router (AFTR) devices
will have to process all the Internet Group Management Protocol (IGMP)
Report messages that have been forwarded by the Customer
Premises Equipment (CPE) into the IPv4-in-IPv6 tunnels. From that
standpoint, AFTR devices are likely to behave as a replication point for
downstream multicast traffic, and the multicast packets will be
replicated for each tunnel endpoint that IPv4 receivers are connected
to.This kind of DS-Lite environment raises two major issues: The IPv6 network loses the benefits of the multicast traffic
forwarding efficiency because it is unable to deterministically
replicate the data as close to the receivers as possible. As a
consequence, the downstream bandwidth in the IPv6 network will be
vastly consumed by sending multicast data over a unicast
infrastructure.The AFTR is responsible for replicating multicast traffic and
forwarding it into each tunnel endpoint connecting IPv4 receivers
that have explicitly asked for the corresponding contents. This
process may significantly consume the AFTR’s resources and
overload the AFTR.This document specifies an extension to the DS-Lite model to deliver
IPv4 multicast services to IPv4 clients over an IPv6 multicast-enabled
network.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.This document makes use of the following terms:an IPv6 address which
embeds a 32-bit-encoded IPv4 address. An IPv4-embedded IPv6 address
can be unicast or multicast.a dedicated multicast IPv6 prefix for
constructing IPv4-embedded IPv6 multicast addresses. mPrefix64 can
be of two types: ASM_mPrefix64 used in Any Source Multicast (ASM)
mode or SSM_mPrefix64 used in Source Specific Multicast (SSM) mode
.a dedicated IPv6 unicast prefix for
constructing IPv4-embedded IPv6 unicast addresses .a functional entity which
supports an IPv4-IPv6 multicast interworking function (refer to
). It receives and
encapsulates the IPv4 multicast packets into IPv4-in-IPv6 packets
and behaves as the corresponding IPv6 multicast source for the
encapsulated IPv4-in-IPv6 packets.a functional entity which supports
an IGMP-MLD interworking function (refer to ) that relays information
conveyed in IGMP messages by forwarding the corresponding Multicast
Listener Discovery (MLD) messages towards the MLD Querier in the
IPv6 network. In addition, the mB4 decapsulates IPv4-in-IPv6
multicast packets.refers to Protocol Independent Multicast (PIM)
when deployed in an IPv4 infrastructure (i.e., IPv4 transport
capabilities are used to exchange PIM messages).refers to PIM when deployed in an IPv6
infrastructure (i.e., IPv6 transport capabilities are used to
exchange PIM messages).This document focuses only on the subscription to an IPv4 multicast
group and the delivery of IPv4-formatted content to IPv4 receivers over
an IPv6-only network. In particular, only the following case is
covered:An IPv4 receiver accesses IPv4 multicast contents over an IPv6-
only multicast-enabled network.This document does not cover the source/receiver heuristics, where an
IPv4 receiver can also behave as an IPv4 multicast source. This document
assumes that hosts behind the mB4 are IPv4 multicast receivers only.In the DS-Lite specification , an
IPv4-in-IPv6 tunnel is used to carry bidirectional IPv4 unicast traffic
between a B4 and an AFTR. The solution specified in this document
provides an IPv4-in-IPv6 encapsulation scheme to deliver unidirectional
IPv4 multicast traffic from an mAFTR to an mB4.An overview of the solution is provided in this section which is
intended as an introduction to how it works, but is not normative. For
the normative specifications of the two new functional elements: mB4 and
mAFTR (), refer to and .In order to map the addresses of IPv4 multicast traffic with IPv6
multicast addresses, an IPv6 multicast prefix (mPrefix64) and an IPv6
unicast prefix (uPrefix64) are provided to the mAFTR and the mB4
elements, both of which contribute to the computation and the
maintenance of the IPv6 multicast distribution tree that extends the
IPv4 multicast distribution tree into the IPv6 multicast network.The mAFTR and the mB4 use mPrefix64 to convert an IPv4 multicast
address (G4) into an IPv4-embedded IPv6 multicast address (G6). The
mAFTR and the mB4 use uPrefix64 to convert an IPv4 multicast source
address (S4) into an IPv4-embedded IPv6 address (S6). The mAFTR and
the mB4 must use the same mPrefix64 and uPrefix64, and also run the
same algorithm for building IPv4-embedded IPv6 addresses. Refer to
for more details about the address
mapping.When an IPv4 receiver connected to the device that embeds the mB4
capability wants to subscribe to an IPv4 multicast group, it sends an
IGMP Report message to the mB4. The mB4 creates the IPv6 multicast
group (G6) address using mPrefix64 and the original IPv4 multicast
group address. If the receiver sends a source-specific IGMPv3 Report
message, the mB4 will create the IPv6 source address (S6) using
uPrefix64 and the original IPv4 source address.The mB4 uses the G6 (and both S6 and G6 in SSM) to create the
corresponding MLD Report message. The mB4 sends the Report message to
the MLD Querier in the IPv6 network. The MLD Querier (which usually
acts as the PIMv6 Designated Router too) receives the MLD Report
message and sends the PIMv6 Join to join the IPv6 multicast
distribution tree. The MLD Querier can send either PIMv6 Join (*,G6)
in ASM or PIMv6 Join (S6,G6) in SSM to the mAFTR.The mAFTR acts as the DR to which the uPrefix64-derived S6 is
connected. The mAFTR will receive the source-specific PIMv6 Join
message (S6,G6) from the IPv6 multicast network. If the mAFTR is the
Rendezvous Point (RP) of G6, it will receive the any-source PIMv6 Join
message (*,G6) from the IPv6 multicast network. If the mAFTR is not
the RP of G6, it will send the PIM Register message to the RP of G6
located in the IPv6 multicast network.When the mAFTR receives the PIMv6 Join message (*,G6), it will
extract the IPv4 multicast group address (G4). If the mAFTR is the RP
of G4 in the IPv4 multicast network, it will create a (*,G4) entry (if
such entry does not already exist) in its own IPv4 multicast routing
table. If the mAFTR is not the RP of G4, it will send the
corresponding PIMv4 Join message (*,G4) towards the RP of G4 in the
IPv4 multicast network.When the mAFTR receives the PIMv6 Join message (S6,G6), it will
extract the IPv4 multicast group address (G4) and IPv4 source address
(S4) and send the corresponding (S4,G4) PIMv4 Join message directly to
the IPv4 source.A branch of the multicast distribution tree is thus constructed,
comprising both an IPv4 part (from the mAFTR upstream) and an IPv6
part (from mAFTR downstream to the mB4).The mAFTR advertises the route of uPrefix64 with an IPv6 Interior
Gateway Protocol (IGP), so as to represent the IPv4-embedded IPv6
source in the IPv6 multicast network, and to run the Reverse Path
Forwarding (RPF) check procedure on incoming multicast traffic.When the mAFTR receives an IPv4 multicast packet, it will
encapsulate the packet into an IPv6 multicast packet using the
IPv4-embedded IPv6 multicast address as the destination address and an
IPv4-embedded IPv6 unicast address as the source address. The
encapsulated IPv6 multicast packet will be forwarded down the IPv6
multicast distribution tree and the mB4 will eventually receive the
packet.The IPv6 multicast network treats the IPv4-in-IPv6 encapsulated
multicast packets as native IPv6 multicast packets. The IPv6 multicast
routers use the outer IPv6 header to make their forwarding
decisions.When the mB4 receives the IPv6 multicast packet (to G6) derived by
mPrefix64, it decapsulates it and forwards the original IPv4 multicast
packet to the receivers subscribing to G4.Note: At this point, only IPv4-in-IPv6 encapsulation is defined;
however, other types of encapsulation could be defined in the
future.A dedicated IPv6 multicast prefix (mPrefix64) is provisioned to the
mAFTR and the mB4. The mAFTR and the mB4 use the mPrefix64 to form an
IPv6 multicast group address from an IPv4 multicast group address. The
mPrefix64 can be of two types: ASM_mPrefix64 (a mPrefix64 used in ASM
mode) or SSM_mPrefix64 (a mPrefix64 used in SSM mode). The mPrefix64
MUST be derived from the corresponding IPv6 multicast address space
(e.g., the SSM_mPrefix64 must be in the range of multicast address
space specified in ).The IPv6 part of the multicast distribution tree can be seen as an
extension of the IPv4 part of the multicast distribution tree. The
IPv4 multicast source address MUST be mapped to an IPv6 multicast
source address. An IPv6 unicast prefix (uPrefix64) is provisioned to
the mAFTR and the mB4. The mAFTR and the mB4 use the uPrefix64 to form
an IPv6 multicast source address from an IPv4 multicast source
address. The uPrefix-formed IPv6 multicast source address will
represent the original IPv4 multicast source in the IPv6 multicast
network. The uPrefix64 MUST be derived from the IPv6 unicast address
space.The address translation MUST follow the algorithm defined in .The mPrefix64 and uPrefix64 can be configured in the mB4 using a
variety of methods, including an out-of-band mechanism, manual
configuration, or a dedicated provisioning protocol (e.g., using
DHCPv6 ).IPv4-Embedded IPv6 multicast addresses are composed according to
the following algorithm:Concatenate the mPrefix64 and the 32 bits of the IPv4 address
to obtain a 128-bit address.The IPv4 multicast addresses are extracted from the IPv4-Embedded
IPv6 Multicast Addresses according to the following algorithm:If the multicast address has a pre-configured mPrefix64,
extract the last 32 bits of the IPv6 multicast address.An IPv4 source is represented in the IPv6 realm with its
IPv4-converted IPv6 address .The embedded IPv4 address in an IPv6 multicast address is included
in the last 32 bits; therefore, dotted decimal notation can be
used.IPv4 and IPv6 addresses used in this example are derived from the
IPv4 and IPv6 blocks reserved for documentation, as per . The unicast IPv4 address of the above
example is derived from the documentation address block defined in
.The IGMP-MLD Interworking Function combines the IGMP/MLD Proxying
function and the address synthesizing operations. The IGMP/MLD
Proxying function is specified in . The
address translation is stateless and MUST follow the address mapping
specified in .The mB4 performs the host portion of the MLD protocol on the
upstream interface. The composition of IPv6 membership in this context
is constructed through address synthesizing operations and MUST
synchronize with the membership database maintained in the IGMP
domain. MLD messages are forwarded natively towards the MLD Querier
located upstream in the IPv6 network. The mB4 also performs the router
portion of the IGMP protocol on the downstream interface(s). Refer to
for more details.If SSM is deployed, the mB4 MUST construct the IPv6 source address
(or retrieve the IPv4 source address) using the uPrefix64. The mB4 may
create a membership database which associates the IPv4-IPv6 multicast
groups with the interfaces (e.g., WLAN and Wired Ethernet) facing IPv4
multicast receivers.When the mB4 receives an IPv6 multicast packet, it MUST check the
group address and the source address. If the IPv6 multicast group
prefix is mPrefix64 and the IPv6 source prefix is uPrefix64, the mB4
MUST decapsulate the IPv6 header and forward the IPv4 multicast packet
through each relevant interface. Otherwise, the mB4 MUST silently drop
the packet.As an illustration, if a packet is received from source
2001:db8::192.0.2.33 and needs to be forwarded to group
ff3x:1000::233.252.0.1, the mB4 decapsulates it into an IPv4 multicast
packet using 192.0.2.33 as the IPv4 multicast source address and using
233.252.0.1 as the IPv4 destination address.Encapsulating IPv4 multicast packets into IPv6 multicast packets
that will be forwarded by the mAFTR to the mB4 along the IPv6
multicast distribution tree reduces the effective MTU size by the size
of an IPv6 header. In this specification, the data flow is
unidirectional from the mAFTR to the mB4. The mAFTR MUST fragment the
oversized IPv6 packet after the encapsulation into two IPv6 packets.
The mB4 MUST reassemble the IPv6 packets, decapsulate the IPv6 packet,
and forward the IPv4 packet to the hosts that have subscribed to the
corresponding multicast group. Further considerations about
fragmentation issues are documented in .If the mB4 function is implemented in the host which is directly
connected to an IPv6-only network, the host MUST implement , , and . The host
MAY optimize the implementation to provide an Application Programming
Interface (API) or kernel module to skip the IGMP-MLD Interworking
Function. Optimization considerations are out of scope of this
specification.When several mPrefix64s are available, if each enclosed
IPv4-embedded IPv6 multicast prefix has a distinct scope, the mB4 MUST
select the appropriate IPv4-embedded IPv6 multicast prefix whose scope
matches the IPv4 multicast address used to synthesize an IPv4-embedded
IPv6 multicast address.The mB4 MAY be configured to not preserve the scope when enforcing
the address translation algorithm.The mAFTR is responsible for interconnecting the IPv4 multicast
distribution tree with the corresponding IPv6 multicast distribution
tree. The mAFTR MUST use the uPrefix64 to build the IPv6 source
addresses of the multicast group address derived from mPrefix64. In
other words, the mAFTR MUST be the multicast source whose address is
derived from uPrefix64.The mAFTR MUST advertise the route towards uPrefix64 with the IPv6
IGP. This is needed by the IPv6 multicast routers so that they acquire
the routing information to discover the source.The mAFTR MUST interwork PIM Join/Prune messages for (*, G6) and
(S6, G6) on their corresponding (*, G4) and (S4, G4). The following
text specifies the expected behavior of the mAFTR for PIM Join
messages.The mAFTR contains two separate Tree Information Bases (TIBs): the
IPv4 Tree Information Base (TIB4) and the IPv6 Tree Information Base
(TIB6), which are bridged by one IPv4-in-IPv6 virtual interface. It
should be noted that TIB implementations may vary (e.g., some may rely
upon a single integrated TIB without any virtual interface), but they
should follow this specification for the sake of global and functional
consistency.When a mAFTR receives a PIMv6 Join message (*,G6) with an IPv6
multicast group address (G6) that is derived from the mPrefix64, it
MUST check its IPv6 Tree Information Base (TIB6). If there is an entry
for this G6 address, it MUST check whether the interface through which
the PIMv6 Join message has been received is in the outgoing interface
(oif) list. If not, the mAFTR MUST add the interface to the oif list.
If there is no entry in the TIB6, the mAFTR MUST create a new entry
(*,G6) for the multicast group. Whether or not the IPv4-in-IPv6
virtual interface is set as the incoming interface of the newly
created entry is up to the implementation but it should comply with
the mAFTR's multicast data forwarding behavior, see .The mAFTR MUST extract the IPv4 multicast group address (G4) from
the IPv4-embedded IPv6 multicast address (G6) contained in the PIMv6
Join message. The mAFTR MUST check its IPv4 Tree Information Base
(TIB4). If there is an entry for G4, it MUST check whether the
IPv4-in-IPv6 virtual interface is in the outgoing interface list. If
not, the mAFTR MUST add the interface to the oif list. If there is no
entry for G4, the mAFTR MUST create a new (*,G4) entry in its TIB4 and
initiate the procedure for building the shared tree in the IPv4
multicast network without any additional requirement.If the mAFTR receives a source-specific Join message, the (S6, G6)
is processed rather than (*,G6). The procedures of processing (S6,G6)
and (*,G6) are almost the same. Differences have been detailed in
.When the mAFTR receives the first IPv4 multicast packet, it may
extract the multicast source address (S4) from the packet and send an
Explicit PIMv4 (S4,G4) Join message directly to S4. The mAFTR switches
from the shared Rendezvous Point Tree (RPT) to the Shortest Path Tree
(SPT) for G4.For IPv6 multicast routers to switch to the SPT, there is no new
requirement. IPv6 multicast routers may send an Explicit PIMv6 Join to
the mAFTR once the first (S6,G6) multicast packet arrives from
upstream multicast routers.When the mAFTR receives an IPv4 multicast packet, it checks its
TIB4 to find a matching entry and then forwards the packet to the
interface(s) listed in the outgoing interface list. If the
IPv4-in-IPv6 virtual interface also belongs to this list, the packet
is encapsulated with the mPrefix64-derived and uPrefix64-derived
IPv4-embedded IPv6 addresses to form an IPv6 multicast packet. Then
another lookup is made by the mAFTR to find a matching entry in the
TIB6. Whether the RPF check for the second lookup is performed or not
is up to the implementation and is out of the scope of this document.
The IPv6 multicast packet is then forwarded along the IPv6 multicast
distribution tree, based upon the outgoing interface list of the
matching entry in the TIB6.As an illustration, if a packet is received from source 192.0.2.33
and needs to be forwarded to group 233.252.0.1, the mAFTR encapsulates
it into an IPv6 multicast packet using ff3x:1000::233.252.0.1 as the
IPv6 destination address and using 2001:db8::192.0.2.33 as the IPv6
multicast source address.The Scope field of IPv4-in-IPv6 multicast addresses should be
valued accordingly (e.g, to “E”, Global scope;) in the
deployment environment. This specification does not discuss the scope
value that should be used.Nevertheless, when several mPrefix64s are available, if each
enclosed IPv4-embedded IPv6 multicast prefix has a distinct scope, the
mAFTR MUST select the appropriate IPv4-embedded IPv6 multicast prefix
whose scope matches the IPv4 multicast address used to synthesize an
IPv4-embedded IPv6 multicast address.An mAFTR MAY be configured to not preserve the scope when enforcing
the address translation algorithm.Besides multicast scoping considerations (see and ), this
document does not introduce any new security concern in addition to what
is discussed in Section 5 of , Section 10
of and Section 6 of .An mB4 SHOULD be provided with appropriate configuration information
to preserve the scope of a multicast message when mapping an IPv4
multicast address into an IPv4-embedded IPv6 multicast address and vice
versa.The CPE that embeds the mB4 function SHOULD be configured to accept
incoming MLD messages and traffic forwarded to multicast groups
subscribed by receivers located in the customer premises.The authors would like to thank Dan Wing for his guidance in the
early discussions which initiated this work. We also thank Peng Sun, Jie
Hu, Qiong Sun, Lizhong Jin, Alain Durand, Dean Cheng, Behcet Sarikaya,
Tina Tsou, Rajiv Asati, Xiaohong Deng, and Stig Venaas for their
valuable comments.Many thanks to Ian Farrer for the review.This document includes no request to IANA.IPTV generally includes two categories of service offerings:Video on Demand (VoD) that unicast video content to
receivers.Multicast live TV broadcast services.Two players intervene in the delivery of this service:Content Providers, who usually own the contents that is multicast
to receivers. Content providers may contractually define an
agreement with network providers to deliver contents to
receivers.Network Providers, who provide network connectivity services
(e.g., network providers are responsible for carrying multicast
flows from head-ends to receivers).Note that some contract agreements prevent a network provider from
altering the content as sent by the content provider for various
reasons. Depending on these contract agreements, multicast streams
should be delivered unaltered to the requesting users.Many current IPTV contents are likely to remain IPv4-formatted and
out of control of the network providers. Additionally, there are
numerous legacy receivers (e.g., IPv4-only Set Top Boxes (STB)) that
can't be upgraded or be easily replaced to support IPv6. As a
consequence, IPv4 service continuity must be guaranteed during the
transition period, including the delivery of multicast services such as
Live TV Broadcasting to users.The mAFTR can embed the MLD Querier function (as well as the
PIMv6 DR) for optimization purposes. When the mB4 sends a MLD Report
message to this mAFTR, the mAFTR should process the MLD Report
message that contains the IPv4-embedded IPv6 multicast group address
and then send the corresponding PIMv4 Join message. ()Discussions about the location of the mAFTR capability and
related ASM or SSM multicast design considerations are out of the
scope of this document.If the mAFTR is co-located with the DR connected to the original
IPv4 source, it may simply use the uPrefix64 and mPrefix64 prefixes
to build the IPv4-embedded IPv6 multicast packets, and the sending
of PIMv4 Join messages becomes unnecessary.Given the multiple versions of group membership management
protocols, mismatch issues may arise at the mB4 (refer to ).If IGMPv2 operates on the IPv4 receivers while MLDv2 operates on
the MLD Querier, or if IGMPv3 operates on the IPv4 receivers while
MLDv1 operates on the MLD Querier, the issue mentioned above will be
encountered. To solve this problem, the mB4 should perform the router
portion of IGMP which is similar to the corresponding MLD version
(IGMPv2 as of MLDv1, or IGMPv3 as of MLDv2) operating in the IPv6
domain. Then, the protocol interaction approach specified in Section 7
of can be applied to exchange signaling
messages with the IPv4 receivers on which the different version of
IGMP is operating.For robustness and load distribution purposes, several nodes in the
network can embed the mAFTR function. In such case, the same IPv6
prefixes (i.e., mPrefix64 and uPrefix64) and algorithm to build IPv4-
embedded IPv6 addresses must be configured on those nodes.For the sake of simplicity, it is recommended to configure the
mAFTR as the RP for the IPv4-embedded IPv6 multicast groups it
manages. No registration procedure is required under this
configuration.The mAFTR may be configured with a list of IPv4 multicast groups
and sources. Only multicast flows bound to the configured addresses
should be handled by the mAFTR. Otherwise, packets are silently
dropped.To optimize the usage of network resources in current deployments,
all multicast streams are conveyed in the core network while only the
most popular ones are forwarded in the aggregation/access networks
(static mode). Less popular streams are forwarded in the access
network upon request (dynamic mode). Depending on the location of the
mAFTR in the network, two modes can be envisaged: static and dynamic.
the mAFTR is configured to instantiate
permanent (S6, G6) and (*, G6) entries in its TIB6 using a
pre-configured (S4, G4) list.the instantiation or withdrawal of
(S6, G6) or (*, G6) entries is triggered by the receipt of PIMv6
messages.