mboned Working Group                                           P. Savola
Internet Draft                                                 CSC/FUNET
Expiration Date: April 2004
                                                             B. Haberman
                                                        Caspian Networks

                                                            October 2003

         Embedding the Address of RP in IPv6 Multicast Address


Status of this Memo

   This document is an Internet-Draft and is subject to all provisions
   of Section 10 of RFC2026.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
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   There exists a huge deployment problem with global, interdomain IPv6
   multicast: Protocol Independent Multicast - Sparse Mode (PIM-SM)
   Rendezvous Points (RPs) have no way of communicating the information
   about multicast sources to other multicast domains, as there is no
   Multicast Source Discovery Protocol (MSDP), and the whole interdomain
   Any Source Multicast (ASM) model is rendered unusable; Source
   Specific Multicast (SSM) avoids these problems but is not considered
   readily deployable at the moment.  This memo defines a PIM-SM group-
   to-RP mapping which encodes the address of the RP in the IPv6
   multicast address. In consequence, there would be no need for
   interdomain MSDP, and even intra-domain RP configuration could be
   simplified.  This memo updates RFC 3306.

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Table of Contents

   1.  Introduction  ...............................................   2
   2.  Unicast-Prefix-based Address Format  ........................   4
   3.  Modified Unicast-Prefix-based Address Format  ...............   4
   4.  Embedding the Address of the RP in the Multicast Address  ...   5
   5.  Examples  ...................................................   6
     5.1.  Example 1  ..............................................   6
     5.2.  Example 2  ..............................................   6
     5.3.  Example 3  ..............................................   6
     5.4.  Example 4  ..............................................   7
   6.  Operational Requirements  ...................................   7
     6.1.  Anycast-RP  .............................................   7
     6.2.  Guidelines for Assigning IPv6 Addresses to RPs  .........   7
   7.  Required PIM-SM Modifications  ..............................   7
     7.1.  Overview of the Model  ..................................   9
   8.  Scalability/Usability Analysis  .............................   9
   9.  Acknowledgements  ...........................................  11
   10.  Security Considerations  ...................................  11
   11.  References  ................................................  12
     11.1.  Normative References  ..................................  12
     11.2.  Informative References  ................................  12
   Authors' Addresses  .............................................  13
   A.  Discussion about Design Tradeoffs  ..........................  13
   Intellectual Property Statement  ................................  14
   Full Copyright Statement  .......................................  15

1. Introduction

   As has been noticed [V6MISSUES], there exists a huge deployment
   problem with global, interdomain IPv6 multicast: PIM-SM [PIM-SM] RPs
   have no way of communicating the information about multicast sources
   to other multicast domains, as there is no MSDP [MSDP], and the whole
   interdomain Any Source Multicast model is rendered unusable; SSM
   [SSM] avoids these problems.

   It has been noted that there are some problems with SSM deployment
   and support: it seems unlikely that SSM could be usable as the only
   interdomain multicast routing mechanism in the short term.  This memo
   proposes a fix to interdomain multicast routing, and provides an
   additional method for the RP discovery with the intra-domain case.

   This document proposes a solution to the group-to-RP mapping problem
   which leverages and extends [RFC3306] by encoding the RP address of
   the IPv6 multicast group into the group address itself.

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   This mechanism not only provides a simple solution for IPv6
   interdomain ASM but can be used as a simple solution for IPv6
   intradomain ASM on scoped addresses, as well. The use as a substitute
   for Bootstrap Router protocol (BSR) [BSR] is also possible.

   The solution consists of two elements applicable to a subrange of
   [RFC3306] IPv6 multicast group addresses which are defined by setting
   one previously unused bit of the Flags field to "1":

     o A specification of the mapping by which such a group address
       encodes the RP address that is to be used with this group, and

     o A specification of optional and mandatory procedures to operate
       ASM with PIM-SM on these IPv6 multicast groups.

   Addresses in this  subrange will be called embedded-RP addresses.  If
   used in the interdomain, a mechanism similar to MSDP is not required
   for these addresses and RP configuration for these addresses can be
   as simple as zero configuration for routers supporting this

   It is self-evident that a 128 bit RP address can in general not be
   embedded into a 128-bit group address with space left to carry a
   group identity itself. An appropriate form of encoding is thus
   defined, and it is assumed that the Interface-ID of RPs in the
   embedded-RP range can be assigned to be specific values.

   If these assumptions can't be followed, either operational procedures
   and configuration must be slightly changed or this mechanism can not
   be used.

   The assignment of multicast addresses is outside the scope of this
   document; however, the mechanisms are very probably similar to ones
   used with [RFC3306].

   This memo updates the addressing format presented in RFC 3306.

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

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2. Unicast-Prefix-based Address Format

   As described in [RFC3306], the multicast address format is as

        |   8    |  4 |  4 |   8    |    8   |       64       |    32    |
        |11111111|flgs|scop|reserved|  plen  | network prefix | group ID |

   Where flgs are "0011".  (The first two bits are yet undefined and
   thus zero.)

3. Modified Unicast-Prefix-based Address Format

   This memo proposes a modification to the unicast-prefix-based address

      1. If the second high-order bit in "flgs" is set to 1, the address
         of the RP is embedded in the multicast address, as described in
         this memo.

      2. If the second high-order bit in "flgs" was set to 1, interpret
         the last low-order 4 bits of "reserved" field as signifying the
         RP interface ID, as described in this memo.

   In consequence, the address format becomes:

        |   8    |  4 |  4 |  4 |  4 |    8   |       64       |    32    |
        |11111111|flgs|scop|rsvd|RPad|  plen  | network prefix | group ID |
        flgs is a set of 4 flags:       |0|R|P|T|

   R = 1 indicates a multicast address that embeds the address of the
   PIM-SM RP.  Then P MUST BE set to 1, and consequently T MUST be set
   to 1, as specified in [RFC3306].

   In the case that R = 1, the last 4 bits of previously reserved field
   ("RPad") are interpreted as embedding the interface ID of the RP, as
   specified in this memo.

   R = 0 indicates a multicast address that does not embed the address
   of the PIM-SM RP and follows the semantics defined in [ADDRARCH] and
   [RFC3306].  In this context, the value of "RPad" has no meaning.

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4. Embedding the Address of the RP in the Multicast Address

   The address of the RP can only be embedded in unicast-prefix -based
   ASM addresses.

   To identify whether an address is a multicast address as specified in
   this memo and to be processed any further, it must satisfy all of the

     o it MUST be a multicast address and have R, P, and T flag bits set
       to 1 (that is, be part of the prefix FF7::/12 or FFF::/12),

     o "plen" MUST NOT be 0 (ie. not SSM), and

     o "plen" MUST NOT be greater than 64.

   The address of the RP can be obtained from a multicast address
   satisfying the above criteria by taking the following steps:

      1. take the last 96 bits of the multicast address add 32 zero bits
         at the end,

      2. zero the last 128-"plen" bits, and

      3. replace the last 4 bits with the contents of "RPad".

   One should note that there are several operational scenarios when
   [RFC3306] statement "all non-significant bits of the network prefix
   field SHOULD be zero" is ignored -- and why the second step, above,
   is necessary.  This is to allow multicast address assignments to
   third parties which still use your RP; see example 2 below.

   "plen" higher than 64 MUST NOT be used as that would overlap with the
   upper bits of multicast group-id.

   The implementation MUST perform at least the same address validity
   checks to the calculated RP address as to one received via other
   means (like BSR [BSR] or MSDP for IPv4), to avoid e.g. the address
   being "::" or "::1".

   One should note that the 4 bits reserved for "RPad" set the upper
   bound for RPs per multicast group address; not the number of RPs in a
   subnet, PIM-SM domain or large-scale network.

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5. Examples

5.1. Example 1

   The network administrator of 3FFE:FFFF::/32 wants to set up an RP for
   the network and all of his customers.  He chooses network
   prefix=3FFE:FFFF and plen=32, and wants to use this addressing
   mechanism.  The multicast addresses he will be able to use are of the


   Where "x" is the multicast scope, "y" the interface ID of the RP
   address, and "zzzz:zzzz" will be freely assignable within the PIM-SM
   domain. In this case, the address of the PIM-SM RP would be:


   (and "y" could be anything from 0 to F); the address 3FFE:FFFF::y/128
   is added as a Loopback address and injected to the routing system.

5.2. Example 2

   As above, the network administrator can also allocate multicast
   addresses like "FF7x:y20:3FFE:FFFF:DEAD::/80" to some of his
   customers within the PIM-SM domain.  In this case the RP address
   would still be "3FFE:FFFF::y".

   Note the second rule of deriving the RP address: the "plen" field in
   the multicast address, (hex)20 = 32, refers to the length of "network
   prefix" field considered when obtaining the RP address.  In this
   case, only the first 32 bits of the network prefix field, "3FFE:FFFF"
   are preserved: the value of "plen" takes no stance on actual
   unicast/multicast prefix lengths allocated or used in the networks,
   here from 3FFE:FFFF:DEAD::/48.

5.3. Example 3

   In the above network, the network admin sets up addresses as above,
   but an organization wants to have their own PIM-SM domain; that's
   reasonable.  The organization can pick multicast addresses like
   "FF7x:y30:3FFE:FFFF:BEEF::/80", and then their RP address would be

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5.4. Example 4

   In the above networks, if the admin wants to specify the RP to be in
   a non-zero /64 subnet, he could always use something like
   "FF7x:y40:3FFE:FFFF:BEEF:FEED::/96", and then their RP address would
   be "3FFE:FFFF:BEEF:FEED::y".  There are still 32 bits of multicast
   group-id's to assign to customers and self.

6. Operational Requirements

6.1. Anycast-RP

   One should note that MSDP is also used, in addition to interdomain
   connections between RPs, in anycast-RP [ANYCASTRP] -technique, for
   sharing the state information between different RPs in one PIM-SM
   domain.  However, there are other propositions, like [ANYPIMRP].

   Anycast-RP mechanism is incompatible with this addressing method
   unless MSDP is specified and implemented.  Alternatively, another
   method for sharing state information could be used.

   Anycast-RP and other possible RP failover mechanisms are outside of
   the scope of this memo.

6.2. Guidelines for Assigning IPv6 Addresses to RPs

   With this mechanism, the RP can be given basically any network prefix
   up to /64. The interface identifier will have to be manually
   configured to match "RPad".

   RPad = 0 SHOULD NOT be used as using it would cause ambiguity with
   the Subnet-Router Anycast Address [ADDRARCH].

   If an administrator wishes to use an RP address that does not conform
   to the addressing topology but is still from the network provider's
   prefix (e.g. an additional loopback address assigned on a router),
   that address can be injected into the routing system via a host

7. Required PIM-SM Modifications

   The use of multicast addresses with embedded RP addresses requires
   additional PIM-SM processing.  Namely, a PIM-SM router will need to
   be able to recognize the encoding and derive the RP address from the
   address using the rules in section 4 and to be able to use the
   embedded RP, instead of its own for multicast addresses in this
   specified range.

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   The three key places where these modifications are used are the
   Designated Routers (DRs) on the receiver/sender networks, the
   backbone networks, and the RPs in the domain where the embdedded
   address has been derived from (see figure below).

   For the foreign DRs (rtrR1, rtrR23, and rtrR4), this means sending
   PIM-SM Join/Prune/Register messages towards the foreign RP (rtrRP_S).
   Naturally, PIM-SM Register-Stop and other messages must also be
   allowed from the foreign RP.  DRs in the local PIM-SM domain (rtrS)
   do the same.

   For the RP (rtrRP_S), this means being able to recognize and validate
   PIM-SM messages which use RP-embedded addressing originated from any
   DR at all.

   For the other routers on the path (rtrBB), this means recognizing and
   validating that the Join/Prune PIM-SM messages using the embedded RP
   addressing are on the right path towards the RP they think is in
   charge of the particular address.

        nodeS - rtrS - rtrRP_S - rtrBB -----+--- rtrR1 - node1
                         |         |        |
        node2_S ---------+         |        +-- rtrR23 - node2
                                   |               |
                                   |               +---- node3
                                   +------------ rtrR4 - node4

   In addition, the administration of the PIM-SM domains MAY have an
   option to manually override the RP selection for the embedded RP
   multicast addresses: the default policy SHOULD be to use the embedded

   The extraction of the RP information from the multicast address
   should be done during forwarding state creation.  That is, if no
   state exists for the multicast address, PIM-SM must take the embedded
   RP information into account when creating forwarding state.  Unless
   otherwise dictated by the administrative policy, this would result in
   a receiver's DR initiating a PIM-SM Join towards the foreign RP or a
   source's DR sending PIM-SM Register messages towards the foreign RP.

   It should be noted that this approach removes the need to run inter-
   domain MSDP.  Multicast distribution trees in foreign networks can be
   joined by issuing a PIM-SM Join/Prune/Register to the RP address
   encoded in the multicast address.

   Also, the addressing model described here could be used to replace or
   augment the intra-domain Bootstrap Router mechanism (BSR), as the RP-

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   mappings can be communicated by the multicast address assignment.

7.1. Overview of the Model

   The steps when a receiver wishes to join a group are:

      1. A receiver finds out a group address from some means (e.g. SDR
         or a web page).
      2. The receiver issues an MLD Report, joining the group.
      3. The receiver's DR will initiate the PIM-SM Join process towards
         the RP embedded in the multicast address.

   The steps when a sender wishes to send to a group are:

      1. A sender finds out a group address from some means, whether in
         an existing group (e.g. SDR, web page) or in a new group (e.g.
         a call to the administrator for group assignment, use of a
         multicast address assignment protocol).
      2. The sender sends to the group.
      3. The sender's DR will send the packets unicast-encapsulated in
         PIM-SM Register-messages to the RP address encoded in the
         multicast address (in the special case that DR is the RP, such
         sending is only conceptual).

   In both cases, the messages then go on as specified in [PIM-SM] and
   other specifications (e.g.  Register-Stop and/or SPT Join); there is
   no difference in them except for the fact that the RP address is
   derived from the multicast address.

   Sometimes, some information, using conventional mechanisms, about
   another RP exists in the PIM-SM domain.  The embedded RP SHOULD be
   used by default, but there MAY be an option to switch the preference.
   This is because especially when performing PIM-SM forwarding in the
   transit networks, the routers must have the same notion of the RP, or
   else the messages may be dropped.

8. Scalability/Usability Analysis

   Interdomain MSDP model for connecting PIM-SM domains is mostly
   hierarchical.  The "embedded RP address" changes this to a mostly
   flat, sender-centered, full-mesh virtual topology.

   This may or may not cause some effects; it may or may not be
   desirable.  At the very least, it makes many things much more robust
   as the number of third parties is minimized.  A good scalability
   analysis is needed.

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   In some cases (especially if e.g. every home user is employing site-
   local multicast), some degree of hierarchy would be highly desirable,
   for scalability (e.g. to take the advantage of shared multicast
   state) and administrative point-of-view.

   Being able to join/send to remote RPs has security considerations
   that are considered below, but it has an advantage too: every group
   has a "home RP" which is able to control (to some extent) who are
   able to send to the group.

   One should note that the model presented here simplifies the PIM-SM
   multicast routing model slightly by removing the RP for senders and
   receivers in foreign domains.  One scalability consideration should
   be noted: previously foreign sources sent the unicast-encapsulated
   data to their local RP, now they do so to the foreign RP responsible
   for the specific group.  This is especially important with large
   multicast groups where there are a lot of heavy senders --
   particularly if implementations do not handle unicast-decapsulation

   This model increases the amount of Internet-wide multicast state
   slightly: the backbone routers might end up with (*, G) and (S, G,
   rpt) state between receivers and the RP, in addition to (S, G) states
   between the receivers and senders.  Certainly, the amount of inter-
   domain multicast traffic between sources and the embedded-RP will
   increase compared to the ASM model with MSDP; however, the domain
   responsible for the RP is expected to be able to handle this.

   As the address of the RP is tied to the multicast address, in the
   case of RP failure PIM-SM BSR mechanisms cannot pick a new RP; the
   failover mechanisms, if used, for backup RPs are different, and
   typically would depend on sharing one address.  The failover
   techniques are outside of the scope of this memo.

   The PIM-SM specification states, "Any RP address configured or
   learned MUST be a domain-wide reachable address".  What this means is
   not clear, even without embedded-RP.  However, typically this
   statement cannot be proven especially with the foreign RPs (typically
   one can not even guarantee that the RP exists!).  The bottom line is
   that while traditionally the configuration of RPs and DRs was
   typically a manual process, and e.g. configuring a non-existant RP
   was possible, but here the hosts and users which use multicast
   indirectly specify the RP.

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9. Acknowledgements

   Jerome Durand commented on an early draft of this memo.  Marshall
   Eubanks noted an issue regarding short plen values.  Tom Pusateri
   noted problems with earlier SPT-join approach.  Rami Lehtonen pointed
   out issues with the scope of SA-state and provided extensive
   commentary.  Nidhi Bhaskar gave the draft a thorough review.  The
   whole MboneD working group is also acknowledged for the continued
   support and comments.

10. Security Considerations

   The address of the PIM-SM RP is embedded in the multicast address.
   RPs may be a good target for Denial of Service attacks -- as they are
   a single point of failure (excluding failover techniques) for a
   group. In this way, the target would be clearly visible.  However, it
   could be argued that if interdomain multicast was to be made work
   e.g. with MSDP, the address would have to be visible anyway (through
   via other channels, which may be more easily securable).

   As any RP will have to accept PIM-SM Join/Prune/Register messages
   from any DR, this might cause a potential DoS attack scenario.
   However, this can be mitigated by the fact that the RP can discard
   all such messages for all multicast addresses that do not embed the
   address of the RP, and if deemed important, the implementation could
   also allow manual configuration of which multicast addresses or
   prefixes embedding the RP could be used, so that only the pre-agreed
   sources could use the RP.

   In a similar fashion, when a receiver joins to an RP, the DRs must
   accept similar PIM-SM messages back RPs.

   One consequence of the usage model is that it allows Internet-wide
   multicast state creation (from receiver(s) in another domain to the
   RP in another domain) compared to the domain wide state creation in
   the MSDP model.

   One should observe that the embedded RP threat model is actually
   pretty similar to SSM; both mechanisms significantly reduce the
   threats at the sender side, but have new ones in the receiver side,
   as any receiver can try to join any non-existant group or channel,
   and the local DR or RP cannot readily reject such joins (based on
   MSDP information).

   RPs may become a bit more single points of failure as anycast-RP
   mechanism is not (at least immediately) available.  This can be
   partially mitigated by the fact that some other forms of failover are
   still possible, and there should be less need to store state as with

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   The implementation MUST perform at least the same address validity
   checks to the embedded RP address as to one received via other means
   (like BSR or MSDP), to avoid the address being e.g. "::" or "::1".

11. References

11.1. Normative References

   [ADDRARCH]  Hinden, R., Deering, S., "IP Version 6 Addressing
               Architecture", RFC3513, April 2003.

   [RFC2119]   Bradner, S., "Key words for use in RFCs to Indicate
               Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC3306]   Haberman, B., Thaler, D., "Unicast-Prefix-based IPv6
               Multicast Addresses", RFC3306, August 2002.

11.2. Informative References

   [ANYCASTRP] Kim, D. et al, "Anycast RP mechanism using PIM and
               MSDP", RFC 3446, January 2003.

   [ANYPIMRP]  Farinacci, D., Cai, Y., "Anycast-RP using PIM",
               work-in-progress, draft-farinacci-pim-anycast-rp-00.txt,
               January 2003.

   [BSR]       Fenner, B., et al., "Bootstrap Router (BSR) Mechanism for
               PIM Sparse Mode", work-in-progress, draft-ietf-pim-sm-
               bsr-03.txt, February 2003.

   [MSDP]      Meyer, D., Fenner, B, (Eds.), "Multicast Sourc
               Discovery Protocol (MSDP)", work-in-progress,
               draft-ietf-msdp-spec-20.txt, May 2003.

   [PIM-SM]    Fenner, B. et al, "Protocol Independent Multicast -
               Sparse Mode (PIM-SM): Protocol Specification (Revised),
               work-in-progress, draft-ietf-pim-sm-v2-new-08.txt,
               October 2003.

   [SSM]       Holbrook, H. et al, "Source-Specific Multicast for IP",
               work-in-progress, draft-ietf-ssm-arch-03.txt,
               May 2003.

   [V6MISSUES] Savola, P., "IPv6 Multicast Deployment Issues",
               work-in-progress, draft-savola-v6ops-multicast-
               issues-02.txt, October 2003.

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Authors' Addresses

   Pekka Savola
   Espoo, Finland
   EMail: psavola@funet.fi

   Brian Haberman
   Caspian Networks
   One Park Drive, Suite 300
   Research Triangle Park, NC  27709
   EMail: brian@innovationslab.net
   Phone: +1-919-949-4828

A. Discussion about Design Tradeoffs

   The initial thought was to use only SPT join from local RP/DR to
   foreign RP, rather than a full PIM Join to foreign RP.  However, this
   turned out to be problematic, as this kind of SPT joins where
   disregarded because the path had not been set up before sending them.
   A full join to foreign PIM domain is a much clearer approach.

   One could argue that there should be more RPs than the 4-bit "RPad"
   allows for, especially if anycast-RP cannot be used.  In that light,
   extending "RPad" to take full advantage of whole 8 bits would seem
   reasonable.  However, this would use up all of the reserved bits, and
   leave no room for future flexibility.  In case of large number of
   RPs, an operational workaround could be to split the PIM domain: for
   example, using two /33's instead of one /32 would gain another 16 (or
   15, if zero is not used) RP addresses.  Note that the limit of 4 bits
   worth of RPs just depends on the prefix the RP address is derived
   from; one can use multiple prefixes in a domain, and the limit of 16
   (or 15) RPs should never really be a problem.

   Some hierarchy (e.g. two-level, "ISP/customer") for RPs could
   possibly be added if necessary, but that would be torturing one 128
   bits even more.

   One particular case, whether in the backbone or in the sender's
   domain, is where the regular PIM-SM RP would be X, and the embedded
   RP address would be Y.  This could e.g. be a result of a default all-
   multicast-to-one-RP group mapping, or a local policy decision.  The
   embedded RP SHOULD be used by default, but there MAY be an option to
   change this preference.

   Values 64 < "plen" < 96 would overlap with upper bits of the
   multicast group-id; due to this restriction, "plen" must not exceed
   64 bits.  This is in line with RFC 3306.

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   The embedded RP addressing could be used to convey other information
   (other than RP address) as well, for example, what should be the RPT
   threshold for PIM-SM.  These could be encoded in the RP address
   somehow, or in the multicast group address.  However, such
   modifications are beyond the scope of this memo.

   Some kind of rate-limiting functions, ICMP message responses, or
   similar could be defined for the case of when the RP embedded in the
   address is not willing to serve for the specific group (or doesn't
   even exist).  Typically this would result in the datagrams getting
   blackholed or rejected with ICMP.  In particular, a case for
   "rejection" or "source quench" -like messages would be in the case
   that a source keeps transmitting a huge amount of data, which is sent
   to a foreign RP using Register message but is discarded if the RP
   doesn't allow the source host to transmit: the RP should be able to
   indicate to the DR, "please limit the amount of Register messages",
   or "this source sending to my group is bogus".  Note that such "kiss-
   of-death" packets have an authentication problem; spoofing them could
   result in an entirely different kind of Denial of Service, for
   legitimate sources.  One possibility here would be to specify some
   form of "return routability" check for DRs and RPs; for example, if a
   DR receives packets from a host to group G G (resulting in RP address
   R), the DR would send only a limited amount of packets to R until it
   has heard back from R (a "positive acknowledgement").  It is not
   clear whether this needs to be considered or specified in more

   Could this model work with bidir-PIM?  Is it feasible?  Not sure, not
   familiar enough with bidir-PIM.

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Internet Draft     draft-ietf-mboned-embeddedrp-00.txt      October 2003

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