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


                                                              April 2004

Embedding the Rendezvous Point (RP) Address in an 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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   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."

   The list of current Internet-Drafts can be accessed at

   To view the list Internet-Draft Shadow Directories, see


   This memo defines an address allocation policy in which the address
   of the Rendezvous Point (RP) is encoded in an IPv6 multicast group
   address.  For Protocol Independent Multicast - Sparse Mode (PIM-SM),
   this can be seen as a specification of a group-to-RP mapping
   mechanism.  This allows an easy deployment of scalable inter-domain
   multicast, and simplifies the intra-domain multicast configuration as
   well.  This memo updates the addressing format presented in RFC 3306.

Table of Contents

   1.  Introduction  ...............................................   3
     1.1.  Background  .............................................   3
     1.2.  Solution  ...............................................   3
     1.3.  Assumptions and Scope  ..................................   4
     1.4.  Keywords  ...............................................   4
   2.  Unicast-Prefix-based Address Format  ........................   4
   3.  Modified Unicast-Prefix-based Address Format  ...............   5
   4.  Embedding the Address of the RP in the Multicast Address  ...   5   6
   5.  Examples  ...................................................   7
     5.1.  Example 1  ..............................................   7
     5.2.  Example 2  ..............................................   7   8
     5.3.  Example 3  ..............................................   8
     5.4.  Example 4  ..............................................   8
   6.  Operational Considerations  .................................   8
     6.1.  RP Redundancy  ..........................................   8
     6.2.  RP Deployment  ..........................................   8   9
     6.3.  Guidelines for Assigning IPv6 Addresses to RPs  .........   9
     6.4.  Use as a Substitute for BSR  ............................   9  10
   7.  The Embedded-RP Group-to-RP Mapping Mechanism  ..............   9  10
     7.1.  PIM-SM Group-to-RP Mapping  .............................   9  10
     7.2.  Overview of the Model  ..................................  10
   8.  Scalability Analysis  .......................................  11
   9.  Acknowledgements  ...........................................  12
   10.  Security Considerations  ...................................  12  13
   11.  References  ................................................  13  14
     11.1.  Normative References  ..................................  13  14
     11.2.  Informative References  ................................  13  14
   Authors' Addresses  .............................................  14  15
   A.  Discussion about Design Tradeoffs  ..........................  14  15
   B.  Changes  ....................................................  15  16
     B.1  Changes since -01 -02  .......................................  15  16
     B.2  Changes since -01  .......................................  16
     B.3  Changes since -00  .......................................  15  16
   Intellectual Property Statement  ................................  16  17
   Full Copyright Statement  .......................................  16  17

1. Introduction

1.1. Background

   As has been noticed [V6MISSUES], there exists a deployment problem
   with global, interdomain IPv6 multicast: PIM-SM [PIM-SM] RPs have no
   way of communicating the information about (active) multicast sources
   to other multicast domains, as Multicast Source Discovery Protocol
   (MSDP) [MSDP] has not been, on purpose, specified for IPv6.
   Therefore the whole interdomain Any Source Multicast model is
   rendered unusable; Source-Specific Multicast (SSM) [SSM] avoids these
   problems but is not a complete solution for several reasons.

   Further, it has been noted that there are some problems with the
   support and deployment of mechanisms SSM would require [V6MISSUES]:
   it seems unlikely that SSM could be usable as the only interdomain
   multicast routing mechanism in the short term.

1.2. Solution

   This memo describes a multicast address allocation policy in which
   the address of the RP is encoded in the IPv6 multicast group address,
   and specifies a PIM-SM group-to-RP mapping to use the encoding,
   leveraging and extending unicast-prefix -based addressing [RFC3306].

   This mechanism not only provides a simple solution for IPv6
   interdomain Any Source Multicast (ASM) but can be used as a simple
   solution for IPv6 intradomain ASM with scoped multicast addresses as
   well.  It can also be used as an automatic RP discovery mechanism in
   those deployment scenarios which would have previously used the
   Bootstrap Router protocol (BSR) [BSR].

   The solution consists of three elements:

     o A specification of a subrange of [RFC3306] IPv6 multicast group
       addresses 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 description of operational procedures to operate ASM with PIM-
       SM on these IPv6 multicast groups.

   Addresses in the subrange will be called embedded-RP addresses.

   This scheme obviates the need for MSDP, and the routers are not
   required to include any multicast configuration, except when they act
   as an RP.

   This memo updates the addressing format presented in RFC 3306.

1.3. Assumptions and Scope

   In general, a 128-bit RP address can't be embedded into a 128-bit
   group address with space left to carry the group identity itself. An
   appropriate form of encoding is thus defined by requiring that the
   Interface-IDs of RPs in the embedded-RP range can be assigned to be a
   specific value.

   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; it is up to the RP and applications to ensure that group
   addresses are unique using some unspecified method.  However, the
   mechanisms are very probably similar to ones used with [RFC3306].

   Similarly, RP failure management methods, such as Anycast-RP, are out
   of scope for this document.  These do not work without additional
   specification or deployment.  This is covered briefly in Section 6.1.

1.4. Keywords

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

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  |
        |11111111|flgs|scop|reserved|plen| network prefix | group ID |

   Where flgs are "0011".  (The first two bits have been yet undefined,
   sent as zero and ignored on receipt.)

3. Modified Unicast-Prefix-based Address Format

   This memo specifies a modification to the unicast-prefix-based
   address format:

      1. If the second two high-order bit bits in "flgs" is are set to 1, 01, the address
         of the RP is embedded in the multicast address, as described in
         this memo.

      2. If the second two high-order bit in "flgs" is are set to 1, 01, interpret
         the last low-order 4 bits of "reserved" field as signifying the
         RP interface ID ("RIID"), as described in this memo.

   The encoding and the protocol mode used when the two high-order bit
   in "flgs" are set to 11 is intentionally unspecified until such time
   that the highest-order bit is defined.

   In consequence, the address format becomes:

        |   8    |  4 |  4 |  4 |  4 | 8  |       64       |    32    |
        |11111111|flgs|scop|rsvd|RIID|  plen  |
        |11111111|flgs|scop|rsvd|RIID|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 on the
   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 the previously reserved
   field are interpreted as embedding the RP interface ID, as specified
   in this memo.

   R = 0 indicates a multicast address that does not embed the address
   of the RP and follows the semantics defined in [ADDRARCH] and
   [RFC3306].  In this context, the value of "RIID" MUST be sent as zero
   and MUST be ignored on receipt.

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.

   That is, 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 below:

     o it MUST be a multicast address and have R, P, and T flag bits set
       to 1 (that -- that is, be part of the prefixes prefix FF70::/12 or FFF0::/12), (note that
       FFF0::/12 is unspecified),

     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 two steps:

      1. copy the first "plen" bits of the "network prefix" to a zeroed
         128-bit address structure, and
      2. replace the last 4 bits with the contents of "RIID".

   These two steps could be illustrated as follows:

        | 20 bits | 4  | 8  |       64       |    32    |
        |xtra bits|RIID|plen| network prefix | group ID |
                    ||    \\  vvvvvvvvvvv
                    ||     ``====> copy plen bits of "network prefix"
                    ||       +------------+------------------------+
                    ||       | network pre| 0000000000000000000000 |
                    ||       +------------+------------------------+
                      ``=================> copy RIID to the last 4 bits
                             | network pre| 0000000000000000000 |ID|

   One should note that there are several operational scenarios (see
   Example 2 below) when [RFC3306] statement "all non-significant bits
   of the network prefix field SHOULD be zero" is ignored.  This is to
   allow multicast group address allocations to be consistent with
   unicast prefixes, while the multicast addresses would still use the
   RP associated with the network prefix.

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

   When processing an encoding to get the RP address, the multicast
   routers 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).  At least fe80::/10, ::/16, and ff00::/8
   MUST be excluded.  This is particularly important as the information
   is obtained from an untrusted source, i.e., any Internet user's

   One should note that the 4 bits reserved for "RIID" set the upper
   bound for RPs for the combination of scope, network prefix, and group
   ID -- without varying any of these, you can have 4 bits worth of 2^4-1 = 15 different RPs. RPs
   (as RIID=0 is reserved).  However, each of these is an IPv6 group
   address of its own (i.e., there can be only one RP per multicast

5. Examples

   Four examples of multicast address allocation and resulting group-to-
   RP mappings are described here, to better illustrate the
   possibilities provided by the encoding.

5.1. Example 1

   The network administrator of 2001:DB8::/32 wants to set up an RP for
   the network and all the customers.  (S)he chooses network
   prefix=2001:DB8 and plen=32, and wants to use this addressing
   mechanism.  The multicast addresses (s)he will be able to use are of
   the form:


   Where "x" is the multicast scope, "y" the interface ID of the RP
   address, and "zzzz:zzzz" will be freely assignable to anyone. In this
   case, the address of the RP would be:


   (and "y" could be anything from 1 to F, as 0 must not be used); the
   address 2001:DB8::y/128 is added on assigned to a router as a loopback address
   and injected to the routing system.

5.2. Example 2

   As in Example 1, the network administrator can also allocate
   multicast addresses like "FF7x:y20:2001:DB8:DEAD::/80" to some of
   customers.  In this case the RP address would still be "2001:DB8::y".

   Note the second rule of deriving the RP address: the "plen" field in
   the multicast address, 0x20 = 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, "2001:DB8"
   are preserved: the value of "plen" takes no stance on actual
   unicast/multicast prefix lengths allocated or used in the networks,
   here from 2001:DB8:DEAD::/48.

   In short, this distinction allows more flexible RP address
   configuration in the scenarios where it is desirable to have the
   group addresses to be consistent with the unicast prefix allocations.

5.3. Example 3

   In the network of Examples 1 and 2, the network admin sets up
   addresses for use by their customers, but an organization wants to
   have their own PIM-SM domain.  The organization can pick multicast
   addresses like "FF7x:y30:2001:DB8:BEEF::/80", and then their RP
   address would be "2001:DB8:BEEF::y".

5.4. Example 4

   In the above networks, if the administrator wants to specify the RP
   to be in a non-zero /64 subnet, (s)he could always use something like
   "FF7x:y40:2001:DB8:BEEF:FEED::/96", and then their RP address would
   be "2001:DB8:BEEF:FEED::y".  There are still 32 bits of multicast
   group-id's to assign to customers and self.

6. Operational Considerations

   This desction section describes the major operational considerations for those
   deploying this mechanism.

6.1. RP Redundancy

   A technique called "Anycast RP" is used within a PIM-SM domain to
   share an address and multicast state information between a set of
   RP's mainly for redundancy purposes.  Typically, MSDP has been used
   for that as well [ANYCASTRP].  There are also other approaches, like
   using PIM for sharing this information [ANYPIMRP].

   RP failover cannot be used with this specification without additional
   mechanisms or techniques such as MSDP, PIM-SM extensions, or
   "anycasting" (i.e., the shared-unicast model [ANYCAST]) the RP
   address in the IGP without state sharing (depending on the redundancy
   requirements, this may or may not be enough, though).  However, the
   redundancy mechanisms are outside of the scope of this memo.

6.2. RP Deployment

   As there is no need to share inter-domain state with MSDP, each DR
   connecting multicast sources could act as an RP without scalability
   concerns about setting up and maintaining MSDP sessions.

   This might be particularly attractive when concerned about RP
   redundancy.  In the case where the DR close to a major source for a
   group acts as the RP, a certain amount of fate-sharing properties can
   be obtained without using any RP failover mechanisms: if the DR goes
   down, the multicast transmission may not work anymore in any case.

   Along the same lines, it's may also be desirable to distribute the RP
   responsibilities to multiple RPs.  As long as different RPs serve
   different groups, this is is trivial: each group could map to a
   different RP (or sufficiently many different RPs that the load on one
   RP is not a problem).  However, load sharing one group faces the
   similar challenges as Anycast-RP.

6.3. 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 "RIID".

   RIID = 0 must 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, as
   described in example 1 in Section 5.1), that address can be injected
   into the routing system via a host route.

6.4. Use as a Substitute for BSR

   With embedded-RP, use of BSR or other RP configuration mechanisms
   throughout the PIM domain is not necessary, as each group address
   specifies how the RP to be used.

7. The Embedded-RP Group-to-RP Mapping Mechanism

   This section specifies the group-to-RP mapping mechanism works for
   Embedded RP.

7.1. PIM-SM Group-to-RP Mapping

   The only PIM-SM modification required is implementing this mechanism
   as one group-to-RP mapping method.

   The implementation will have to recognize the address format and
   derive and use the RP address using the rules in Section 4.  This
   information is used at least when performing Reverse Path Forwarding
   (RPF) lookups, when processing Join/Prune messages, or performing

   To avoid loops and inconsistancies, the group-to-RP mapping specified
   in this memo MUST be used for all embedded-RP groups (i.e., addresses
   with prefix FF70::/12 or FFF0::/12). FF70::/12).

   It is worth noting that compared to the other group-to-RP mapping
   mechanisms, which can be precomputed, the embedded-RP mapping must be
   redone for every new IPv6 group address which would map to a
   different RP.  For efficiency, the results may be cached in an
   implementation-specific manner, to avoid computation for every
   embedded-RP packet.

   This group-to-RP mapping mechanism must be supported by the RP, the
   DR adjacent to the senders and any router on the path from any
   receiver to the RP.  Further, as the switch-over to  Paths for Shortest Path Tree (SPT)
   is also possible, it must be supported on the path between the
   receivers formation and the senders as well.  It also must be supported by any
   router on the path from any sender to the RP -- in case the RP issues
   Register-Stop and Joins the sources.  So, in practice, the
   mechanism must be supported by all routers on any path between do not require the
   RP, receivers, and senders. support, as those are accomplished
   with an (S,G) Join.

7.2. Overview of the Model

   This section gives a high-level, non-normative overview of how
   Embedded RP operates, as specified in the previous section.

   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 encoded in the multicast address, irrespective of
         whether it is in the "local" or "remote" PIM domain.

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

      1. A sender finds out a group address using an unspecified method
         (e.g, by contacting the administrator for group assignment or
         using 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 fact, all the messages go as specified in [PIM-SM] -- embedded-RP
   just acts as a group-to-RP mapping mechanism; instead of obtaining
   the address of the RP from local configuration or configuration
   protocols (e.g., BSR), it is derived transparently from the encoded
   multicast address.

8. Scalability Analysis

   Interdomain MSDP model for connecting PIM-SM domains is mostly
   hierarchical in configuration and deployment, but flat with regard to
   information distribution.  The embedded-RP inter-domain model behaves
   as if all of the Internet was a single PIM domain, with just one RP
   per group.  So, the inter-domain multicast becomes a flat, RP-
   centered topology.  The scaling issues are described below.

   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 (i.e., the prefix where the group address was
   derived from).  This is especially important with large multicast
   groups where there are a lot of heavy senders -- particularly if
   implementations do not handle unicast-decapsulation well.

   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 past receivers, for PIM Prunes) and
   the RP, in addition to (S, G) states between the receivers and
   senders, if SPT is used.  However, the traditional ASM model also
   requires MSDP state to propagate everywhere in inter-domain, so the
   total amount of state is smaller.

   The embedded-RP model is practically identical in both inter-domain
   and intra-domain cases to the traditional PIM-SM in intra-domain.  On
   the other hand, PIM-SM has been deployed (in IPv4) in inter-domain
   using MSDP; compared to that inter-domain model, this specification
   simplifies the multicast routing by removing the RP for senders and
   receivers in foreign domains, and eliminating the MSDP information

   As the address of the RP is tied to the multicast address, the RP
   failure management becomes more difficult, as failover or redundancy
   mechanisms (e.g., BSR, Anycast-RP with MSDP) cannot be used as-is.
   On the other hand, Anycast-RP using PIM could be used.  This
   described briefly in Section 6.1.

   The PIM-SM specification states, "Any RP address configured or
   learned MUST be a domain-wide reachable address".  What "reachable"
   precisely means is not clear, even without embedded-RP.  This
   statement cannot be proven especially with the foreign RPs as one can
   not even guarantee that the RP exists.  Instead of manually
   configuring RPs and DRs with a manual process (configuring a non-existent RP was possible
   though rare), with this specification the hosts and users using
   multicast indirectly specify the RP themselves, lowering the
   expectancy of the RP reachability.  This is a relatively significant
   problem but not much different from the current multicast deployment:
   e.g., MLDv2 (S,G) joins, whether ASM or SSM, yield the same result

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

   A more extensive description and comparison of the inter-domain
   multicast routing models (traditional ASM with MSDP, embedded-RP,
   SSM) and their security properties has been described in [PIMSEC].

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 an 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.
   Toerless Eckert, Hugh Holbrook, and Dave Meyer provided very
   extensive feedback.  In particular, Pavlin Radoslavov, Dino
   Farinacci, and Nidhi Bhaskar provided good comments during and after
   WG last call.  The whole MboneD working group is also acknowledged
   for the continued support and comments.

10. Security Considerations

   The address addresses of the RP is RPs are encoded in the multicast address addresses -- and
   thus become more visible as single points of failure.  Even though
   this does not significantly affect the multicast routing security, it
   may expose the RP to other kinds of attacks.  The operators are
   encouraged to pay special attention to securing these routers.  See
   Section 6.1 on considerations regarding failover and Section 6.2 on
   placement of RPs leading to a degree of fate-sharing properties.

   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 encode the
   address of the RP.  The  Both the sender- and receiver-based attacks are
   described at more length in [PIMSEC].

   Additionally the implementation MAY also allow manual configuration
   of which multicast addresses or prefixes embedding the RP could are allowed
   to be used.  This can be used to limit the use of the RP to
   designated groups only.  In a similar fashion, when a receiver joins some cases, it is desirable to an RP, be able to
   restrict (at the DRs must
   accept similar PIM-SM RP) which unicast addresses are allowed to send or
   join to a group. (However, note that Join/Prune messages back from RPs.  However, this would still
   leave state in the network, and Register messages can be spoofed
   [PIMSEC].)  Obviously, these controls are only possible at the RP,
   not at the intermediate routers or the DR.

   It is recommended that routers supporting this specification do not a
   considerable threat.
   act as RPs unless explicitly configured to do so; as becoming an RP
   does not require any advertisement (e.g., through BSR or manually),
   otherwise any router could potentially become an RP (and be abused as

   One should observe that the embedded-RP threat model is actually
   rather similar to SSM; both mechanisms significantly reduce the
   threats at the sender side.  On the receiver side, the threats are
   somewhat comparable, as an attacker could do an MLDv2 (S,G) join
   towards a non-existent source, which the local RP could not block
   based on the MSDP information.

   The implementation MUST perform at least the same address validity
   checks to the embedded-RP address as to one received via other means;
   at least fe80::/10, ::/16, and ff00::/8 should be excluded.  This is
   particularly important as the information is derived from the
   untrusted source (i.e., any user in the Internet), not from the local

   A more extensive description and comparison of the inter-domain
   multicast routing models (traditional ASM with MSDP, embedded-RP,
   SSM) and their security properties has been done separately in

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

   [ANYCAST]   Hagino, J., Ettikan, K., "An analysis of IPv6
               anycast", work-in-progress,
               draft-ietf-ipngwg-ipv6-anycast-analysis-02.txt, draft-ietf-ipngwg-ipv6-
               anycast-analysis-02.txt, June 2003.

   [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-ietf-pim-anycast-rp-00.txt,
               November 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 Source
               Discovery Protocol (MSDP)", RFC 3618, October 2003.

   [PIMSEC]    Savola, P., Lehtonen, R., Meyer, D., "PIM-SM Multicast
               Routing Security Issues and Enhancements",
               work-in-progress, draft-savola-mboned-mroutesec-00.txt,
               January draft-ietf-mboned-mroutesec-00.txt,
               April 2004.

   [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-09.txt,
               February 2004.

   [SSM]       Holbrook, H. et al, "Source-Specific Multicast for IP",
               work-in-progress, draft-ietf-ssm-arch-04.txt,
               October 2003.

   [V6MISSUES] Savola, P., "IPv6 Multicast Deployment Issues",
               work-in-progress, draft-savola-v6ops-multicast-
               issues-03.txt, February 2004.

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 document only specifies FF70::/12 for now; if/when the upper-most
   bit is used, one must specify how FFF0::/12 applies to Embedded-RP.
   For example, a different mode of PIM or another protocol might use
   that range, in contrast to FF70::/12, as currently specified, being
   for PIM-SM only.

   Instead of using flags bits ("FF70::/12"), one could have used the
   left-most reserved bits instead ("FF3x:8000::/17").

   It has been argued that instead of allowing the operator to specify
   RIID, the value could be pre-determined (e.g., "1").  However, this
   has not been adopted, as this eliminates address assignment
   flexibility from the operator.

   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.

   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, whether feasible or not,
   encoded in the RP address somehow, or in the multicast group address.
   In any case, such modifications are beyond the scope of this memo.

   For the cases where the RPs do not exist or are unreachable, or too
   much state is being generated to reach in a resource exhaustion DoS
   attack, some forms of rate-limiting or other mechanisms could be
   deployed to mitigate the threats while trying not to disturb the
   legitimate usage.  However, as the threats are generic, they are
   considered out of scope and discussed separately in [PIMSEC].

   The mechanism is not usable with Bidirectional PIM without protocol
   extensions, as pre-computing the Designated Forwarder is not

B. Changes

   [[ RFC-Editor: please remove before publication ]]

  B.1 Changes since -02

     o Clarified security considerations, wrt. RPs being abused by third
       parties and policy controls at the RP.
     o Clarified that only RPs, DRs next to sources sending to embedded-
       RP groups, and routers between the receivers and the RPs need to
       have support this mapping.
     o Try to be clearer that FF70::/12 is meant for PIM-SM at the
       moment, while FFF0::/12 is unspecified.
     o Minor miscellaneous changes.

  B.2 Changes since -01

     o Lots of editorial cleanups and some reorganization, without
       technical changes.
     o Remove the specification that RIID=0 SHOULD NOT be accepted, but
       state that they "must not" be used (implementation vs.
       operational wording).
     o Specify that the RP address MUST NOT be of prefixes fe80::/10,
       ::/16, or ff00::/8.


  B.3 Changes since -00

     o Lots of editorial cleanups, or cleanups without techinical
     o Reinforce the notion of Embedded RP just being a group-to-RP
       mapping mechanism (causing substantive rewriting in section 7);
       highlight the fact that precomputing the group-to-RP mapping is
       not possible.
     o Add (a bit) more text on RP redundancy and deployment tradeoffs
       wrt. RPs becoming SPoF.
     o Clarify the usability/scalability issues in section 8.
     o Clarify the security issues in Sections 8, Security
       Considerations and Appendix A, mainly by referring to a separate

     o Add a MUST that embedded-RP mappings must be honored by

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