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Network Working Group P. Savola Request for Comments: 3956 CSC/FUNET Updates: 3306 B. Haberman Category: Standards Track JHU APL

                                                         November 2004
            Embedding the Rendezvous Point (RP) Address
                    in an IPv6 Multicast Address

Status of this Memo

 This document specifies an Internet standards track protocol for the
 Internet community, and requests discussion and suggestions for
 improvements.  Please refer to the current edition of the "Internet
 Official Protocol Standards" (STD 1) for the standardization state
 and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

 Copyright (C) The Internet Society (2004).


 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  ...............................................   2
     1.1.  Background ............................................   2
     1.2.  Solution  .............................................   2
     1.3.  Assumptions and Scope .................................   3
     1.4.  Terminology  ..........................................   4
     1.5.  Abbreviations  ........................................   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
 5.  Examples  ...................................................   7
     5.1.  Example 1  ............................................   7
     5.2.  Example 2  ............................................   7
     5.3.  Example 3  ............................................   8
     5.4.  Example 4  ............................................   8

Savola & Haberman Standards Track [Page 1] RFC 3956 The RP Address in IPv6 Multicast Address November 2004

 6.  Operational Considerations  .................................   8
     6.1.  RP Redundancy .........................................   8
     6.2.  RP Deployment  ........................................   9
     6.3.  Guidelines for Assigning IPv6 Addresses to RPs ........   9
     6.4.  Use as a Substitute for BSR ...........................   9
     6.5.  Controlling the Use of RPs ............................   9
 7.  The Embedded-RP Group-to-RP Mapping Mechanism  ..............  10
     7.1.  PIM-SM Group-to-RP Mapping ............................  10
     7.2.  Overview of the Model .................................  11
 8.  Scalability Analysis  .......................................  12
 9.  Acknowledgements  ...........................................  13
 10. Security Considerations .....................................  13
 11. References ..................................................  15
     11.1. Normative References ..................................  15
     11.2. Informative References ................................  15
 A.  Discussion about Design Tradeoffs ...........................  16
 Authors' Addresses ..............................................  17
 Full Copyright Statement ......................................... 18

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 deliberately not been specified for IPv6.
 Therefore the whole interdomain Any Source Multicast (ASM) model is
 rendered unusable; Source-Specific Multicast (SSM) [SSM] avoids these
 problems but is not a complete solution for several reasons, as noted
 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 but can be used as a simple solution
 for IPv6 intra-domain ASM with scoped multicast addresses as well.

Savola & Haberman Standards Track [Page 2] RFC 3956 The RP Address in IPv6 Multicast Address November 2004

 It can also be used as an automatic RP discovery mechanism in those
 deployment scenarios that 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.
 Some design tradeoffs are discussed in Appendix A.

1.3. Assumptions and Scope

 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
 If these assumptions can't be followed, operational procedures and
 configuration must be slightly changed, or this mechanism can't be
 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 by using some unspecified method.  However, the
 mechanisms are probably similar to those 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.

Savola & Haberman Standards Track [Page 3] RFC 3956 The RP Address in IPv6 Multicast Address November 2004

1.4. Terminology

 Embedded-RP behaves as if all the members of the group were intra-
 domain to the information distribution. However, as it gives a
 solution for the global IPv6 multicast Internet, spanning multiple
 administrative domains, we say it is a solution for inter-domain
 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
 document are to be interpreted as described in [RFC2119].

1.5. Abbreviations

    ASM     Any Source Multicast
    BSR     Bootstrap Router
    DR      Designated Router
    IGP     Interior Gateway Protocol
    MLD     Multicast Listener Discovery
    MSDP    Multicast Source Discovery Protocol
    PIM     Protocol Independent Multicast
    PIM-SM  Protocol Independent Multicast - Sparse Mode
    RIID    RP Interface ID (as specified in this memo)
    RP      Rendezvous Point
    RPF     Reverse Path Forwarding
    SPT     Shortest Path Tree
    SSM     Source-Specific Multicast

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 as yet undefined,
 sent as zero and ignored on receipt.)

Savola & Haberman Standards Track [Page 4] RFC 3956 The RP Address in IPv6 Multicast Address November 2004

3. Modified Unicast-Prefix-based Address Format

 This memo specifies a modification to the unicast-prefix-based
 address format by specifying the second high-order bit ("R-bit") as
    |   8    |  4 |  4 |  4 |  4 | 8  |       64       |    32    |
    |11111111|flgs|scop|rsvd|RIID|plen| network prefix | group ID |
    flgs is a set of four flags:    |0|R|P|T|
 When the highest-order bit is 0, 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
 effect, this implies the prefix FF70::/12.  In this case, the last 4
 bits of the previously reserved field are interpreted as embedding
 the RP interface ID, as specified in this memo.
 The behavior is unspecified if P or T is not set to 1, as then the
 prefix would not be FF70::/12.  Likewise, the encoding and the
 protocol mode used when the two high-order bits in "flgs" are set to
 11 ("FFF0::/12") is intentionally unspecified until such time that
 the highest-order bit is defined.  Without further IETF
 specification, implementations SHOULD NOT treat the FFF0::/12 range
 as Embedded-RP.
 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 it is a multicast address as specified
 in this memo and to be processed any further, an address must satisfy
 all of the following:
 o It MUST be a multicast address with "flgs" set to 0111, that is, to
    be of the prefix FF70::/12,
 o  "plen" MUST NOT be 0 (i.e., not SSM), and

Savola & Haberman Standards Track [Page 5] RFC 3956 The RP Address in IPv6 Multicast Address November 2004

 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 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 |RIID|
 One should note that there are several operational scenarios (see
 Example 3 below) when the [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; 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, one can have 2^4-1 = 15 different

Savola & Haberman Standards Track [Page 6] RFC 3956 The RP Address in IPv6 Multicast Address November 2004

 RPs (as RIID=0 is reserved, see section 6.3).  However, each of these
 is an IPv6 group address of its own (i.e., there can be only one RP
 per multicast address).

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, by placing it on an existing
 subnet, e.g., 2001:DB8:BEEF:FEED::/64.
 In that case, the group addresses would be 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-ids to assign to customers and self ("y" could be anything from
 1 to F, as 0 must not be used).

5.2. Example 2

 As in Example 1, the network administrator of 2001:DB8::/32 wants to
 set up the RP but, to make it more flexible, wants to place it on a
 specifically routed subnet and wants to keep larger address space for
 group allocations.  That is, the administrator selects the least
 specific part of the unicast prefix, with plen=32, and the group
 addresses will be from the multicast prefix:
 where "x" is the multicast scope, "y" is the interface ID of the RP
 address, and there are 64 bits for group-ids or assignments.  In this
 case, the address of the RP would be:
 The address 2001:DB8::y/128 is assigned to a router as a loopback
 address and is injected into the routing system; if the network
 administrator sets up only one or two RPs (and, e.g., not one RP per
 subnet), this approach may be preferable to the one described in
 Example 1.

Savola & Haberman Standards Track [Page 7] RFC 3956 The RP Address in IPv6 Multicast Address November 2004

5.3. Example 3

 As in Example 2, the network administrator can also assign multicast
 prefixes such as "FF7x:y20:2001:DB8:DEAD::/80" to some of customers.
 In this case the RP address would still be "2001:DB8::y".  (Note that
 this is just a more specific subcase of Example 2, where the
 administrator assigns a multicast prefix, not just individual group-
 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 be consistent with the unicast prefix allocations.

5.4. Example 4

 In the network of Examples 1, 2, and 3, the network admin sets up
 addresses for use by customers, but an organization wants to have its
 own PIM-SM domain.  The organization can pick multicast addresses
 such as "FF7x:y30:2001:DB8:BEEF::/80", and then the RP address would
 be "2001:DB8:BEEF::y".

6. Operational Considerations

 This 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 RPs
 mainly for redundancy purposes.  Typically, MSDP has been used for
 this as well [ANYCASTRP].  There are also other approaches, such as
 using PIM for sharing this information [ANYPIMRP].
 The most feasible candidate for RP failover is using PIM for Anycast
 RP or "anycasting" (i.e., the shared-unicast model [ANYCAST]) the RP
 address in the Interior Gateway Protocol (IGP) without state sharing
 (although depending on the redundancy requirements, this may or may
 not be enough).  However, the redundancy mechanisms are outside of
 the scope of this memo.

Savola & Haberman Standards Track [Page 8] RFC 3956 The RP Address in IPv6 Multicast Address November 2004

6.2. RP Deployment

 As there is no need to share inter-domain state with MSDP, each
 Designated Router connecting multicast sources could act as an RP
 without scalability concerns about setting up and maintaining MSDP
 This might be particularly attractive when one is 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, its may also be desirable to distribute the RP
 responsibilities to multiple RPs.  As long as different RPs serve
 different groups, this 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 challenges one group
 faces are similar to those of Anycast-RP.

6.3. Guidelines for Assigning IPv6 Addresses to RPs

 With this mechanism, the RP can be given basically any unicast
 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
 unicast prefix (e.g., an additional loopback address assigned on a
 router, as described in Example 2 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 the RP to be used.

6.5. Controlling the Use of RPs

 Compared to the MSDP inter-domain ASM model, the control and
 management of who can use an RP, and how, changes slightly and
 deserves explicit discussion.

Savola & Haberman Standards Track [Page 9] RFC 3956 The RP Address in IPv6 Multicast Address November 2004

 MSDP advertisement filtering typically includes at least two
 capabilities: filtering who is able to create a global session
 ("source filtering") and filtering which groups should be globally
 accessible ("group filtering").  These are done to prevent local
 groups from being advertised to the outside or unauthorized senders
 from creating global groups.
 However, such controls do not yet block the outsiders from using such
 groups, as they could join the groups even without Source Active
 advertisement with a (Source, Group) or (S,G) Join by
 guessing/learning the source and/or the group address.  For proper
 protection, one should set up, for example, PIM multicast scoping
 borders at the border routers.  Therefore, embedded-RP has by default
 a roughly equivalent level of "protection" as MSDP with SA filtering.
 A new issue with control is that nodes in a "foreign domain" may
 register to an RP, or send PIM Join to an RP.  (These have been
 possible in the past as well, to a degree, but only through willful
 attempts or purposeful RP configuration at DRs.)  The main threat in
 this case is that an outsider may illegitimately use the RP to host
 his/hers own group(s).  This can be mitigated to an extent by
 filtering which groups or group ranges are allowed at the RP; more
 specific controls are beyond the scope of this memo.  Note that this
 does not seem to be a serious threat in the first place, as anyone
 with a /64 unicast prefix can create their own RP without having to
 illegitimately get it from someone else.

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

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

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 by 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 inconsistencies, for addresses in the range
 FF70::/12, the Embedded-RP mapping MUST be considered the longest
 possible match and higher priority than any other mechanism.

Savola & Haberman Standards Track [Page 10] RFC 3956 The RP Address in IPv6 Multicast Address November 2004

 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 that 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.  Paths for Shortest Path Tree (SPT) formation and
 Register-Stop do not require the 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 as follows:
 1. A receiver finds out a group address by some means (e.g., SDR or a
    web page).
 2. The receiver issues an Multicast Listener Discovery (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 as follows:
 1. A sender finds out a group address by 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), the algorithm derives it transparently from
 the encoded multicast address.

Savola & Haberman Standards Track [Page 11] RFC 3956 The RP Address in IPv6 Multicast Address November 2004

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 every group formed its own Internet-wide PIM domain, with the
 group mapping to a single RP, wherever the receivers or senders are
 located.  Hence, 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 are sent 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
 With IPv4 ASM multicast, there are roughly two kinds of Internet-wide
 state: MSDP (propagated everywhere), and multicast routing state (on
 the receiver or sender branches).  The former is eliminated, but 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 total amount of state is smaller.
 In both inter-domain and intra-domain cases, the embedded-RP model is
 practically identical 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 tree construction (i.e., multicast routing) by
 removing the RP for senders and receivers in foreign domains and
 eliminating the MSDP information distribution.
 As the address of the RP is tied to the multicast address, the RP
 failure management becomes more difficult, as the deployed failover
 or redundancy mechanisms (e.g., BSR, Anycast-RP with MSDP) cannot be
 used as-is.  However, Anycast-RP using PIM provides equal redundancy;
 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
 cannot even guarantee that the RP exists.  Instead of manually
 configuring RPs and DRs (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

Savola & Haberman Standards Track [Page 12] RFC 3956 The RP Address in IPv6 Multicast Address November 2004

 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" that is able to control (to some extent) who
 is 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 version 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 document a thorough review.
 Toerless Eckert, Hugh Holbrook, and Dave Meyer provided very
 extensive feedback.  In particular, Pavlin Radoslavov, Dino
 Farinacci, Nidhi Bhaskar, and Jerome Durand provided good comments
 during and after WG last call.  Mark Allman, Bill Fenner, Thomas
 Narten, and Alex Zinin provided substantive comments during the IESG
 evaluation.  The whole MboneD working group is also acknowledged for
 continued support and comments.

10. Security Considerations

 The addresses of RPs are encoded in the multicast addresses, thus
 becoming 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 for considerations regarding failover and Section 6.2 for
 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 Denial of Service attack
 scenario.  However, this can be mitigated, as the RP can discard all
 such messages for all multicast addresses that do not encode the
 address of the RP.  Both the sender- and receiver-based attacks are
 described at greater length in [PIMSEC].

Savola & Haberman Standards Track [Page 13] RFC 3956 The RP Address in IPv6 Multicast Address November 2004

 Additionally, the implementation SHOULD also allow manual
 configuration of which multicast prefixes are allowed to be used.
 This can be used to limit the use of the RP to designated groups
 only.  In some cases, being able to restrict (at the RP) which
 unicast addresses are allowed to send or join to a group is
 desirable.  (However, note that Join/Prune messages 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
 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 such).  Further, multicast groups or group ranges to-be-served MAY
 need to be explicitly configured at the RPs, to protect them from
 being used unwillingly.  Note that the more specific controls (e.g.,
 "insider-must-create" or "invite-outsiders" models) as to who is
 allowed to use the groups are beyond the scope of this memo.
 Excluding internal-only groups from MSDP advertisements does not
 protect the groups from outsiders but only offers security by
 obscurity; embedded-RP offers similar level of protection.  When real
 protection is desired, PIM scoping for example, should be set up at
 the borders. This is described at more length in Section 6.5.
 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 it would 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 configuration.
 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

Savola & Haberman Standards Track [Page 14] RFC 3956 The RP Address in IPv6 Multicast Address November 2004

11. References

11.1. Normative References

 [ADDRARCH]  Hinden, R. and S. Deering, "Internet Protocol Version 6
             (IPv6) Addressing Architecture", RFC 3513, April 2003.
 [RFC2119]   Bradner, S., "Key words for use in RFCs to Indicate
             Requirement Levels", BCP 14, RFC 2119, March 1997.
 [RFC3306]   Haberman, B. and D. Thaler, "Unicast-Prefix-based IPv6
             Multicast Addresses", RFC 3306, August 2002.

11.2. Informative References

 [ANYCAST]   Hagino, J. and K. Ettikan, "An analysis of IPv6 anycast",
             Work in Progress, June 2003.
 [ANYCASTRP] Kim, D., Meyer, D., Kilmer, H., and D. Farinacci,
             "Anycast Rendevous Point (RP) mechanism using Protocol
             Independent Multicast (PIM) and Multicast Source
             Discovery Protocol (MSDP)", RFC 3446, January 2003.
 [ANYPIMRP]  Farinacci, D. and Y. Cai, "Anycast-RP using PIM", Work in
             Progress, June 2004.
 [BSR]       Fenner, B., et al., "Bootstrap Router (BSR) Mechanism for
             PIM Sparse Mode", Work in Progress, July 2004.
 [MSDP]      Fenner, B. and D. Meyer, "Multicast Source Discovery
             Protocol (MSDP)", RFC 3618, October 2003.
 [PIMSEC]    Savola, P., Lehtonen, R., and D. Meyer, "PIM-SM Multicast
             Routing Security Issues and Enhancements", Work in
             Progress, October 2004.
 [PIM-SM]    Fenner, B. et al, "Protocol Independent Multicast -
             Sparse Mode (PIM-SM): Protocol Specification (Revised)",
             Work in Progress, July 2004.
 [SSM]       Holbrook, H. et al, "Source-Specific Multicast for IP",
             Work in Progress, September 2004.
 [V6MISSUES] Savola, P., "IPv6 Multicast Deployment Issues", Work in
             Progress, September 2004.

Savola & Haberman Standards Track [Page 15] RFC 3956 The RP Address in IPv6 Multicast Address November 2004

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
 leftmost 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
 Denial of Service 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

Savola & Haberman Standards Track [Page 16] RFC 3956 The RP Address in IPv6 Multicast Address November 2004

Authors' Addresses

 Pekka Savola
 Espoo, Finland
 Brian Haberman
 Johns Hopkins University Applied Physics Lab
 11100 Johns Hopkins Road
 Laurel, MD  20723-6099
 Phone: +1 443 778 1319

Savola & Haberman Standards Track [Page 17] RFC 3956 The RP Address in IPv6 Multicast Address November 2004

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Savola & Haberman Standards Track [Page 18]

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