mboned Working Group P. Savola
Internet Draft CSC/FUNET
Expiration Date: August 2004
B. Haberman
Caspian Networks
February 2004
Embedding the Rendezvous Point (RP) Address in an IPv6 Multicast Address
draft-ietf-mboned-embeddedrp-01.txt
Status of this Memo
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of Section 10 of RFC2026.
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Abstract
A very difficult deployment problem with global, interdomain IPv6
multicast using Protocol Independent Multicast - Sparse Mode (PIM-SM)
has been identified. This memo defines an address allocation policy
in which the address of the Rendezvous Point (RP) is encoded in the
IPv6 multicast group address. For 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.
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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 .............................................. 7
5.3. Example 3 .............................................. 7
5.4. Example 4 .............................................. 7
6. Operational Considerations ................................. 7
6.1. RP Redundancy .......................................... 7
6.2. RP Deployment .......................................... 8
6.3. Guidelines for Assigning IPv6 Addresses to RPs ......... 8
7. PIM-SM Protocol Modifications .............................. 8
7.1. PIM-SM Group-to-RP Mapping ............................. 9
7.2. Overview of the Model .................................. 9
8. Scalability/Usability Analysis ............................. 10
9. Acknowledgements ........................................... 11
10. Security Considerations ................................... 11
11. References ................................................ 13
11.1. Normative References .................................. 13
11.2. Informative References ................................ 13
Authors' Addresses ............................................. 14
A. Discussion about Design Tradeoffs .......................... 14
B. Changes since -00 .......................................... 15
Intellectual Property Statement ................................ 15
Full Copyright Statement ....................................... 16
1. Introduction
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 multicast sources to other
multicast domains, as there is no Multicast Source Discovery Protocol
(MSDP) [MSDP] (at least yet). 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: it seems
unlikely that SSM could be usable as the only interdomain multicast
routing mechanism in the short term.
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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 the 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 on scoped addresses as well. It
can also be used 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 specification of optional and mandatory 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 inter-domain MSDP, and the routers are
not required to include any multicast configuration, except when they
act as an RP.
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, and it is assumed that
the Interface-ID 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.
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This memo updates the addressing format presented in RFC 3306.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
2. Unicast-Prefix-based Address Format
As described in [RFC3306], the multicast address format is as
follows:
| 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, 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 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" is 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|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 ("RIID"), as
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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 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.
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 is, be part of the prefixes FF70::/12 or FFF0::/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 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|
+------------+---------------------+--+
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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 address assignments to third parties which still use
the RP associated with the network prefix.
"plen" higher than 64 MUST NOT be used as that would overlap with the
upper 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), to avoid e.g., the address being "::",
"::1", or a link-local address.
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
different RPs. 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 of his 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:
FF7x:y20:2001:DB8:zzzz:zzzz:<group-id>
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:
2001:DB8::y
(and "y" could be anything from 0 to F); the address 2001:DB8::y/128
is added as a Loopback address and injected to the routing system.
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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 his
customers within the PIM-SM domain. 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.
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; that's reasonable. 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 admin 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 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 the RP address in the IGP without state sharing (depending
on the redundancy requirements, this may or may not be enough,
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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 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 be all that interesting
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 should map to a
different RP (or enough 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 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
route.
7. PIM-SM Protocol Modifications
This section describes how PIM-SM is modified, i.e., how the group-
to-RP mapping mechanism works for Embedded RP.
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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 RPF lookups and when
processing Join/Prune messages, or performing Register-encapsulation.
To avoid loops and inconsistancies, the group-to-RP mapping specified
in this memo MUST be used for all embedded RP groups (i.e., with
prefix FF70::/12 or FFF0::/12).
It is worth noting that compared to the other group-to-RP mappings,
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.
This group-to-RP mapping mechanism must be supported by the DR
adjacent to senders and any router on the path from any receiver to
the RP. It also must be supported by any router on the path from any
sender to the RP -- in case the RP issues a Register-Stop and Joins
the sources.
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-
mappings can be derived from the application of multicast address
assignmen policies.
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).
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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 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/Usability 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 be 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. 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. Certainly, the amount of inter-domain multicast traffic
between sources and the embedded RP will increase compared to the ASM
model with MSDP.
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
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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.
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.
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 (one can
not even guarantee that the RP exists!). Instead of 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.
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 "home 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 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. The whole MboneD working group is also
acknowledged for the continued support and comments.
10. Security Considerations
The address of the RP is encoded 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 to work e.g.,
with MSDP, the address would have to be visible anyway (through via
other channels).
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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, 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 from RPs.
One consequence of the embedded RP 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 traditional ASM model. However, the traditional ASM
model also requires MSDP state to propagate everywhere in inter-
domain, so the total amount of state is smaller.
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-existent group or channel,
and the local DR or RP cannot readily reject (e.g., based on MSDP
information) such joins.
RPs become single points of failure as anycast-RP mechanism is not
(at least immediately) available. However, some other forms of
failover are still possible (see Section 6.1) and one can obtain some
forms of fate-sharing properties with a proper placement of RPs (see
Section 6.2).
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., "::", "::1", or
a link-local address.
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].
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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-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 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-08.txt,
October 2003.
[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-02.txt, October 2003.
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Authors' Addresses
Pekka Savola
CSC/FUNET
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
One could argue that there should be more RPs than the 4-bit "RIID"
allows for, especially if anycast-RP cannot be used. In that light,
extending "RIID" 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.
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 encoded in the RP address
somehow, or in the multicast group address. Whether this is a good
idea is another thing. 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. This has been described at more length in
[PIMSEC].
The mechanism is not usable with Bidirectional PIM without protocol
extensions, as pre-computing the Designated Forwarder is not
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possible.
B. Changes since -00
[[ RFC-Editor: please remove before publication ]]
o Lots of editorial cleanups, or cleanups without techinical
changes.
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
document.
o Add a MUST that embedded RP mappings must be honored by
implementations.
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Savola & Haberman [Expires August 2004] [Page 15]
Internet Draft draft-ietf-mboned-embeddedrp-01.txt February 2004
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Savola & Haberman [Expires August 2004] [Page 16]
