Internet Engineering Task Force P. Savola
Internet Draft CSC/FUNET
Expiration Date: September 2003
B. Haberman
Caspian Networks
March 2003
Embedding the Address of RP in IPv6 Multicast Address
draft-savola-mboned-mcast-rpaddr-02.txt
Status of this Memo
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Abstract
As has been noticed, there is exists a huge deployment problem with
global, interdomain IPv6 multicast: PIM Renzesvous Points (RPs) have
no way of communicating the information about multicast sources to
other multicast domains, as there is no MSDP, and the whole
interdomain Any Source Multicast model is rendered unusable; SSM
avoids these problems. This memo outlines a way to embed the address
of the RP in the multicast address, solving the interdomain multicast
problem. The problem is three-fold: specify an address format, adjust
the operational procedures and configuration if necessary, and modify
PIM implementations of those who want to join or send to a group
(Designated Routers) or provide one (Rendezvous Points). In
consequence, there would be no need for interdomain MSDP.
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Table of Contents
1. Introduction ............................................... 2
2. Unicast-Prefix-based Address Format ........................ 3
3. Modified Unicast-Prefix-based Address Format ............... 3
4. Embedding the Address of the RP in the Multicast Address ... 4
5. Examples ................................................... 5
5.1. Example 1 .............................................. 5
5.2. Example 2 .............................................. 5
5.3. Example 3 .............................................. 6
5.4. Example 4 .............................................. 6
6. Operational Requirements ................................... 6
6.1. Anycast-RP ............................................. 6
6.2. Guidelines for Assigning IPv6 Addresses to RPs ......... 6
7. Required PIM Modifications ................................. 7
7.1. Overview of the Model .................................. 8
8. Scalability/Usability Analysis ............................. 8
9. Acknowledgements ........................................... 9
10. Security Considerations ................................... 10
11. References ................................................ 11
11.1. Normative References .................................. 11
11.2. Informative References ................................ 11
Authors' Addresses ............................................. 11
A. Open Issues/Discussion ..................................... 12
1. Introduction
As has been noticed [V6MISSUES], there is exists a huge deployment
problem with global, interdomain IPv6 multicast: PIM [PIM] RPs have
no way of communicating the information about multicast sources to
other multicast domains, as there is no MSDP [MSDP], and the whole
interdomain Any Source Multicast model is rendered unusable; SSM
[SSM] avoids there problems.
This memo outlines a way to embed the address of the RP in the
multicast address, solving the interdomain multicast problem. The
problem is three-fold: specify an address format, adjust the
operational procedures and configuration if necessary, and modify PIM
implementations of DR's where receivers/senders are expected use the
multicast addressing as described in this memo. In consequence,
there would be no need for interdomain MSDP.
The solution is founded upon unicast-prefix-based IPv6 multicast
addressing [UNIPRFXM] and making some assumptions about IPv6 address
assignment for the RPs in the PIM domain.
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Further, a change in how interdomain PIM operates with these
addresses is presented: multicast receivers' and senders' DR's join
or send to (respectively) the RP embedded in the address -- not their
locally configured RP.
It is self-evident that one can't embed, in the general case, two
128-bit addresses in one 128-bit address. In this memo, some
assumptions on how this could be done are made. If these assumptions
can't be followed, either operational procedures and configuration
must be slightly changed or this mechanism not be used.
The assignment of multicast addresses is outside the scope of this
document; however, the mechanisms are very probably similar to ones
used with [UNIPRFXM].
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 [UNIPRFXM], 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 and
thus zero.)
3. Modified Unicast-Prefix-based Address Format
This memo proposes 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" was set to 1, interpret
the last low-order 4 bits of "reserved" field as signifying the
RP interface ID, as described in this memo.
In consequence, the address format becomes:
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| 8 | 4 | 4 | 4 | 4 | 8 | 64 | 32 |
+--------+----+----+----+----+--------+----------------+----------+
|11111111|flgs|scop|rsvd|RPad| plen | network prefix | group ID |
+--------+----+----+----+----+--------+----------------+----------+
+-+-+-+-+
flgs is a set of 4 flags: |0|R|P|T|
+-+-+-+-+
R = 1 indicates a multicast address that embeds the address of the
PIM RP. Then P MUST BE set to 1, and consequently T MUST be set to
1, as specified in [UNIPRFXM].
In the case that R = 1, the last 4 bits of previously reserved field
("RPad") are interpreted as embedding the interface ID of the RP, as
specified in this memo.
R = 0 indicates a multicast address that does not embed the address
of the PIM RP and follows the semantics defined in [ADDRARCH] and
[UNIPRFXM]. In this context, the value of "RPad" has no meaning.
4. Embedding the Address of the RP in the Multicast Address
The address of the RP can only be embedded in unicast-prefix -based
addresses, but the scheme could be extended to other forms of
multicast addresses as well. Further, the mechanism cannot be
combined with SSM, as SSM has no RP's.
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 prefix FF7::/12 or FFF::/12)
o "plen" MUST NOT be 0 (ie. not SSM)
o "plen" MUST NOT be greater than 96
The address of the RP can be obtained from a multicast address
satisfying the above criteria by taking the following steps:
1. take the last 96 bits of the multicast address add 32 zero bits
at the end,
2. zero the last 128-"plen" bits, and
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3. replace the last 4 bits with the contents of "RPad".
One should note that there are several operational scenarios when
[UNIPRFXM] 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 your RP; see example 2
below.
"Plen" higher than 64 SHOULD NOT be used as that would overlap with
the upper bits of multicast group-id.
The implementation MUST perform at least the same address validity
checks to the calculated RP address as to one received via other
means (like MSDP), to avoid e.g. the address being "::" or "::1".
One should note that the 4 bits reserved for "RPad" set the upper
bound for RP's per multicast group address; not the number of RP's in
a subnet, PIM domain or large-scale network.
5. Examples
5.1. Example 1
The network administrator of 3FFE:FFFF::/32 wants to set up an RP for
the network and all of his customers. He chooses network
prefix=3FFE:FFFF and plen=32, and wants to use this addressing
mechanism. The multicast addresses he will be able to use are of the
form:
FF7x:y20:3FFE:FFFF: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
domain. In this case, the address of the PIM RP would be:
3FFE:FFFF::y
(and "y" could be anything from 0 to F); the address 3FFE:FFFF::y/128
is added as a Loopback address and injected to the routing system.
5.2. Example 2
As above, the network administrator can also allocate multicast
addresses like "FF7x:y20:3FFE:FFFF:DEAD::/80" to some of his
customers within the PIM domain. In this case the RP address would
still be "3FFE:FFFF::y" (note the prefix length rule: "plen" does not
need to have anything to do with real unicast/multicast address
prefix lengths).
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5.3. Example 3
In the above network, the network admin sets up addresses as above,
but an organization wants to have their own PIM domain; that's
reasonable. The organization can pick multicast addresses like
"FF7x:y30:3FFE:FFFF:BEEF::/80", and then their RP address would be
"3FFE:FFFF: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, he could always use something like
"FF7x:y40:3FFE:FFFF:BEEF:FEED::/96", and then their RP address would
be "3FFE:FFFF:BEEF:FEED::y". There are still 32 bits of multicast
group-id's to assign to customers and self.
6. Operational Requirements
6.1. Anycast-RP
One should note that MSDP is also used, in addition to interdomain
connections between RPs, in anycast-RP [ANYCASTRP] -technique, for
sharing the state information between different RPs in one PIM
domain. However, there are other propositions, like [ANYPIMRP].
Anycast-RP mechanism is incompatible with this addressing method
unless MSDP is specified and implemented. Alternatively, another
method for sharing state information could be used.
Anycast-RP and other possible RP failover mechanisms are outside of
the scope of this memo.
6.2. Guidelines for Assigning IPv6 Addresses to RPs
With this mechanism, the RP can be given basically any network prefix
up to /64 (and even beyond, by using the upper bits of multicast
group-id). The interface identifier will have to be manually
configured to match "RPad".
If an administrator wishes to use an RP address that does not conform
to the addressing topology, that address can be injected into the
routing system via a host route. This RP address SHOULD be assigned
out of the network's prefix in order to ensure aggregation at the
border.
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7. Required PIM Modifications
The use of multicast addresses with embedded RP addresses requires
additional PIM processing. Namely, a PIM router will need to be able
to recognize the encoding and derive the RP address from the address
using the rules in section 4 and to be able to use the embedded RP,
instead of its own for multicast addresses in this specified range.
The two key places where these modifications are used are the
Designated Routers (DRs) on the receiver/sender networks and the RPs
in the domain where the embdedded address has been derived from (see
figure below).
For the foreign DR's (rtrR1, rtrR23, and rtrR4), this means sending
PIM Join/Prune/Register messages towards the foreign RP (rtrRP_S).
Naturally, PIM Register-Stop and other messages must also be allowed
from the foreign RP. DR's in the local PIM domain (rtrS) do the
same, but the RP used should the same as with regular Any-Source
Multicast (ASM); however, see the appendix for more.
For the RP (rtrRP_S), this means being able to recognize and validate
PIM messages which use RP-embedded addressing originated from any DR
at all.
In particular, there is no need to have all routers (like rtrBB) on
the path modified: this is a major benefit for quick deployment.
nodeS - rtrS - rtrRP_S - rtrBB -----+--- rtrR1 - node1
| | |
node2_S ---------+ | +-- rtrR23 - node2
| |
| +---- node3
|
+------------ rtrR4 - node4
In addition, the administration of the PIM domain will require a
policy decision on where the PIM messages to the encoded RP be sent;
this is typically assumed to everywhere unless explicitly configured
otherwise.
The extraction of the RP information from the multicast address
should be done during forwarding state creation. That is, if no
state exists for the multicast address, PIM must take the embedded RP
information into account when creating forwarding state. Depending
on administrative policy, this would result in a receiver's DR
initiating a PIM Join towards the foreign RP or a source's DR sending
PIM Register messages towards the foreign RP.
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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 Join/Prune/Register to the RP address encoded
in the multicast address.
7.1. Overview of the Model
The steps when a receiver wishes to join a group are:
1. A receiver finds out a group address from some means (e.g. SDR
or a web page).
2. The receiver issues an MLD Report, joining the group.
3. The receiver's DR will initiate the PIM 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 unicast-encapsulated in PIM Register-messages to the RP
address encoded in the multicast address (in the special case
that DR is the RP, such sending is only conceptual).
In both cases, the messages then go on as specified in [PIM] and
other specifications (e.g. Register-Stop and/or SPT Join); there is
no difference in them except for the fact that the RP address is
derived from the multicast address.
When sending or receiving, there is a special case when the DR is in
local domain, and information about RP to be used with the group is
available with conventional mechanisms, and that differs from the RP
embedded in the address; see the appendix for more information.
8. Scalability/Usability Analysis
Interdomain MSDP model for connecting PIM domains is mostly
hierarchical. The "embedded RP address" changes this to a mostly
flat, sender-centered, full-mesh virtual topology.
This may or may not cause some effects; it may or may not be
desirable. At the very least, it makes many things much more robust
as the number of third parties is minimized. A good scalability
analysis is needed.
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In some cases (especially if e.g. every home user is employing site-
local multicast), some degree of hierarchy would be highly desirable,
for scalability (e.g. take the advantage of shared multicast state)
and administrative point-of-view.
Being able to join/send to remote RP's has security considerations
that are considered below, but it has an advantage too: every group
has a "home RP" which is able to control (to some extent) who are
able to send to the group.
One should note that the model presented here simplifies the PIM
multicast routing model slightly by removing the RP for senders and
receivers in foreign domains. One scalability consideration should
be noted: previously foreign sources sent the unicast-encapsulated
data to their local RP, now they do so to the foreign RP responsible
for the specific group. This is especially important with large
multicast groups where there are a lot of heavy senders --
particularly if implementations do not handle unicast-decapsulation
well.
This model increases the amount of Internet-wide multicast state
slightly: the backbone routers might end up with at least temporary
(*, G) and (S, G, rpt) state in addition to (S, G) states between the
receivers and senders. Certainly, the amount of inter-domain
multicast traffic between sources and the embedded-RP will increase
compared to the ASM model with MSDP; however, the domain responsible
for the RP is expected to be able to handle this.
As the address of the RP is tied to the multicast address, in the
case of RP failure, PIM BSR mechanisms cannot pick a new RP; the
failover mechanisms, if used, for backup RP's are different, and
typically would depend on sharing one address. The failover
techniques are outside of the scope of this memo.
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. The whole MboneD working group is also acknowledged for
the continued support and comments.
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10. Security Considerations
The address of the PIM RP is embedded in the multicast address. RPs
may be a good target for Denial of Service attacks -- as they are a
single point of failure (excluding failover techniques) for a group.
In this way, the target would be clearly visible. However, it could
be argued that if interdomain multicast was to be made work e.g. with
MSDP, the address would have to be visible anyway (through via other
channels, which may be more easily securable).
As any RP will have to accept PIM Join/Prune/Register messages from
any DR's, this might cause a potential DoS attack scenario. However,
this can be mitigated by the fact that the RP can discard all such
messages for all multicast addresses that do not embed the address of
the RP, and if deemed important, the implementation could also allow
manual configuration of which multicast addresses or prefixes
embedding the RP could be used; however, at least with addresses,
this would increase the need for coordination between multicast
sources and administration.
In a similar fashion, DR's must accept similar PIM messages back from
the foreign RP's for which a receiver in DR's network has joined.
One consequence of the usage model is that it allows Internet-wide
multicast state creation (from receiver(s) in another domain to the
RP in another domain) compared to the domain wide state creation in
the MSDP model.
RPs may become a bit more single points of failure as anycast-RP
mechanism is not (at least immediately) available. This can be
partially mitigated by the fact that some other forms of failover are
still possible, and there should be less need to store state as with
MSDP.
The implementation MUST perform at least the same address validity
checks to the embedded RP address as to one received via other means
(like MSDP), to avoid the address being e.g. "::" or "::1".
TBD: the implications (if any) with regard to embedding the RP
address in the packets (e.g. packet laundering and DoS do not seem
possible due to the way multicast works, but more analysis is
needed).
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11. References
11.1. Normative References
[ADDRARCH] Hinden, R., Deering, S., "IP Version 6
Addressing Architecture", RFC2373, July 1998.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[UNIPRFXM] 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", work-in-progress, draft-ietf-mboned-anycast-
rp-08.txt, May 2001.
[ANYPIMRP] Farinacci, D., Cai, Y., "Anycast-RP using PIM",
work-in-progress, draft-farinacci-pim-anycast-rp-00.txt,
January 2003.
[MSDP] Meyer, D., Fenner, B, (Eds.), "Multicast Sourc
Discovery Protocol (MSDP)", work-in-progress,
draft-ietf-msdp-spec-14.txt, November 2002.
[PIM] Fenner, B. et al, "Protocol Independent Multicast -
Sparse Mode (PIM-SM): Protocol Specification (Revised),
work-in-progress, draft-ietf-pim-sm-v2-new-06.txt,
December 2002.
[SSM] Holbrook, H. et al, "Source-Specific Multicast for IP",
work-in-progress, draft-ietf-ssm-arch-02.txt,
February 2003.
[V6MISSUES] Savola, P., "IPv6 Multicast Deployment Issues",
work-in-progress, draft-savola-v6ops-multicast-
issues-01.txt, November 2002.
Authors' Addresses
Pekka Savola
CSC/FUNET
Espoo, Finland
EMail: psavola@funet.fi
Brian Haberman
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Caspian Networks
One Park Drive
Suite 400
Research Triangle Park, NC 27709
EMail: bkhabs@nc.rr.com
Phone: +1-919-949-4828
A. Open Issues/Discussion
The initial thought was to use only SPT join from local RP/DR to
foreign RP, rather than a full PIM Join to foreign RP. However, this
turned out to be problematic, as this kind of SPT joins where
disregarded because the path had not been set up before sending them.
A full join to foreign PIM domain is a much clearer approach.
One could argue that there can be more RPs than the 4-bit "RPad"
allows for, especially if anycast-RP cannot be used. In that light,
extending "RPad" to take full advantage of whole 8 bits would seem
reasonable. However, this would use up all of the reserved bits, and
leave no room for future flexibility. In case of large number of
RPs, an operational workaround could be to split the PIM domain: for
example, using two /33's instead of one /32 would gain another 16 RP
addresses.
Some hierarchy (e.g. two-level, "ISP/customer") for RPs could
possibly be added if necessary, but that would be torturing one 128
bits even more.
One particular case with a sender in the local domain is where
regular ASM RP would be X, and the embedded RP address would be Y.
This would typically be due to a misconfiguration, but the DR SHOULD
be conservative and use the configured address X. However, the
simplest approach, and one which would typically be least surprising,
would be the one where one would always use the embedded RP address
by default. Any other thoughts on that?
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