mboned Working Group P. Savola
Internet Draft CSC/FUNET
Expiration Date: April 2004
B. Haberman
Caspian Networks
October 2003
Embedding the Address of RP in IPv6 Multicast Address
draft-ietf-mboned-embeddedrp-00.txt
Status of this Memo
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of Section 10 of RFC2026.
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Abstract
There exists a huge deployment problem with global, interdomain IPv6
multicast: Protocol Independent Multicast - Sparse Mode (PIM-SM)
Rendezvous Points (RPs) have no way of communicating the information
about multicast sources to other multicast domains, as there is no
Multicast Source Discovery Protocol (MSDP), and the whole interdomain
Any Source Multicast (ASM) model is rendered unusable; Source
Specific Multicast (SSM) avoids these problems but is not considered
readily deployable at the moment. This memo defines a PIM-SM group-
to-RP mapping which encodes the address of the RP in the IPv6
multicast address. In consequence, there would be no need for
interdomain MSDP, and even intra-domain RP configuration could be
simplified. This memo updates RFC 3306.
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Table of Contents
1. Introduction ............................................... 2
2. Unicast-Prefix-based Address Format ........................ 4
3. Modified Unicast-Prefix-based Address Format ............... 4
4. Embedding the Address of the RP in the Multicast Address ... 5
5. Examples ................................................... 6
5.1. Example 1 .............................................. 6
5.2. Example 2 .............................................. 6
5.3. Example 3 .............................................. 6
5.4. Example 4 .............................................. 7
6. Operational Requirements ................................... 7
6.1. Anycast-RP ............................................. 7
6.2. Guidelines for Assigning IPv6 Addresses to RPs ......... 7
7. Required PIM-SM Modifications .............................. 7
7.1. Overview of the Model .................................. 9
8. Scalability/Usability Analysis ............................. 9
9. Acknowledgements ........................................... 11
10. Security Considerations ................................... 11
11. References ................................................ 12
11.1. Normative References .................................. 12
11.2. Informative References ................................ 12
Authors' Addresses ............................................. 13
A. Discussion about Design Tradeoffs .......................... 13
Intellectual Property Statement ................................ 14
Full Copyright Statement ....................................... 15
1. Introduction
As has been noticed [V6MISSUES], there exists a huge deployment
problem with global, interdomain IPv6 multicast: PIM-SM [PIM-SM] RPs
have no way of communicating the information about multicast sources
to other multicast domains, as there is no MSDP [MSDP], and the whole
interdomain Any Source Multicast model is rendered unusable; SSM
[SSM] avoids these problems.
It has been noted that there are some problems with SSM deployment
and support: it seems unlikely that SSM could be usable as the only
interdomain multicast routing mechanism in the short term. This memo
proposes a fix to interdomain multicast routing, and provides an
additional method for the RP discovery with the intra-domain case.
This document proposes a solution to the group-to-RP mapping problem
which leverages and extends [RFC3306] by encoding the RP address of
the IPv6 multicast group into the group address itself.
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This mechanism not only provides a simple solution for IPv6
interdomain ASM but can be used as a simple solution for IPv6
intradomain ASM on scoped addresses, as well. The use as a substitute
for Bootstrap Router protocol (BSR) [BSR] is also possible.
The solution consists of two elements applicable to a subrange of
[RFC3306] IPv6 multicast group addresses which are defined by setting
one previously unused bit of the Flags field to "1":
o A specification of the mapping by which such a group address
encodes the RP address that is to be used with this group, and
o A specification of optional and mandatory procedures to operate
ASM with PIM-SM on these IPv6 multicast groups.
Addresses in this subrange will be called embedded-RP addresses. If
used in the interdomain, a mechanism similar to MSDP is not required
for these addresses and RP configuration for these addresses can be
as simple as zero configuration for routers supporting this
specification.
It is self-evident that a 128 bit RP address can in general not be
embedded into a 128-bit group address with space left to carry a
group identity itself. An appropriate form of encoding is thus
defined, and it is assumed that the Interface-ID of RPs in the
embedded-RP range can be assigned to be specific values.
If these assumptions can't be followed, either operational procedures
and configuration must be slightly changed or this mechanism can not
be used.
The assignment of multicast addresses is outside the scope of this
document; however, the mechanisms are very probably similar to ones
used with [RFC3306].
This memo updates the addressing format presented in RFC 3306.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
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2. Unicast-Prefix-based Address Format
As described in [RFC3306], the multicast address format is as
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:
| 8 | 4 | 4 | 4 | 4 | 8 | 64 | 32 |
+--------+----+----+----+----+--------+----------------+----------+
|11111111|flgs|scop|rsvd|RPad| plen | network prefix | group ID |
+--------+----+----+----+----+--------+----------------+----------+
+-+-+-+-+
flgs is a set of 4 flags: |0|R|P|T|
+-+-+-+-+
R = 1 indicates a multicast address that embeds the address of the
PIM-SM RP. Then P MUST BE set to 1, and consequently T MUST be set
to 1, as specified in [RFC3306].
In the case that R = 1, the last 4 bits of previously reserved field
("RPad") are interpreted as embedding the interface ID of the RP, as
specified in this memo.
R = 0 indicates a multicast address that does not embed the address
of the PIM-SM RP and follows the semantics defined in [ADDRARCH] and
[RFC3306]. In this context, the value of "RPad" has no meaning.
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4. Embedding the Address of the RP in the Multicast Address
The address of the RP can only be embedded in unicast-prefix -based
ASM addresses.
To identify whether an address is a multicast address as specified in
this memo and to be processed any further, it must satisfy all of the
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), and
o "plen" MUST NOT be greater than 64.
The address of the RP can be obtained from a multicast address
satisfying the above criteria by taking the following steps:
1. take the last 96 bits of the multicast address add 32 zero bits
at the end,
2. zero the last 128-"plen" bits, and
3. replace the last 4 bits with the contents of "RPad".
One should note that there are several operational scenarios when
[RFC3306] statement "all non-significant bits of the network prefix
field SHOULD be zero" is ignored -- and why the second step, above,
is necessary. This is to allow multicast address assignments to
third parties which still use your RP; see example 2 below.
"plen" higher than 64 MUST NOT be used as that would overlap with the
upper bits of multicast group-id.
The implementation MUST perform at least the same address validity
checks to the calculated RP address as to one received via other
means (like BSR [BSR] or MSDP for IPv4), to avoid e.g. the address
being "::" or "::1".
One should note that the 4 bits reserved for "RPad" set the upper
bound for RPs per multicast group address; not the number of RPs in a
subnet, PIM-SM domain or large-scale network.
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5. Examples
5.1. Example 1
The network administrator of 3FFE:FFFF::/32 wants to set up an RP for
the network and all of his customers. He chooses network
prefix=3FFE:FFFF and plen=32, and wants to use this addressing
mechanism. The multicast addresses he will be able to use are of the
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-SM
domain. In this case, the address of the PIM-SM 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-SM domain. In this case the RP address
would still be "3FFE:FFFF::y".
Note the second rule of deriving the RP address: the "plen" field in
the multicast address, (hex)20 = 32, refers to the length of "network
prefix" field considered when obtaining the RP address. In this
case, only the first 32 bits of the network prefix field, "3FFE:FFFF"
are preserved: the value of "plen" takes no stance on actual
unicast/multicast prefix lengths allocated or used in the networks,
here from 3FFE:FFFF:DEAD::/48.
5.3. Example 3
In the above network, the network admin sets up addresses as above,
but an organization wants to have their own PIM-SM domain; that's
reasonable. The organization can pick multicast addresses like
"FF7x:y30:3FFE:FFFF:BEEF::/80", and then their RP address would be
"3FFE:FFFF:BEEF::y".
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5.4. Example 4
In the above networks, if the admin wants to specify the RP to be in
a non-zero /64 subnet, he could always use something like
"FF7x:y40:3FFE:FFFF:BEEF:FEED::/96", and then their RP address would
be "3FFE:FFFF:BEEF:FEED::y". There are still 32 bits of multicast
group-id's to assign to customers and self.
6. Operational Requirements
6.1. Anycast-RP
One should note that MSDP is also used, in addition to interdomain
connections between RPs, in anycast-RP [ANYCASTRP] -technique, for
sharing the state information between different RPs in one PIM-SM
domain. However, there are other propositions, like [ANYPIMRP].
Anycast-RP mechanism is incompatible with this addressing method
unless MSDP is specified and implemented. Alternatively, another
method for sharing state information could be used.
Anycast-RP and other possible RP failover mechanisms are outside of
the scope of this memo.
6.2. Guidelines for Assigning IPv6 Addresses to RPs
With this mechanism, the RP can be given basically any network prefix
up to /64. The interface identifier will have to be manually
configured to match "RPad".
RPad = 0 SHOULD NOT be used as using it would cause ambiguity with
the Subnet-Router Anycast Address [ADDRARCH].
If an administrator wishes to use an RP address that does not conform
to the addressing topology but is still from the network provider's
prefix (e.g. an additional loopback address assigned on a router),
that address can be injected into the routing system via a host
route.
7. Required PIM-SM Modifications
The use of multicast addresses with embedded RP addresses requires
additional PIM-SM processing. Namely, a PIM-SM router will need to
be able to recognize the encoding and derive the RP address from the
address using the rules in section 4 and to be able to use the
embedded RP, instead of its own for multicast addresses in this
specified range.
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The three key places where these modifications are used are the
Designated Routers (DRs) on the receiver/sender networks, the
backbone networks, and the RPs in the domain where the embdedded
address has been derived from (see figure below).
For the foreign DRs (rtrR1, rtrR23, and rtrR4), this means sending
PIM-SM Join/Prune/Register messages towards the foreign RP (rtrRP_S).
Naturally, PIM-SM Register-Stop and other messages must also be
allowed from the foreign RP. DRs in the local PIM-SM domain (rtrS)
do the same.
For the RP (rtrRP_S), this means being able to recognize and validate
PIM-SM messages which use RP-embedded addressing originated from any
DR at all.
For the other routers on the path (rtrBB), this means recognizing and
validating that the Join/Prune PIM-SM messages using the embedded RP
addressing are on the right path towards the RP they think is in
charge of the particular address.
nodeS - rtrS - rtrRP_S - rtrBB -----+--- rtrR1 - node1
| | |
node2_S ---------+ | +-- rtrR23 - node2
| |
| +---- node3
|
+------------ rtrR4 - node4
In addition, the administration of the PIM-SM domains MAY have an
option to manually override the RP selection for the embedded RP
multicast addresses: the default policy SHOULD be to use the embedded
RP.
The extraction of the RP information from the multicast address
should be done during forwarding state creation. That is, if no
state exists for the multicast address, PIM-SM must take the embedded
RP information into account when creating forwarding state. Unless
otherwise dictated by the administrative policy, this would result in
a receiver's DR initiating a PIM-SM Join towards the foreign RP or a
source's DR sending PIM-SM Register messages towards the foreign RP.
It should be noted that this approach removes the need to run inter-
domain MSDP. Multicast distribution trees in foreign networks can be
joined by issuing a PIM-SM Join/Prune/Register to the RP address
encoded in the multicast address.
Also, the addressing model described here could be used to replace or
augment the intra-domain Bootstrap Router mechanism (BSR), as the RP-
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mappings can be communicated by the multicast address assignment.
7.1. Overview of the Model
The steps when a receiver wishes to join a group are:
1. A receiver finds out a group address from some means (e.g. SDR
or a web page).
2. The receiver issues an MLD Report, joining the group.
3. The receiver's DR will initiate the PIM-SM Join process towards
the RP embedded in the multicast address.
The steps when a sender wishes to send to a group are:
1. A sender finds out a group address from some means, whether in
an existing group (e.g. SDR, web page) or in a new group (e.g.
a call to the administrator for group assignment, use of a
multicast address assignment protocol).
2. The sender sends to the group.
3. The sender's DR will send the packets unicast-encapsulated in
PIM-SM Register-messages to the RP address encoded in the
multicast address (in the special case that DR is the RP, such
sending is only conceptual).
In both cases, the messages then go on as specified in [PIM-SM] and
other specifications (e.g. Register-Stop and/or SPT Join); there is
no difference in them except for the fact that the RP address is
derived from the multicast address.
Sometimes, some information, using conventional mechanisms, about
another RP exists in the PIM-SM domain. The embedded RP SHOULD be
used by default, but there MAY be an option to switch the preference.
This is because especially when performing PIM-SM forwarding in the
transit networks, the routers must have the same notion of the RP, or
else the messages may be dropped.
8. Scalability/Usability Analysis
Interdomain MSDP model for connecting PIM-SM domains is mostly
hierarchical. The "embedded RP address" changes this to a mostly
flat, sender-centered, full-mesh virtual topology.
This may or may not cause some effects; it may or may not be
desirable. At the very least, it makes many things much more robust
as the number of third parties is minimized. A good scalability
analysis is needed.
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In some cases (especially if e.g. every home user is employing site-
local multicast), some degree of hierarchy would be highly desirable,
for scalability (e.g. to take the advantage of shared multicast
state) and administrative point-of-view.
Being able to join/send to remote RPs has security considerations
that are considered below, but it has an advantage too: every group
has a "home RP" which is able to control (to some extent) who are
able to send to the group.
One should note that the model presented here simplifies the PIM-SM
multicast routing model slightly by removing the RP for senders and
receivers in foreign domains. One scalability consideration should
be noted: previously foreign sources sent the unicast-encapsulated
data to their local RP, now they do so to the foreign RP responsible
for the specific group. This is especially important with large
multicast groups where there are a lot of heavy senders --
particularly if implementations do not handle unicast-decapsulation
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 the RP, in addition to (S, G) states
between the receivers and senders. Certainly, the amount of inter-
domain multicast traffic between sources and the embedded-RP will
increase compared to the ASM model with MSDP; however, the domain
responsible for the RP is expected to be able to handle this.
As the address of the RP is tied to the multicast address, in the
case of RP failure PIM-SM BSR mechanisms cannot pick a new RP; the
failover mechanisms, if used, for backup RPs are different, and
typically would depend on sharing one address. The failover
techniques are outside of the scope of this memo.
The PIM-SM specification states, "Any RP address configured or
learned MUST be a domain-wide reachable address". What this means is
not clear, even without embedded-RP. However, typically this
statement cannot be proven especially with the foreign RPs (typically
one can not even guarantee that the RP exists!). The bottom line is
that while traditionally the configuration of RPs and DRs was
typically a manual process, and e.g. configuring a non-existant RP
was possible, but here the hosts and users which use multicast
indirectly specify the RP.
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9. Acknowledgements
Jerome Durand commented on an early draft of this memo. Marshall
Eubanks noted an issue regarding short plen values. Tom Pusateri
noted problems with earlier SPT-join approach. Rami Lehtonen pointed
out issues with the scope of SA-state and provided extensive
commentary. Nidhi Bhaskar gave the draft a thorough review. The
whole MboneD working group is also acknowledged for the continued
support and comments.
10. Security Considerations
The address of the PIM-SM RP is embedded in the multicast address.
RPs may be a good target for Denial of Service attacks -- as they are
a single point of failure (excluding failover techniques) for a
group. In this way, the target would be clearly visible. However, it
could be argued that if interdomain multicast was to be made work
e.g. with MSDP, the address would have to be visible anyway (through
via other channels, which may be more easily securable).
As any RP will have to accept PIM-SM Join/Prune/Register messages
from any DR, this might cause a potential DoS attack scenario.
However, this can be mitigated by the fact that the RP can discard
all such messages for all multicast addresses that do not embed the
address of the RP, and if deemed important, the implementation could
also allow manual configuration of which multicast addresses or
prefixes embedding the RP could be used, so that only the pre-agreed
sources could use the RP.
In a similar fashion, when a receiver joins to an RP, the DRs must
accept similar PIM-SM messages back RPs.
One consequence of the usage model is that it allows Internet-wide
multicast state creation (from receiver(s) in another domain to the
RP in another domain) compared to the domain wide state creation in
the MSDP model.
One should observe that the embedded RP threat model is actually
pretty similar to SSM; both mechanisms significantly reduce the
threats at the sender side, but have new ones in the receiver side,
as any receiver can try to join any non-existant group or channel,
and the local DR or RP cannot readily reject such joins (based on
MSDP information).
RPs may become a bit more single points of failure as anycast-RP
mechanism is not (at least immediately) available. This can be
partially mitigated by the fact that some other forms of failover are
still possible, and there should be less need to store state as with
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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 BSR or MSDP), to avoid the address being e.g. "::" or "::1".
11. References
11.1. Normative References
[ADDRARCH] Hinden, R., Deering, S., "IP Version 6 Addressing
Architecture", RFC3513, April 2003.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3306] Haberman, B., Thaler, D., "Unicast-Prefix-based IPv6
Multicast Addresses", RFC3306, August 2002.
11.2. Informative References
[ANYCASTRP] Kim, D. et al, "Anycast RP mechanism using PIM and
MSDP", RFC 3446, January 2003.
[ANYPIMRP] Farinacci, D., Cai, Y., "Anycast-RP using PIM",
work-in-progress, draft-farinacci-pim-anycast-rp-00.txt,
January 2003.
[BSR] Fenner, B., et al., "Bootstrap Router (BSR) Mechanism for
PIM Sparse Mode", work-in-progress, draft-ietf-pim-sm-
bsr-03.txt, February 2003.
[MSDP] Meyer, D., Fenner, B, (Eds.), "Multicast Sourc
Discovery Protocol (MSDP)", work-in-progress,
draft-ietf-msdp-spec-20.txt, May 2003.
[PIM-SM] Fenner, B. et al, "Protocol Independent Multicast -
Sparse Mode (PIM-SM): Protocol Specification (Revised),
work-in-progress, draft-ietf-pim-sm-v2-new-08.txt,
October 2003.
[SSM] Holbrook, H. et al, "Source-Specific Multicast for IP",
work-in-progress, draft-ietf-ssm-arch-03.txt,
May 2003.
[V6MISSUES] Savola, P., "IPv6 Multicast Deployment Issues",
work-in-progress, draft-savola-v6ops-multicast-
issues-02.txt, October 2003.
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Authors' Addresses
Pekka Savola
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
The initial thought was to use only SPT join from local RP/DR to
foreign RP, rather than a full PIM Join to foreign RP. However, this
turned out to be problematic, as this kind of SPT joins where
disregarded because the path had not been set up before sending them.
A full join to foreign PIM domain is a much clearer approach.
One could argue that there should be more RPs than the 4-bit "RPad"
allows for, especially if anycast-RP cannot be used. In that light,
extending "RPad" to take full advantage of whole 8 bits would seem
reasonable. However, this would use up all of the reserved bits, and
leave no room for future flexibility. In case of large number of
RPs, an operational workaround could be to split the PIM domain: for
example, using two /33's instead of one /32 would gain another 16 (or
15, if zero is not used) RP addresses. Note that the limit of 4 bits
worth of RPs just depends on the prefix the RP address is derived
from; one can use multiple prefixes in a domain, and the limit of 16
(or 15) RPs should never really be a problem.
Some hierarchy (e.g. two-level, "ISP/customer") for RPs could
possibly be added if necessary, but that would be torturing one 128
bits even more.
One particular case, whether in the backbone or in the sender's
domain, is where the regular PIM-SM RP would be X, and the embedded
RP address would be Y. This could e.g. be a result of a default all-
multicast-to-one-RP group mapping, or a local policy decision. The
embedded RP SHOULD be used by default, but there MAY be an option to
change this preference.
Values 64 < "plen" < 96 would overlap with upper bits of the
multicast group-id; due to this restriction, "plen" must not exceed
64 bits. This is in line with RFC 3306.
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The embedded RP addressing could be used to convey other information
(other than RP address) as well, for example, what should be the RPT
threshold for PIM-SM. These could be encoded in the RP address
somehow, or in the multicast group address. However, such
modifications are beyond the scope of this memo.
Some kind of rate-limiting functions, ICMP message responses, or
similar could be defined for the case of when the RP embedded in the
address is not willing to serve for the specific group (or doesn't
even exist). Typically this would result in the datagrams getting
blackholed or rejected with ICMP. In particular, a case for
"rejection" or "source quench" -like messages would be in the case
that a source keeps transmitting a huge amount of data, which is sent
to a foreign RP using Register message but is discarded if the RP
doesn't allow the source host to transmit: the RP should be able to
indicate to the DR, "please limit the amount of Register messages",
or "this source sending to my group is bogus". Note that such "kiss-
of-death" packets have an authentication problem; spoofing them could
result in an entirely different kind of Denial of Service, for
legitimate sources. One possibility here would be to specify some
form of "return routability" check for DRs and RPs; for example, if a
DR receives packets from a host to group G G (resulting in RP address
R), the DR would send only a limited amount of packets to R until it
has heard back from R (a "positive acknowledgement"). It is not
clear whether this needs to be considered or specified in more
detail.
Could this model work with bidir-PIM? Is it feasible? Not sure, not
familiar enough with bidir-PIM.
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Internet Draft draft-ietf-mboned-embeddedrp-00.txt October 2003
rights which may cover technology that may be required to practice
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