One document matched: draft-ietf-pim-group-rp-mapping-01.txt
Differences from draft-ietf-pim-group-rp-mapping-00.txt
PIM Working Group B. Joshi
Internet-Draft Infosys Technologies Ltd.
Expires: December 27, 2009 A. Kessler
Cisco Systems, Inc.
D. McWalter
Data Connection Ltd
June 25, 2009
PIM Group-to-RP Mapping
draft-ietf-pim-group-rp-mapping-01.txt
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Abstract
Each PIM-SM router in a PIM Domain which supports ASM maintains
Group-to-RP mappings which are used to identify a RP for a specific
multicast group. PIM-SM has defined an algorithm to choose a RP from
the Group-to-RP mappings learned using various mechanisms. This
algorithm does not allow administrator to override a specific Group-
to-RP mapping with the static Group-to-RP mapping which an
administrator would want to use. This algorithm also does not
consider the PIM mode and the mechanism through which a Group-to-RP
mapping was learned.
The intention of this document is to suggest a standard algorithm to
deterministically choose between several group-to-rp mappings for a
specific group. This document first explains the requirements to
extend the Group-to-RP mapping algorithm and then proposes the new
algorithm.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Existing algorithm . . . . . . . . . . . . . . . . . . . . . . 6
4. Assumptions . . . . . . . . . . . . . . . . . . . . . . . . . 7
5. Common use cases . . . . . . . . . . . . . . . . . . . . . . . 8
6. Proposed algorithm . . . . . . . . . . . . . . . . . . . . . . 10
7. Deprecation of MIB Objects . . . . . . . . . . . . . . . . . . 12
8. Clarification for MIB Objects . . . . . . . . . . . . . . . . 13
9. Migration to the new algorithm . . . . . . . . . . . . . . . . 14
10. Security Consideration . . . . . . . . . . . . . . . . . . . . 15
11. IANA Consideration . . . . . . . . . . . . . . . . . . . . . . 16
12. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 17
13. Normative References . . . . . . . . . . . . . . . . . . . . . 18
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 19
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1. Introduction
Multiple mechanisms exist today to create and distribute Group-to-RP
mappings. Each PIM-SM router may learn Group-to-RP mappings through
various mechanisms.
It is critical that each router select the same 'RP' for a specific
multicast group address. This is even true in the case of Anycast RP
for redundancy. Routers should select the same RP address to use for
a given group address. This RP address may correspond to a different
physical router but it is one logical RP address and must be
consistent across the PIM domain. This is usually achieved by using
the same algorithm to select the RP in all the PIM routers in a
domain.
PIM-SM[RFC4601] has defined an algorithm to select a 'RP' for a given
multicast group address but it is not flexible enough for an
administrator to apply various policies. Please refer to section 3
for more details.
PIM-STD-MIB [RFC5060] has defined an algorithm that allows
administrators to override Group-to-RP mappings with static
configuration. But this algorithm is not completely deterministic,
because it includes an implementation-specific 'precedence' value.
Embedded-RP as defined in section-7.1 of Embedded-RP address in IPv6
Multicast address [RFC3956], mentions that 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.
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2. Terminology
In this document, the key words "MUST", "MUST NOT", "REQUIRED",
"SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY",
and "OPTIONAL" are to be interpreted as described in RFC 2119. This
document also uses following terms:
o PIM Mode
PIM Mode is the mode of operation a particular multicast group is
used for. Wherever this term in used in this document, it refers to
either Sparse Mode or BIDIR Mode.
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3. Existing algorithm
Existing algorithm defined in PIM-SM (Section 4.7.1 in [RFC4601])
does not consider following constraints:
o It does not consider the origin of a Group-to-RP mapping and
therefore will treat all of them equally.
o It does not provide the flexibility that a specific statically
created Group-to-RP mapping can override any dynamically learned
mappings.
o It does not provide the flexibility to give higher priority to a
specific PIM mode. For example, an entry learned for PIM-BIDIR
mode is treated with same priority as an entry learned for PIM-SM.
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4. Assumptions
We have made following assumptions in defining this algorithm:
o PIM-SM [RFC4601] and BSR [RFC5059] suggested use of a hash
function as the last step to select a RP from multiple Group-to-RP
mappings. There seems to be no requirement for this function, so
this draft assumes that the step to apply hash function can be
removed.
o A static Group-to-RP mapping entry can be configured with
override-dynamic flag. If this flag is set, the static
Group-to-RP mapping entry will be preferred instead of dynamically
learned entries.
o Group-to-RP mappings created with the embedded RP extracted from
Multicast Group addresses are special and always has the highest
priority. These mappings can not be overridden by a static Group-
to-RP mapping with override-dynamic flag set.
o A Group-to-RP mapping can be learned from various mechanisms. We
assume that following list is in the decreasing preferences of
these mechanism:
* Embedded Group-to-RP mappings
* Bootstrap Router Mechanism [PIM-BSR]
* Auto-RP [Cisco]
* Static configuration.
* Other mapping method
o A Group-to-RP mapping learned for PIM-BIDIR mode is preferred to
an entry learned for PIM-SM mode.
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5. Common use cases
o Default static Group-to-RP mappings with dynamically learned
entries
Many network operators will have a dedicated infrastructure for the
standard multicast group range (224/4) and so might be using
statically configured Group-to-RP mappings for this range. In this
case, to support some specific applications, they might like to learn
Group-to-RP mappings dynamically using either BSR or Auto-RP
mechanism. In this case to select Group-to-RP mappings for these
specific applications, a longer prefix match should be given
preference over statically configured Group-to-RP mappings. For
example 239.100.0.0/16 could be learned for a corporate
communications application. Network operators may change the Group-
to-RP mappings for these applications more often and would need to be
learned dynamically.
o Static Group-to-RP mappings with override-dynamic flag
Many Network operators would like to statically configure one or
multiple Group-to-RP mappings and would always want to ignore any
dynamically learned mappings through either BSR, AutoRP or embedded
RP for these group prefixes. This is accomplished by providing a
'override-dynamic' flag for Group-to-RP mapping configuration. When
this flag is enabled for a static Group-to-RP mapping, it will have
the highest precedence and would always be use for the specified
group prefix. For example: 224.1.0.0/16 is configured with override-
dynamic flag enabled and uses RP address RP1. If the router learns
the more specific group prefix 224.1.1.0/24 which uses RP2 through
BSR, it will choose the RP1 for any group falling under 224.1.0.0/16
range.
o Migration situations
Network operators occasionally go through a migration due to an
acquisition or a change in their network design. In order to
facilitate this migration there is a needs to have a deterministic
behavior of Group-to-RP mapping selection for entries learned using
BSR and AutoRP mechanism. This will help in avoiding any unforeseen
interoperability issues between different vendor's network elements.
o Use by management systems
A network management system [or a stand alone box] can find out RP
for a specific group in a specific router by running this algorithm
on the Group-to-RP mapping table fetched using SNMP MIB objects.
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o More use cases
By no means, the above list is complete. Please drop a mail to
'authors' if you see any other use case for this.
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6. Proposed algorithm
We propose following algorithm here which addresses the above
mentioned shortcomings in the existing mechanism:
1. If the Multicast Group Address being looked up contains an
embedded RP, RP address extracted from the Group address is
selected as Group-to-RP mapping.
2. If the Multicast Group Address being looked up is in the SSM
range or is configured for Dense mode, no Group-to-RP mapping is
selected, and this algorithm terminates. Alternatively, a RP
with address type 'unknown' can be selected. Please look at
section #8 for more details on this.
3. From the set of all Group-to-RP mapping entries, the subset whose
group prefix contains the multicast group that is being looked
up, are selected.
4. If there are no entries available, then the Group-to-RP mapping
is undefined.
5. If there are multiple entries available, a subset of those Group-
to-RP mapping is selected that are learned using 'static'
configuration and are configured with 'override-dynamic' flag.
* If there is only one entry available then that is selected as
Group-to-RP mapping.
* If there are multiple entries available, we continue with the
algorithm with this smaller set of Group-to-RP Mappings
* If there are no static entries with 'override-dynamic' flag
set then we continue with the original subset of Group-to-RP
Mappings from step 2.
6. A longest prefix match is performed on the subset of Group-to-RP
Mappings.
* If there is only one entry available then that is selected as
Group-to-RP mapping.
* If there are multiple entries available, we continue with the
algorithm with this smaller set of Group-to-RP Mappings
7. From the remaining set of Group-to-RP Mappings we select the
subset of entries based on the preference for the PIM modes which
they are assigned. A Group-to-RP mapping entry with PIM Mode
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'BIDIR' will be preferred to an entry with PIM Mode 'PIM-SM'
* If there is only one entry available then that is selected as
Group-to-RP mapping.
* If there are multiple entries available, we continue with the
algorithm with this smaller set of Group-to-RP Mappings
8. From the remaining set of Group-to-RP Mappings we select the
subset of the entries based on the origin. Origin preference
will be 'bsr', 'auto-rp', 'static' and 'other'.
* If there is only one entry available then that is selected as
Group-to-RP mapping.
* If there are multiple entries available, we continue with the
algorithm with this smaller set of Group-to-RP Mappings
9. From the remaining set of Group-to-RP Mappings we will select the
RP with the highest IP address. This will serve as a final
tiebreaker.
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7. Deprecation of MIB Objects
Group-to-RP mapping algorithm defined in PIM-STD-MIB [RFC5060] does
not specify the usage of 'pimGroupMappingPrecedence' and
'pimStaticRPPrecedence' objects in 'pimGroupMappingTable' table
clearly. With the newly proposed algorithm in this document, these
MIB objects would not be required. So we propose to deprecate these
MIB objects from PIM-STD-MIB. Also the newly proposed algorithm in
this document MUST be preferred over Group-to-RP mapping algorithm
defined in either PIM-SM[RFC4601] or in PIM-STD-MIB[RFC5060].
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8. Clarification for MIB Objects
When an Group-to-RP mapping entry is created in the
pimGroupMappingTable in the PIM-STD MIB[RFC5060], it would be
acceptable to have an entry with an RP with address type 'unknown'
and a PimMode of Dense Mode or SSM. These entries would represent
group ranges for Dense mode or SSM.
Also all the entries which are already included in the SSM Range
table in the IP Mcast MIB would be copied over to
pimGroupMappingTable. They would have a type of configSSM and an RP
with address type 'unknown' as described above.
The advantage of keeping all the ranges in the table would be that
this table will contain all the known multicast group ranges.
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9. Migration to the new algorithm
The Group-to-RP mapping algorithm proposed in this document obsoletes
the use of the hash function. With this change, there will be no
interoperability between the old and the new algorithm. So networks
that use multiple RP addresses for a Group Range and use the hash
function for load sharing will need to be migrated to the new
algorithm proposed in this document. A seamless migration to the new
Group-to-RP algorithm can be accomplished by using one RP address
with Anycast RP.
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10. Security Consideration
This document does not suggest any protocol specific functionality so
there is no security related consideration.
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11. IANA Consideration
This draft does not create any namespace for IANA to manage.
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12. Acknowledgments
This draft is created based on the discussion occurred during the
PIM-STD-MIB [RFC5060] work. Many thanks to Stig Vennas and Toerless
Eckert for providing useful comments during that discussion.
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13. Normative References
[RFC4601] Fenner, B., Handley, M., Holbrook, H., and I. Kouvelas,
"Protocol Independent Multicast - Sparse Mode (PIM-SM):
Protocol Specification (Revised)", RFC 4601, August 2006.
[RFC5060] Sivaramu, R., Lingard, J., McWalter, D., Joshi, B., and A.
Kessler, "Protocol Independent Multicast MIB", RFC 5060,
January 2008.
[RFC3956] Savola, P. and B. Haberman, "Embedding the Rendezvous
Point (RP) Address in an IPv6 Multicast Address",
RFC 3956, November 2004.
[RFC5059] Bhaskar, N., Gall, A., Lingard, J., and S. Venaas,
"Bootstrap Router (BSR) Mechanism for Protocol Independent
Multicast (PIM)", RFC 5059, January 2008.
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Authors' Addresses
Bharat Joshi
Infosys Technologies Ltd.
44 Electronics City, Hosur Road
Bangalore 560 100
India
Email: bharat_joshi@infosys.com
URI: http://www.infosys.com/
Andy Kessler
Cisco Systems, Inc.
425 E. Tasman Drive
San Jose, CA 95134
USA
Email: kessler@cisco.com
URI: http://www.cisco.com/
David McWalter
Data Connection Ltd
100 Church Street
Enfield EN2 6BQ
UK
Email: dmcw@dataconnection.com
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