PIM Working Group B. Joshi
Internet-Draft Infosys Technologies Ltd.
Expires: October 28, 2010 A. Kessler
Cisco Systems, Inc.
D. McWalter
Metaswitch Networks
April 26, 2010
PIM Group-to-RP Mapping
draft-ietf-pim-group-rp-mapping-04.txt
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 consider the PIM mode and the mechanism through
which a Group-to-RP mapping was learned.
This document defines 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.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on October 28, 2010.
Copyright Notice
Copyright (c) 2010 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Existing algorithm . . . . . . . . . . . . . . . . . . . . . . 5
4. Assumptions . . . . . . . . . . . . . . . . . . . . . . . . . 6
5. Common use cases . . . . . . . . . . . . . . . . . . . . . . . 7
6. Proposed algorithm . . . . . . . . . . . . . . . . . . . . . . 8
7. Deprecation of MIB Objects . . . . . . . . . . . . . . . . . . 10
8. Clarification for MIB Objects . . . . . . . . . . . . . . . . 11
9. Use of dynamic group-to-rp mapping protocols . . . . . . . . . 12
10. Consideration for Bidirectional-PIM and BSR hash . . . . . . . 13
11. Filtering Group-to-RP mappings at domain boundaries . . . . . 14
12. Security Consideration . . . . . . . . . . . . . . . . . . . . 15
13. IANA Consideration . . . . . . . . . . . . . . . . . . . . . . 16
14. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 17
15. 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. 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.
o Dynamic group-to-RP mapping mechanisms
The term Dynamic group-to-RP mapping mechanisms in this document
refers to BSR and Auto-RP.
o Dynamic mappings or Dynamically learned mappings
The terms Dynamic mappings or Dynamically learned mappings refer to
group-to-RP mappings that have been learned by BSR or Auto-RP.
Group-to-RP mappings that have been learned by embedded RP are
referred to as Embedded Group-to-RP mappings.
o Filtering
Filtering is selective discarding of dynamic Group-to-RP mapping
information, based on the group address, the type of Group-to-RP
mapping message and the interface on which the mapping message was
received.
o Multicast Domain and Boundaries
The term multicast domain used in this document refers to a network
topology that has a consistent set of Group-to-RP Mappings. The
interface between two or more multicast domains is a multicast domain
boundary. The multicast boundaries are usually enforced by filtering
the dynamic mapping messages and/or configuring different static RP
mappings.
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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 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 Embedded Group-to-RP mappings are special and always have the
highest priority. They cannot be overridden either by static
configuration or by dynamic Group-to-RP mappings.
o Dynamic mappings will override a static RP config if they have
overlapping ranges. However, it is possible to override dynamic
Group-to-RP mappings with static configurations, either by
filtering, or by configuring longer static group addresses that
override dynamic mappings when longest prefix matching is applied.
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
* Dynamically learned mappings
* 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.
o Dynamic group-to-RP mapping mechanisms are filtered at domain
boundaries or for policy enforcement inside a domain.
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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 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 need to have a deterministic
behaviour of Group-to-RP mapping selection for entries learned using
BSR and Auto-RP mechanism. This will help in avoiding any unforeseen
interoperability issues between different vendor's network elements.
o Use by management systems
A network management station can determine the 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.
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
The following algorithm 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. 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.
6. 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 '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
7. From the remaining set of Group-to-RP Mappings we select the
subset of the entries based on the origin. Group-to-RP mappings
learned dynamically are preferred over static mappings. If the
remaining dynamic Group-to-RP mappings are from BSR and Auto-RP
then the mappings from BSR SHOULD be preferred.
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* 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. If the remaining Group-to-RP mappings were learned through BSR
then the RP will be selected by comparing the RP Priority in the
Candidate-RP-Advertisement messages. The RP mapping with the
lowest value indicates the highest priority [RFC5059].
* If more than one RP has the same highest priority value we
continue with the algorithm with those Group-to-RP mappings.
* If the remaining Group-to-RP mappings were NOT learned from
BSR we continue the algorithm with the next step.
9. If the remaining Group-to-RP mappings were learned through BSR
and the PIM Mode of the Group is 'PIM-SM' then the hash function
will be used to choose the RP. The RP with the highest
resulting hash value will be selected.
* If more than one RP has the same highest hash value we
continue with the algorithm with those Group-to-RP mappings.
* If the remaining Group-to-RP mappings were NOT learned from
BSR we continue the algorithm with the next step.
10. 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. Use of dynamic group-to-rp mapping protocols
In practice, it is not usually necessary to run several dynamic
Group-to-RP mapping mechanisms in one administrative domain.
Specifically, interoperation of BSR and Auto-RP is OPTIONAL and not
recommended by this document.
However, if a router does receive two overlapping sets of Group-to-RP
mappings, for example from Auto-RP and BSR, then some algorithm is
needed to deterministically resolve the situation. The algorithm in
this document MUST be used. This can be important at domain border
routers, and is likely to improve stability under misconfiguration
and when configuration is changing.
An implementation of PIM that supports only one mechanism for
learning Group-to-RP mappings SHOULD also use this algorithm. The
algorithm has been chosen so that existing standard implementations
are already compliant.
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10. Consideration for Bidirectional-PIM and BSR hash
Bidir-PIM [RFC5015] is designed to avoid any data driven events.
This is especially true in the case of a source only branch. The RP
mapping is determined based on a group mask when the mapping is
received through a dynamic mapping protocol or statically configured.
Therefore the hash in BSR is ignored for PIM-Bidir RP mappings based
on the algorithm defined in this document. It is RECOMMENDED that
network operators configure only one PIM-Bidir RP for each RP
Priority.
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11. Filtering Group-to-RP mappings at domain boundaries
An implementation of PIM SHOULD support configuration to block
specific dynamic mechanism for a valid group prefix range. For
example, it should be possible to allow 239/8 range for Auto-RP
protocol but block the BSR advertisement for the same range.
Similarly it should be possible to filter out all Group-to-RP
mappings learned from BSR or Auto-RP protocol.
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12. Security Consideration
This document does not suggest any protocol specific functionality so
there is no security related consideration.
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13. IANA Consideration
This draft does not create any namespace for IANA to manage.
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14. Acknowledgements
This draft is created based on the discussion occurred during the
PIM-STD-MIB [RFC5060] work. Many thanks to Stig Vennas, Yiqun Cai
and Toerless Eckert for providing useful comments.
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15. 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.
[RFC5015] Handley, M., Kouvelas, I., Speakman, T., and L. Vicisano,
"Bidirectional Protocol Independent Multicast (BIDIR-
PIM)", RFC 5015, October 2007.
[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
Metaswitch Networks
100 Church Street
Enfield EN2 6BQ
UK
Email: dmcw@metaswitch.com
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