One document matched: draft-ietf-pim-group-rp-mapping-08.txt
Differences from draft-ietf-pim-group-rp-mapping-07.txt
PIM Working Group B. Joshi
Internet-Draft Infosys Technologies Ltd.
Updates: 4601 (if approved) A. Kessler
Intended status: Standards Track Cisco Systems, Inc.
Expires: July 1, 2011 D. McWalter
December 28, 2010
PIM Group-to-RP Mapping
draft-ietf-pim-group-rp-mapping-08.txt
Abstract
Each PIM-SM router in a Protocol Independent Multicast (PIM) Domain
which supports Any Source Multicast (ASM) maintains Group-to-RP
mappings which are used to identify a Rendezvous Point(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.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
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Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on July 1, 2011.
Copyright Notice
Copyright (c) 2010 IETF Trust and the persons identified as the
document authors. All rights reserved.
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This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
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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. Interpretation 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] includes a number of objects to allow an
administrator to set the precedence for Group-to-RP mappings which
are learned statically or dynamically and stored in the
'pimGroupMappingTable'. The MIB module also defines an algorithm
that can be applied to the data contained in the
'pimGroupMappingTable' to determine Group-to-RP mappings. However,
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
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 RFC 2119 [RFC2119].
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 Bidirectional (BIDIR) Mode.
o Dynamic group-to-RP mapping mechanisms
The term Dynamic group-to-RP mapping mechanisms in this document
refers to Bootstrap Router (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
The 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.
The algorithm defined in this document updates algorithm defined in
PIM-SM ( Section 4.7.1 in [RFC4601]). The new algorithm is backward
compatible and will produce the same result only if the Group-to-RP
mappings are learned from a single mapping source. The full benefits
of the new algorithm will not be realized until it is widely
deployed.
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4. Assumptions
We have made following assumptions in defining this algorithm:
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 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 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, an administratively scoped multicast address
range, 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.
This is not an issue for IPv6 Multicast address ranges.
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.
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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 Source
Specific Multicast (SSM) range or is configured for Dense mode,
no Group-to-RP mapping is selected, and this algorithm
terminates. The fact that no Group-to-RP mapping has been
selected can be represented in the PIM-STD-MIB module by setting
the address type of the RP to 'unknown' as described in Section
8.
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, is selected.
4. If there are no entries available, then the Group-to-RP mapping
is undefined and this algorithm terminates.
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
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remaining dynamic Group-to-RP mappings are from BSR and Auto-RP
then the mappings from BSR is preferred.
* 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. Please look at section
10 for consideration of hash for BIDIR-PIM and BSR.
* 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. Interpretation of MIB Objects
The algorithm defined in this document does not use the concept of
precedence and therefore the values configured in the
'pimGroupMappingPrecedence' and 'pimStaticRPPrecedence' objects of
the 'pimGroupMappingTable' table in the PIM-STD-MIB module [RFC5060]
are not applicable to the new algorithm. The objects still retain
their meaning for 'legacy' implementations, but since the algorithm
defined in this document is to be used in preference to that found in
PIM-SM [RFC4601] and PIM-STD-MIB [RFC5060], the usage of these fields
will decline as implementations are upgraded to support the new
algorithm.
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8. Clarification for MIB Objects
An implementation of this specification can continue to be managed
using the PIM-STD-MIB [RFC5060]. When a Group-to-RP mapping entry is
created in the pimGroupMappingTable with RP address type in the
pimGroupMappingRPAddressType object is set to unknown(0), and the PIM
Mode in the pimGroupMappingPimMode object is set to either ssm(2) or
dm(5) to represent group ranges for SSM or Dense mode.
Also, all the entries which are already included in the SSM Range
table in the IP Mcast MIB [RFC5132] are copied to the
pimGroupMappingTable. Such entries have their type in the
pimGroupMappingOrigin object set to configSsm(3), and the RP address
type in the pimGroupMappingRPAddressType object set to unknown(0) as
described above.
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9. Use of dynamic group-to-rp mapping protocols
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.
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 MUST 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 an administratively scoped
address range, such as 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 enhances an existing algorithm to deterministically
choose between several group-to-rp mappings for a specific group.
The updated algorithm will not completely prevent the possibility of
different routers selecting different group-to-rp mappings for the
same group. Such situations can be avoided if various mechanisms
used to learn group-to-rp mappings are secure and consistent across
the network.
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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
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[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.
[RFC5132] McWalter, D., Thaler, D., and A. Kessler, "IP Multicast
MIB", RFC 5132, December 2007.
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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
Email: david@mcwalter.eu
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