Internet DRAFT - draft-zhou-pim-vrrp
draft-zhou-pim-vrrp
Network Working Group W. Zhou
Internet-Draft cisco Systems
Intended status: Informational February 23, 2016
Expires: August 26, 2016
VRRP PIM Interoperability
draft-zhou-pim-vrrp-06.txt
Abstract
This document introduces VRRP Aware PIM, a redundancy mechanism for
the Protocol Independent Multicast (PIM) to interoperate with Virtual
Router Redundancy Protocol (VRRP). It allows PIM to track VRRP state
and to preserve multicast traffic upon failover in a redundant
network with virtual routing groups enabled. The mechanism described
in this document is based on Cisco IOS software implementation.
Status of This Memo
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This Internet-Draft will expire on August 26, 2016.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Tracking and Failover . . . . . . . . . . . . . . . . . . . . 3
3. PIM Assert Metric Auto-Adjustment . . . . . . . . . . . . . . 4
4. DF Election for BiDir Group . . . . . . . . . . . . . . . . . 4
5. Tracking Multiple VRRP Groups on an Interface . . . . . . . . 5
6. Support of HSRP . . . . . . . . . . . . . . . . . . . . . . . 5
7. Security Considerations . . . . . . . . . . . . . . . . . . . 5
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5
9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 6
10. Informative References . . . . . . . . . . . . . . . . . . . 6
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 6
1. Introduction
Virtual Router Redundancy Protocol (VRRP) [RFC5798] is a redundancy
protocol for establishing a fault-tolerant default router. The
protocol establishes a framework between network devices in order to
achieve default device failover if the primary devices becomes
inaccessible.
PIM [RFC4601] has no inherent redundancy capabilities and its
operation is completely independent of VRRP group states. As a
result, IP multicast traffic is forwarded not necessarily by the same
device as is elected by VRRP. The VRRP Aware PIM feature provides
consistent IP multicast forwarding in a redundant network with
virtual routing groups enabled.
In a multi-access segment (such as LAN), PIM designated router (DR)
election is unaware of the redundancy configuration, and the elected
DR and VRRP master router (MR) may not be the same router. In order
to ensure that the PIM DR is always able to forward PIM Join/Prune
(J/P) message towards RP or FHR, the VRRP MR becomes the PIM DR (if
there is only one VRRP group). PIM is responsible for adjusting DR
priority based on the group state. When a failover occurs, multicast
states are created on the new MR elected by the VRRP group and the MR
assumes responsibility for the routing and forwarding of all the
traffic addressed to the VRRP virtual IP address (vIP). This ensures
the PIM DR runs on the same router as the VRRP MR and maintains
mroute states. It enables multicast traffic to be forwarded through
the VRRP MR, allowing PIM to leverage VRRP redundancy, avoid
potential duplicate traffic, and enable failover, depending on the
VRRP states in the router.
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This mechanism can be safely deployed into a PIM network without
changing the behavior of other routers. When only a specific set of
routers enabled this feature, user can configure PIM interfaces to
track state change events of desired VRRP group(s). When a router
becomes VRRP MR, PIM component applies the user-defined DR priority
value to the interface in order to make it PIM DR. Other routers
will not break the functionality of this feature, as long as their
configured DR priority do not conflict with the participating
routers. When deployed in PIM transit network, downstream routers
should configure static route to use vIP as next-hop address for PIM
J/P messages in order to take advantage of this feature. If dynamic
routing is used and next-hop address is selected by unicast routing
information as described in [RFC5294], then they cannot leverage the
VRRP redundancy and failover mechanism. These downstream routers,
however, do not have to support this new feature and there is no
additional configuration or coordination required from manageability
point of view. This mechanism does not change any bit on the wire,
and it has been implemented on Cisco IOS software.
2. Tracking and Failover
Without the mechanisms described in this document, a PIM component
will send PIM J/P with DR's IP address to upstream routers. A GenId
in PIM Hello message is randomly selected when the router boots and
remains the same as long as the router is up. A PIM neighbor reboot
can easily be detected if its GenId is different from before, in this
case PIM J/P and RP-Set information can be redistributed to the new
rebooted neighbor. With VRRP aware PIM mechanism enabled, PIM
component listens to the state change notifications from VRRP and
automatically adjusts the priority of the PIM DR based on the VRRP
state, and ensures VRRP MR (if there is only one VRRP group) becomes
the DR of the LAN. If there are multiple VRRP groups, the DR is
determined by user-configured priority value.
Upon failover, PIM component triggers communication between upstream
and downstream routers in order to create mroute states on the new
VRRP MR. PIM component sends an additional PIM Hello message using
the VRRP vIP as the source address for each active VRRP group when a
router becomes VRRP Master. The PIM Hello message with new GenID
will trigger other routers to respond to the VRRP failover event in
the same way of PIM neighbor reboot event as described in [RFC5294].
Specifically, when a downstream router receives this PIM Hello
message, it will add the source IP address (in this case the VRRP
vIP) into its PIM neighbor list, and immediately send triggered PIM
J/P messages towards vIP. Upstream routers will process PIM J/P
based on VRRP group state. If PIM J/P next-hop address matches VRRP
vIP, only the current VRRP MR will process the PIM J/P messages.
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This allows all PIM J/P to reach the VRRP group vIP and minimizes
changes and configurations at the downstream routers.
Alternatively, implementation can choose to have all VRRP passive
routers maintain mroute states and record the GenID of current MR.
When a passive router becomes MR, it uses the existing mroute states
and the recorded MR GenID in its PIM Hello message. This will avoid
resending PIM J/P messages upon failover and eliminates the
requirement of additional PIM Hello with vIP. There is no change in
on-wire behavior or in the PIM and VRRP message format.
3. PIM Assert Metric Auto-Adjustment
It is possible that, after VRRP Master switched from A to B; A is
still forwarding multicast traffic which will result in duplicate
traffic and PIM Assert mechanism will kick in. PIM Assert with
redundancy is enabled.
o If only one VRRP group, passive routers will send arbitrary
penalty metric preference (PIM_ASSERT_INFINITY - 1) and make MR
the Assert winner.
o If there are multiples VRRP groups configured on an interface,
Assert metric preference will be (PIM_ASSERT_INFINITY - 1) if and
only if all VRRP groups are in passive state.
o If there is at least one VRRP group is in Active, then original
Assert metric preference will be used. That is, winner will be
selected between routers using their real Assert metric preference
with at least one active VRRP Group, just like no VRRP is
involved.
4. DF Election for BiDir Group
Change to DF offer/winner metric is handled similarly to PIM Assert
handling with VRRP.
o If only one VRRP group, passive routers will send a large penalty
metric preference in Offer (PIM_BIDIR_INFINITY_PREF- 1) and make
MR the DF winner.
o If there are multiples VRRP groups configured on an interface,
Offer metric preference will be (PIM_BIDIR_INFINITY_PREF- 1) if
and only if all VRRP groups are in passive.
o If there is at least one VRRP group is in Active, then original
Offer metric preference to RP will be used. That is, winner will
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be selected between routers using their real Offer metric, just as
no VRRP is involved.
5. Tracking Multiple VRRP Groups on an Interface
User can configure PIM component to track more than one VRRP groups
on an interface. This allows other applications to exploit the PIM/
VRRP interoperability to achieve various goals (e.g., load
balancing). Since each VRRP groups configured on an interface could
be in different states at any moment, the DR priority is adjusted.
PIM Assert metric and PIM Bidir DF metric if and only if all VRRP
groups configured on an interface are in passive (non-Active) states
to ensure that interfaces with all-passive VRRP groups will not win
in DR, Assert and DF election. In other words, DR, Assert, DF winner
will be elected among the interfaces with at least one Active VRRP
group.
6. Support of HSRP
Although there are differences between VRRP and Hot Standby Router
Protocol (HSRP) [RFC2281] including number of backup (standby)
routers, virtual IP address and timer intervals, the proposed scheme
can also enable HSRP aware PIM with similar failover and tracking
mechanism described in this draft.
7. Security Considerations
The proposed tracking mechanism does not discuss adding
authentication to the protocols and introduces no new negative impact
or threats on security to PIM in either SSM or ASM mode. Note that
VRRP messages from malicious nodes could cause unexpected behaviors
such as multiple Masters and PIM DRs which are associated with VRRP
specific security issues. To mitigate the vulnerability of frequent
VRRP and PIM DR state change from malicious attack, implementation
can choose to enable VRRP preemption such that a higher-priority VRRP
backup router does not take over for a lower-priority MR, this will
reduce the state change notifications to PIM component and subsequent
mroute state change. Detailed analysis of PIM and VRRP security is
provided in [RFC5294] and [RFC5798].
8. IANA Considerations
This document has no IANA actions.
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9. Acknowledgments
I would like to give a special thank you and appreciation to Stig
Venaas for his ideas and comments in this draft.
10. Informative References
[RFC2281] Li, T., Cole, B., Morton, P., and D. Li, "Cisco Hot
Standby Router Protocol (HSRP)", RFC 2281,
DOI 10.17487/RFC2281, March 1998,
<http://www.rfc-editor.org/info/rfc2281>.
[RFC4601] Fenner, B., Handley, M., Holbrook, H., and I. Kouvelas,
"Protocol Independent Multicast - Sparse Mode (PIM-SM):
Protocol Specification (Revised)", RFC 4601,
DOI 10.17487/RFC4601, August 2006,
<http://www.rfc-editor.org/info/rfc4601>.
[RFC5294] Savola, P. and J. Lingard, "Host Threats to Protocol
Independent Multicast (PIM)", RFC 5294,
DOI 10.17487/RFC5294, August 2008,
<http://www.rfc-editor.org/info/rfc5294>.
[RFC5798] Nadas, S., Ed., "Virtual Router Redundancy Protocol (VRRP)
Version 3 for IPv4 and IPv6", RFC 5798,
DOI 10.17487/RFC5798, March 2010,
<http://www.rfc-editor.org/info/rfc5798>.
Author's Address
Wei Zhou
cisco Systems
Tasman Drive
San Jose, CA 95134
USA
Email: weizho2@cisco.com
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