Internet DRAFT - draft-gopal-pim-pfm-forwarding-enhancements
draft-gopal-pim-pfm-forwarding-enhancements
Network Working Group A. Gopal
Internet-Draft S. Venaas
Intended status: Standards Track Cisco Systems, Inc.
Expires: 5 September 2024 4 March 2024
PIM Flooding Mechanism Enhancements
draft-gopal-pim-pfm-forwarding-enhancements-02
Abstract
PIM Flooding Mechanism is a generic PIM message exchange mechanism
that allows multicast information to be exchanged between PIM routers
hop-by-hop. One example is PIM Flooding Mechanism and Source
Discovery which allows Last Hop Routers to learn about new sources
using PFM messages, without the need of initial data registers, RPs
or shared trees.
This document defines a methodology that enhances forwarding
efficiency in PFM deployments. This enhancement can avoid extra
processing at PIM routers when PFM messages are forwarded.
Status of This Memo
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This Internet-Draft will expire on 5 September 2024.
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Please review these documents carefully, as they describe your rights
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Conventions Used in This Document . . . . . . . . . . . . 3
1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
2. Methodology . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.1. RFC 6395 Compliance . . . . . . . . . . . . . . . . . . . 3
2.2. PFM optimization Hello option . . . . . . . . . . . . . . 3
2.3. PFM message sending . . . . . . . . . . . . . . . . . . . 4
2.4. PFM message receiving and RPF check relaxation . . . . . 5
2.5. PFM message forwarding . . . . . . . . . . . . . . . . . 5
3. Operational Considerations . . . . . . . . . . . . . . . . . 5
4. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 6
5. Normative References . . . . . . . . . . . . . . . . . . . . 6
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 6
1. Introduction
PIM Flooding Mechanism [RFC8364] allows a PIM router in the network
to originate a PFM message to distribute multicast information to its
PIM neighbors [RFC7761] . All PIM neighbors then process this PFM
message and flood it further on their PIM-enabled links. To prevent
loops, the originator address as defined in Section 3.1 [RFC8364] is
used for RPF checking at each router. This RPF check is defined in
Section 3.4.1 [RFC8364]. Periodic PFM messages are triggered, see
Section 3.4.2 [RFC8364] and exchanged to keep the multicast
information updated across the PIM domain. At present, a PIM router
will flood a PFM message on all its PIM enabled links. It is the
recipient's responsibility to perform RPF checks on all received PFM
messages and then decide whether to accept or drop a particular
message. This means that if two routers have PIM neighborships over
more than one link, the same PFM messages are exchanged or dropped
over more than one link between the same two routers. This leads to
extra processing at each PIM router, periodically, or every time a
new source is discovered (in case of a PFM-SD implementation). This
document defines a mechanism to exchange PFM messages only on one ONE
link per router-pair, even though they may maintain PIM neighborships
over multiple links. In other words, when there are multiple links
between two PIM routers, routers should not send the same message on
all the links between them. This is achieved by identifying the PIM
routers in the network using Router Identifier that are announced via
PIM hellos, as per RFC 6395.
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1.1. Conventions Used in This Document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
1.2. Terminology
RP: Rendezvous Point
RPF: Reverse Path Forwarding
PFM: PIM Flooding Mechanism
2. Methodology
2.1. RFC 6395 Compliance
For the PFM enhancement specified in this document to be adopted, all
PIM routers MUST be compliant with RFC [RFC6395]. This means that
PIM routers announce a unique domain-wide router ID in their PIM
Hellos. A PIM router announces the same 4-byte Router-ID in PIM
Hellos that it sends to all neighbors on all links. It also caches
of the Router-Ids of its neighbors, when it receives Hellos from RFC
6395 Compliant PIM neighbors. This can be used to determine that
different PIM neighbors are really the same router. In a VRF
context, if the router has multiple interfaces with only one neighbor
per interface, the router SHOULD check if those neighbors announce an
RFC 6395 router ID. If the router can see the same router ID for
multiple neighbors, PFM message exchange is optimized, as discussed
in Section 2.
2.2. PFM optimization Hello option
A PIM router indicates that it supports the mechanism specified in
this document by including the new PFM optimization Hello option.
When this optimization is included in the PIM hello, the router MUST
also include the router-ID Hello Option defined in [RFC6395] with a
non-zero router-ID.
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| OptionType | OptionLength = 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: PFM optimization Hello option
OptionType = TBD
OptionLength = 0
Note that there is no option value included. When a PIM hello with
OptionType TBD is received from a PIM neighbor, the router MUST cache
this information so that it can make forwarding and dropping
decisions for PFM messages for that neighbor. When this option is
included, the router MUST also cache the non-zero router-ID of this
neighbor.
2.3. PFM message sending
Consider a topology where two PIM routers maintain PIM neighborships
over more than one links in the same PIM domain. From each router's
point of view, there is a single neighbor on each link.
Traditionally, each of the routers will send out PFM messages out
over all the links to its neighbor. RPF checks are one of the
commonly used ways to prevents loops, hence the recipient router
performs an RPF check and accepts only on one link, thereby dropping
packets from all the others. Since, the sender does not know which
link will be chosen as the RPF-source on the neighbor, it cannot make
the decision of choosing one of the links, without knowing its
neighbor's decision.
If the optimization specified in this document is advertised by both
routers, the sender should choose one of the links and send and
forward PFM messages to its neighbor using only that link. It is
imperative that the sender does this only when the receiver is
capable of the optimization and is advertising the same. Otherwise,
the messages may be dropped because of RPF failures. Mechanism to
choose a link is left to the implementation.
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2.4. PFM message receiving and RPF check relaxation
Consider a router that is advertising its capability to optimize PFM
exchanges in the network. Upon receiving a PFM message, this router
MUST first check whether this message is sent by a PFM optimization
enabled router. If the check returns true, the receiver should relax
its RPF check and accept the message. When a PFM message is
received, the receiver SHOULD keep track of the router ID of the
sender, so that the receiver does not forward the message back to the
sender on any other link, as explained in the next section. If it
receives this message from a router that is not advertising the PFM
optimization specified in this document, however the optimization is
enabled on the receiver, the receiver SHOULD NOT relax its RPF check.
This is because the sender will be still sending out messages on all
the links between them.
2.5. PFM message forwarding
Traditionally, a PIM router forwards a PFM message on all its PIM
enabled links. However, this is now optimized. Consider a router
that is advertising its capability to optimize PFM exchanges in the
network. When this router receives a PFM message from a router that
also has the PFM optimization enabled, the forwarding mechanism is as
follows. The receiver MUST NOT send the PFM message on out on any
links where there is only one neighbor and the neighbor has the same
router ID as the sender.
3. Operational Considerations
Existing neighbor on a new link: When a new neighbor is detected
announcing support for the optimization and announcing a non-zero
router ID, and it is the only neighbor on the link, a PIM router
needs to check if there is an existing neighbor on another link
with the same router ID (it does not need to be the sole neighbor
on the other link). A mechanism SHOULD be implemented to prevent
PFM messages being sent on this link.
New neighbor on a new link: Appropriate logic SHOULD be implemented
to handle new neighbor additions so that extra messages are not
forwarded to same neighbor, as well as ensuring that a new
neighbor quickly gets the correct state.
Removal of a neighbor: Appropriate logic SHOULD also be implemented
to handle neighbor removals.
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4. Acknowledgments
5. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC7761] Fenner, B., Handley, M., Holbrook, H., Kouvelas, I.,
Parekh, R., Zhang, Z., and L. Zheng, "Protocol Independent
Multicast - Sparse Mode (PIM-SM): Protocol Specification
(Revised)", STD 83, RFC 7761, DOI 10.17487/RFC7761, March
2016, <https://www.rfc-editor.org/info/rfc7761>.
[RFC8364] Wijnands, IJ., Venaas, S., Brig, M., and A. Jonasson, "PIM
Flooding Mechanism (PFM) and Source Discovery (SD)",
RFC 8364, DOI 10.17487/RFC8364, March 2018,
<https://www.rfc-editor.org/info/rfc8364>.
[RFC6395] Gulrajani, S. and S. Venaas, "An Interface Identifier (ID)
Hello Option for PIM", RFC 6395, DOI 10.17487/RFC6395,
October 2011, <https://www.rfc-editor.org/info/rfc6395>.
Authors' Addresses
Ananya Gopal
Cisco Systems, Inc.
Tasman Drive
San Jose, CA 95134
United States of America
Email: ananygop@cisco.com
Stig Venaas
Cisco Systems, Inc.
Tasman Drive
San Jose, CA 95134
United States of America
Email: svenaas@cisco.com
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