draft-ietf-pim-drlb-10.txt		draft-ietf-pim-drlb-11.txt
	Network Working Group Y. Cai		Network Working Group Y. Cai
	Internet-Draft H. Ou		Internet-Draft H. Ou
	Intended status: Standards Track Alibaba Group		Intended status: Standards Track Alibaba Group
	Expires: May 17, 2019 S. Vallepalli		Expires: April 13, 2020 S. Vallepalli
	M. Mishra		M. Mishra
	S. Venaas		S. Venaas
	Cisco Systems, Inc.		Cisco Systems, Inc.
	A. Green		A. Green
	British Telecom		British Telecom
	November 13, 2018		October 11, 2019
	PIM Designated Router Load Balancing		PIM Designated Router Load Balancing
	draft-ietf-pim-drlb-10		draft-ietf-pim-drlb-11
	Abstract		Abstract
	On a multi-access network, one of the PIM routers is elected as a		On a multi-access network, one of the PIM-SM routers is elected as a
	Designated Router (DR). On the last hop LAN, the PIM DR is		Designated Router. One of the responsibilities of the Designated
	responsible for tracking local multicast listeners and forwarding		Router is to track local multicast listeners and forward data to
	traffic to these listeners if the group is operating in PIM-SM. This		these listeners if the group is operating in PIM-SM. This document
	document specifies a modification to the PIM-SM protocol that allows		specifies a modification to the PIM-SM protocol that allows more than
	more than one of these last hop routers to be selected, so that the		one of the PIM-SM routers to take on this responsibility so that the
	forwarding load can be distributed among these routers.		forwarding load can be distributed among multiple routers.
	Status of This Memo		Status of This Memo
	This Internet-Draft is submitted in full conformance with the		This Internet-Draft is submitted in full conformance with the
	provisions of BCP 78 and BCP 79.		provisions of BCP 78 and BCP 79.
	Internet-Drafts are working documents of the Internet Engineering		Internet-Drafts are working documents of the Internet Engineering
	Task Force (IETF). Note that other groups may also distribute		Task Force (IETF). Note that other groups may also distribute
	working documents as Internet-Drafts. The list of current Internet-		working documents as Internet-Drafts. The list of current Internet-
	Drafts is at https://datatracker.ietf.org/drafts/current/.		Drafts is at https://datatracker.ietf.org/drafts/current/.
	Internet-Drafts are draft documents valid for a maximum of six months		Internet-Drafts are draft documents valid for a maximum of six months
	and may be updated, replaced, or obsoleted by other documents at any		and may be updated, replaced, or obsoleted by other documents at any
	time. It is inappropriate to use Internet-Drafts as reference		time. It is inappropriate to use Internet-Drafts as reference
	material or to cite them other than as "work in progress."		material or to cite them other than as "work in progress."
	This Internet-Draft will expire on May 17, 2019.		This Internet-Draft will expire on April 13, 2020.
	Copyright Notice		Copyright Notice
	Copyright (c) 2018 IETF Trust and the persons identified as the		Copyright (c) 2019 IETF Trust and the persons identified as the
	document authors. All rights reserved.		document authors. All rights reserved.
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	carefully, as they describe your rights and restrictions with respect		carefully, as they describe your rights and restrictions with respect
	to this document. Code Components extracted from this document must		to this document. Code Components extracted from this document must
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	described in the Simplified BSD License.		described in the Simplified BSD License.
	Table of Contents		Table of Contents
	1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2		1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
	2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5		2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
	3. Applicability . . . . . . . . . . . . . . . . . . . . . . . . 5		3. Applicability . . . . . . . . . . . . . . . . . . . . . . . . 5
	4. Functional Overview . . . . . . . . . . . . . . . . . . . . . 6		4. Functional Overview . . . . . . . . . . . . . . . . . . . . . 5
	4.1. GDR Candidates . . . . . . . . . . . . . . . . . . . . . 6		4.1. GDR Candidates . . . . . . . . . . . . . . . . . . . . . 6
	4.2. Hash Mask and Hash Algorithm . . . . . . . . . . . . . . 7		5. Protocol Specification . . . . . . . . . . . . . . . . . . . 7
	4.3. Modulo Hash Algorithm . . . . . . . . . . . . . . . . . . 8		5.1. Hash Mask and Hash Algorithm . . . . . . . . . . . . . . 7
	4.3.1. Limitations . . . . . . . . . . . . . . . . . . . . . 9		5.2. Modulo Hash Algorithm . . . . . . . . . . . . . . . . . . 8
	4.4. PIM Hello Options . . . . . . . . . . . . . . . . . . . . 9		5.2.1. Modulo Hash Algorithm Example . . . . . . . . . . . . 9
	5. Hello Option Formats . . . . . . . . . . . . . . . . . . . . 10		5.2.2. Limitations . . . . . . . . . . . . . . . . . . . . . 10
	5.1. PIM DR Load Balancing Capability (DRLBC) Hello Option . . 10		5.3. PIM Hello Options . . . . . . . . . . . . . . . . . . . . 10
	5.2. PIM DR Load Balancing GDR (DRLBGDR) Hello Option . . . . 10		5.3.1. PIM DR Load Balancing Capability (DRLB-Cap) Hello
	6. Protocol Specification . . . . . . . . . . . . . . . . . . . 11		Option . . . . . . . . . . . . . . . . . . . . . . . 10
	6.1. PIM DR Operation . . . . . . . . . . . . . . . . . . . . 11		5.3.2. PIM DR Load Balancing List (DRLB-List) Hello Option . 11
	6.2. PIM GDR Candidate Operation . . . . . . . . . . . . . . . 12		5.4. PIM DR Operation . . . . . . . . . . . . . . . . . . . . 12
	6.2.1. Router Receives New DRLBGDR . . . . . . . . . . . . . 13		5.5. PIM GDR Candidate Operation . . . . . . . . . . . . . . . 13
	6.2.2. Router Receives Updated DRLBGDR . . . . . . . . . . . 13		5.6. DRLB-List Hello Option Processing . . . . . . . . . . . . 13
	6.3. PIM Assert Modification . . . . . . . . . . . . . . . . . 14		5.7. PIM Assert Modification . . . . . . . . . . . . . . . . . 14
	7. Compatibility . . . . . . . . . . . . . . . . . . . . . . . . 15		5.8. Backward Compatibility . . . . . . . . . . . . . . . . . 16
	8. Manageability Considerations . . . . . . . . . . . . . . . . 16		6. Manageability Considerations . . . . . . . . . . . . . . . . 16
	9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16		7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 17
	9.1. Initial registry . . . . . . . . . . . . . . . . . . . . 16		7.1. Initial registry . . . . . . . . . . . . . . . . . . . . 17
	9.2. Assignment of new hash algorithms . . . . . . . . . . . . 16		7.2. Assignment of new hash algorithms . . . . . . . . . . . . 17
	10. Security Considerations . . . . . . . . . . . . . . . . . . . 16		8. Security Considerations . . . . . . . . . . . . . . . . . . . 17
	11. Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . 17		9. Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . 18
	12. References . . . . . . . . . . . . . . . . . . . . . . . . . 17		10. References . . . . . . . . . . . . . . . . . . . . . . . . . 18
	12.1. Normative References . . . . . . . . . . . . . . . . . . 17		10.1. Normative References . . . . . . . . . . . . . . . . . . 18
	12.2. Informative References . . . . . . . . . . . . . . . . . 17		10.2. Informative References . . . . . . . . . . . . . . . . . 18
	Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 17		Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 19
	1. Introduction		1. Introduction
	On a multi-access LAN such as an Ethernet, one of the PIM routers is		On a multi-access LAN, such as an Ethernet, with one or more PIM-SM
	elected as a DR. The PIM DR has two roles in the PIM-SM protocol.		[RFC7761] routers, one of the PIM-SM routers is elected as a
	On the first hop LAN, the PIM DR is responsible for registering an		Designated Router (DR). The PIM DR has two responsibilities in the
	active source with the Rendezvous Point (RP) if the group is		PIM-SM protocol. For any active sources on a LAN, the PIM DR is
	operating in PIM-SM. On the last hop LAN, the PIM DR is responsible		responsible for registering with the Rendezvous Point (RP) if the
	for tracking local multicast listeners and forwarding to these		group is operating in PIM-SM. Also, the PIM DR is responsible for
	listeners if the group is operating in PIM-SM.		tracking local multicast listeners and forwarding to these listeners
			if the group is operating in PIM-SM.
	Consider the following last hop LAN in Figure 1:		Consider the following LAN in Figure 1:
	(core networks)		(core networks)
	| | |		| | |
	| | |		| | |
	R1 R2 R3		R1 R2 R3
	| | |		| | |
	--(last hop LAN)--		----(LAN)----
	|		|
	|		|
	(many receivers)		(many receivers)
	Figure 1: Last Hop LAN		Figure 1: LAN with receivers
	Assume R1 is elected as the Designated Router. According to
	[RFC7761], R1 will be responsible for forwarding traffic to that LAN
	on behalf of any local members. In addition to keeping track of IGMP
	and MLD membership reports, R1 is also responsible for initiating the
	creation of source and/or shared trees towards the senders or the
	RPs.
	Forcing sole data plane forwarding responsibility on the PIM DR		Assume R1 is elected as the DR. According to the PIM-SM protocol, R1
	uncovers a limitation in the protocol. In comparison, even though an		will be responsible for forwarding traffic to that LAN on behalf of
	OSPF DR or an IS-IS DIS handles additional duties while running the		any local members. In addition to keeping track of membership
	OSPF or IS-IS protocols, they are not required to be solely		reports, R1 is also responsible for initiating the creation of source
	responsible for forwarding packets for the network. On the other		and/or shared trees towards the senders or the RPs. The membership
	hand, on a last hop LAN, only the PIM DR is asked to forward packets		reports would be IGMP or MLD messages. This applies to any versions
	while the other routers handle only control traffic (and perhaps drop		of the IGMP and MLD protocols. The most recent versions are IGMPv3
	packets due to RPF failures). Hence the forwarding load of a last		[RFC3376] and MLDv2 [RFC3810].
	hop LAN is concentrated on a single router.
	This leads to several issues. One of the issues is that the		Having a single router acting as DR and being responsible for data
	aggregated bandwidth will be limited to what R1 can handle towards		plane forwarding leads to several issues. One of the issues is that
	this particular interface. It is very common that the last hop LAN		the aggregated bandwidth will be limited to what R1 can handle with
	consists of switches that run IGMP/MLD or PIM snooping. This allows		regards to capacity of incoming links, the interface on the LAN, and
			total forwarding capacity. It is very common that a LAN consists of
			switches that run IGMP/MLD or PIM snooping [RFC4541]. This allows
	the forwarding of multicast packets to be restricted only to segments		the forwarding of multicast packets to be restricted only to segments
	leading to receivers who have indicated their interest in multicast		leading to receivers who have indicated their interest in multicast
	groups using either IGMP or MLD. The emergence of the switched		groups using either IGMP or MLD. The emergence of the switched
	Ethernet allows the aggregated bandwidth to exceed, sometimes by a		Ethernet allows the aggregated bandwidth to exceed, sometimes by a
	large number, that of a single link. For example, let us modify		large number, that of a single link. For example, let us modify
	Figure 1 and introduce an Ethernet switch in Figure 2.		Figure 1 and introduce an Ethernet switch in Figure 2.
	(core networks)		(core networks)
	| | |		| | |
	| | |		| | |
	R1 R2 R3		R1 R2 R3
	| | |		| | |
	+=gi0===gi1===gi2=+		+=gi0===gi1===gi2=+
	+ +		+ +
	+ switch +		+ switch +
	+ +		+ +
	+=gi4===gi5===gi6=+		+=gi4===gi5===gi6=+
	| | |		| | |
	H1 H2 H3		H1 H2 H3
	Figure 2: Last Hop Network with Ethernet Switch		Figure 2: LAN with Ethernet Switch
	Let us assume that each individual link is a Gigabit Ethernet. Each		Let us assume that each individual link is a Gigabit Ethernet. Each
	router, R1, R2 and R3, and the switch have enough forwarding capacity		router, R1, R2 and R3, and the switch have enough forwarding capacity
	to handle hundreds of Gigabits of data.		to handle hundreds of Gigabits of data.
	Let us further assume that each of the hosts requests 500 Mbps of		Let us further assume that each of the hosts requests 500 Mbps of
	unique multicast data. This totals to 1.5 Gbps of data, which is		unique multicast data. This totals to 1.5 Gbps of data, which is
	less than what each switch or the combined uplink bandwidth across		less than what each switch or the combined uplink bandwidth across
	the routers can handle, even under failure of a single router.		the routers can handle, even under failure of a single router.
	On the other hand, the link between R1 and switch, via port gi0, can		On the other hand, the link between R1 and switch, via port gi0, can
	only handle a throughput of 1Gbps. And if R1 is the only DR (the PIM		only handle a throughput of 1Gbps. And if R1 is the only DR (the PIM
	DR elected using the procedure defined by [RFC7761]) at least 500		DR elected using the procedure defined by [RFC7761]) at least 500
	Mbps worth of data will be lost because the only link that can be		Mbps worth of data will be lost because the only link that can be
	used to draw the traffic from the routers to the switch is via gi0.		used to draw the traffic from the routers to the switch is via gi0.
	In other words, the entire network's throughput is limited by the		In other words, the entire network's throughput is limited by the
	single connection between the PIM DR and the switch (or the last hop		single connection between the PIM DR and the switch (or LAN as in
	LAN as in Figure 1).		Figure 1).
	Another important issue is related to failover. If R1 is the only		Another important issue is related to failover. If R1 is the only
	forwarder on the last hop router for a shared LAN, when R1 goes out		forwarder on a shared LAN, when R1 goes out of service, multicast
	of service, multicast forwarding for the entire LAN has to be rebuilt		forwarding for the entire LAN has to be rebuilt by the newly elected
	by the newly elected PIM DR. However, if there was a way that		PIM DR. However, if there was a way that allowed multiple routers to
	allowed multiple routers to forward to the LAN for different groups,		forward to the LAN for different groups, failure of one of the
	failure of one of the routers would only lead to disruption to a		routers would only lead to disruption to a subset of the flows,
	subset of the flows, therefore improving the overall resilience of		therefore improving the overall resilience of the network.
	the network.
	There is a limitation in the hash algorithm used in this document,
	but this document provides the option to have different and more
	consistent hash algorithms in the future.
	This document specifies a modification to the PIM-SM protocol that		This document specifies a modification to the PIM-SM protocol that
	allows more than one of these routers, called Group Designated		allows more than one of these routers, called Group Designated
	Routers (GDR) to be selected so that the forwarding load can be		Routers (GDR) to be selected so that the forwarding load can be
	distributed among a number of routers.		distributed among a number of routers.
	2. Terminology		2. Terminology
	The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",		The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
	"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this		"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
	document are to be interpreted as described in [RFC2119].		"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.
	With respect to PIM, this document follows the terminology that has		With respect to PIM-SM, this document follows the terminology that
	been defined in [RFC7761].		has been defined in [RFC7761].
	This document also introduces the following new acronyms:		This document also introduces the following new acronyms:
	o GDR: GDR stands for "Group Designated Router". For each multicast		o GDR: Group Designated Router. For each multicast flow, either a
	flow, either a (*,G) for ASM, or an (S,G) for SSM, a hash		(*,G) for Any-Source Multicast (ASM), or an (S,G) for Source-
	algorithm (described below) is used to select one of the routers		Specific Multicast (SSM) [RFC4607], a hash algorithm (described
	as a GDR. The GDR is responsible for initiating the forwarding		below) is used to select one of the routers as a GDR. The GDR is
	tree building process for the corresponding multicast flow.		responsible for initiating the forwarding tree building process
			for the corresponding multicast flow.
	o GDR Candidate: a last hop router that has the potential to become		o GDR Candidate: a router that has the potential to become a GDR.
	a GDR. A GDR Candidate must have the same DR priority and must		There might be multiple GDR Candidates on a LAN, but only one can
	run the same GDR election hash algorithm as the DR router. It		become the GDR for a specific multicast flow.
	must send and process new PIM Hello Options as defined in this
	document. There might be more than one GDR Candidate on a LAN,
	but only one can become GDR for a specific multicast flow.
	3. Applicability		3. Applicability
	The extension specified in this document applies to PIM-SM last hop		The extension specified in this document applies to PIM-SM when they
	routers only. It does not alter the behavior of a PIM DR on the		act as last hop routers (there are directly connected receivers). It
	first hop network. This is because the source tree is built using		does not alter the behavior of a PIM DR, or any other routers, on the
	the IP address of the sender, not the IP address of the PIM DR that		first hop network (directly connected sources). This is because the
	sends the registers towards the RP. The load balancing between first		source tree is built using the IP address of the sender, not the IP
	hop routers can be achieved naturally if an IGP provides equal cost		address of the PIM DR that sends the registers towards the RP. The
	multiple paths (which it usually does in practice). Also		load balancing between first hop routers can be achieved naturally if
	distributing the load to do registering does not justify the		an IGP provides equal cost multiple paths (which it usually does in
	additional complexity required to support it.		practice). Also distributing the load to do registering does not
			justify the additional complexity required to support it.
	4. Functional Overview		4. Functional Overview
	In the PIM DR election as defined in [RFC7761], when multiple last		In the PIM DR election as defined in [RFC7761], when multiple routers
	hop routers are connected to a multi-access LAN (for example, an		are connected to a multi-access LAN (for example, an Ethernet), one
	Ethernet), one of them is elected to act as PIM DR. The PIM DR is		of them is elected to act as PIM DR. The PIM DR is responsible for
	responsible for sending local Join/Prune messages towards the RP or		sending local Join/Prune messages towards the RP or source. In order
	source. In order to elect the PIM DR, each PIM router on the LAN		to elect the PIM DR, each PIM router on the LAN examines the received
	examines the received PIM Hello messages and compares its own DR		PIM Hello messages and compares its own DR priority and IP address
	priority and IP address with those of its neighbors. The router with		with those of its neighbors. The router with the highest DR priority
	the highest DR priority is the PIM DR. If there are multiple such		is the PIM DR. If there are multiple such routers, their IP
	routers, their IP addresses are used as the tie-breaker, as described		addresses are used as the tie-breaker, as described in [RFC7761].
	in [RFC7761].
	In order to share forwarding load among last hop routers, besides the		In order to share forwarding load among last hop routers, besides the
	normal PIM DR election, the GDR is also elected on the last hop		normal PIM DR election, the GDR is also elected on the multi-access
	multi-access LAN. There is only one PIM DR on the multi-access LAN,		LAN. There is only one PIM DR on the multi-access LAN, but there
	but there might be multiple GDR Candidates.		might be multiple GDR Candidates.
	For each multicast flow, that is, (*,G) for ASM and (S,G) for SSM, a		For each multicast flow, that is, (*,G) for ASM and (S,G) for SSM, a
	hash algorithm is used to select one of the routers to be the GDR. A		hash algorithm is used to select one of the routers to be the GDR. A
	new DR Load Balancing Capability (DRLBC) PIM Hello Option, which		new DR Load Balancing Capability (DRLB-Cap) PIM Hello Option, which
	contains hash algorithm type, is announced by routers on interfaces		contains hash algorithm type, is announced by routers on interfaces
	where this specification is enabled. Last hop routers with the new		where this specification is enabled. Routers with the new DRLB-Cap
	DRLBC Option advertised in its Hello, and using the same GDR election		Option advertised in their PIM Hello, using the same GDR election
	hash algorithm and the same DR priority as the PIM DR, are considered		hash algorithm and the same DR priority as the PIM DR, are considered
	as GDR Candidates.		as GDR Candidates.
	Hash Masks are defined for Source, Group and RP separately, in order		Hash Masks are defined for Source, Group and RP separately, in order
	to handle PIM ASM/SSM. The masks, as well as a sorted list of GDR		to handle PIM ASM/SSM. The masks, as well as a sorted list of GDR
	Candidate Addresses, are announced by the DR in a new DR Load		Candidate Addresses, are announced by the DR in a new DR Load
	Balancing GDR (DRLBGDR) PIM Hello Option.		Balancing List (DRLB-List) PIM Hello Option.
	A hash algorithm based on the announced Source, Group, or RP masks		A hash algorithm based on the announced Source, Group, or RP masks
	allows one GDR to be assigned to a corresponding multicast state.		allows one GDR to be assigned to a corresponding multicast state.
	And that GDR is responsible for initiating the creation of the		And that GDR is responsible for initiating the creation of the
	multicast forwarding tree for multicast traffic.		multicast forwarding tree for multicast traffic.
	4.1. GDR Candidates		4.1. GDR Candidates
	GDR is the new concept introduced by this specification. GDR		GDR is the new concept introduced by this specification. GDR
	Candidates are routers eligible for GDR election on the LAN. To		Candidates are routers eligible for GDR election on the LAN. To
	become a GDR Candidate, a router MUST support this specification,		become a GDR Candidate, a router must have the same DR priority and
	have the same DR priority and run the same GDR election hash		run the same GDR election hash algorithm as the DR on the LAN.
	algorithm as the DR on the LAN.
	For example, assume there are 4 routers on the LAN: R1, R2, R3 and		For example, assume there are 4 routers on the LAN: R1, R2, R3 and
	R4, which all support this specification. R1, R2 and R3 have the		R4, each announcing a DRLB-Cap option. R1, R2 and R3 have the same
	same DR priority while R4's DR priority is less preferred. In this		DR priority while R4's DR priority is less preferred. In this
	example, R4 will not be eligible for GDR election, because R4 will		example, R4 will not be eligible for GDR election, because R4 will
	not become a PIM DR unless all of R1, R2 and R3 go out of service.		not become a PIM DR unless all of R1, R2 and R3 go out of service.
	Furthermore, assume router R1 wins the PIM DR election, R1 and R2 run		Furthermore, assume router R1 wins the PIM DR election, R1 and R2 run
	the same hash algorithm for GDR election, while R3 runs a different		the same hash algorithm for GDR election, while R3 runs a different
	one. In this case, only R1 and R2 will be eligible for GDR election,		one. In this case, only R1 and R2 will be eligible for GDR election,
	while R3 will not.		while R3 will not.
	As a DR, R1 will include its own Load Balancing Hash Masks and the		As a DR, R1 will include its own Load Balancing Hash Masks and the
	identity of R1 and R2 (the GDR Candidates) in its DRLBGDR Hello		identity of R1 and R2 (the GDR Candidates) in its DRLB-List Hello
	Option.		Option.
	4.2. Hash Mask and Hash Algorithm		5. Protocol Specification
			5.1. Hash Mask and Hash Algorithm
	A Hash Mask is used to extract a number of bits from the		A Hash Mask is used to extract a number of bits from the
	corresponding IP address field (32 for v4, 128 for v6) and calculate		corresponding IP address field (32 for IPv4, 128 for IPv6) and
	a hash value. A hash value is used to select a GDR from GDR		calculate a hash value. A hash value is used to select a GDR from
	Candidates advertised by PIM DR. For example, 0.0.255.0 defines a		GDR Candidates advertised by PIM DR. For example, 0.0.255.0 defines
	Hash Mask for an IPv4 address that masks the first, the second, and		a Hash Mask for an IPv4 address that masks the first, the second, and
	the fourth octets.		the fourth octets. Hash masks allow for certain flows to always be
			forwarded by the same GDR, since the hash values are the same. For
			instance the mask 0.0.255.0 means that only the third octet will be
			considered when hashing.
			In the text below, a hash mask is in some places said to be zero. A
			hash mask is zero if no bits are set. That is, 0.0.0.0 for IPv4 and
			:: for IPv6. Also, a hash mask is said to be an all-bits-set mask if
			it is 255.255.255.255 for IPv4 or
			FFFF:FFFF:FFFF:FFFF:FFFFF:FFFF:FFFF:FFFF for IPv6.
	There are three Hash Masks defined:		There are three Hash Masks defined:
	o RP Hash Mask		o RP Hash Mask
	o Source Hash Mask		o Source Hash Mask
	o Group Hash Mask		o Group Hash Mask
	The hash masks need to be configured on the PIM routers that can		The hash masks need to be configured on the PIM routers that can
	potentially become a PIM DR, unless the implementation provides		potentially become a PIM DR, unless the implementation provides
	default Hash Mask values. An implementation SHOULD provide masks		default hash mask values. An implementation SHOULD have default hash
	with default values 255.255.255.255 (IPv4) and		mask values as follows. The default RP Hash Mask SHOULD be zero (no
	FFFF:FFFF:FFFF:FFFF:FFFFF:FFFF:FFFF:FFFF (IPv6).		bits set). The default Source and Group Hash Masks SHOULD both be
			all-bits-set masks. These default values are likely acceptable for
			most deployments, and simplify configuration.
			The DRLB-List Hello Option contains a list of GDR Candidates. The
			first one listed has ordinal number 0, the second listed ordinal
			number 1, and the last one has ordinal number N - 1 if there are N
			candidates listed. The hash value computed will be the ordinal
			number of the GDR Candidate that is acting as GDR.
	o If the group is in ASM mode and the RP Hash Mask announced by the		o If the group is in ASM mode and the RP Hash Mask announced by the
	PIM DR is not 0, calculate the value of hashvalue_RP [Section 4.3]		PIM DR is not zero (at least one bit is set), calculate the value
	to determine GDR.		of hashvalue_RP [Section 5.2] to determine the GDR.
	o If the group is in ASM mode and the RP Hash Mask announced by the		o If the group is in ASM mode and the RP Hash Mask announced by the
	PIM DR is 0, obtain the value of hashvalue_Group [Section 4.3 ] to		PIM DR is zero (no bits are set), obtain the value of
	determine GDR.		hashvalue_Group [Section 5.2] to determine the GDR.
	o If the group is in SSM mode, use hashvalue_SG [Section 4.3] to		o If the group is in SSM mode, use hashvalue_SG [Section 5.2] to
	determine GDR.		determine the GDR.
	A simple Modulo hash algorithm is defined in this document. However,		A simple Modulo hash algorithm is defined in this document. However,
	to allow another hash algorithms to be used, a 1-octet "Hash		to allow another hash algorithms to be used, a 1-octet "Hash
	Algorithm" field is included in DRLBC Hello Option to specify the		Algorithm" field is included in the DRLB-Cap Hello Option to specify
	hash algorithm used by a last hop router.		the hash algorithm used by the router.
	If different hash algorithms are advertised among last hop routers,		If different hash algorithms are advertised among the routers on a
	only last hop routers running the same hash algorithm as the DR (and		LAN, only the outers advertising the same hash algorithm as the DR
	having the same DR priority as the DR) are eligible for GDR election.		(as well as having the same DR priority as the DR) are eligible for
			GDR election.
	4.3. Modulo Hash Algorithm		5.2. Modulo Hash Algorithm
	The Modulo hash algorithm is discussed here with a detailed		As part of computing the hash, the notation LSZC(hash_mask) is used
	description on hashvalue_RP. The same algorithm is described in		to denote the number of zeroes counted from the least significant bit
	brief for hashvalue_Group using the group address instead of the RP		of a Hash Mask hash_mask. As an example, LSZC(255.255.128) is 7 and
	address for an ASM group with zero RP_hashmask, and also with		also LSZC(FFFF:8000::) is 111. If all bits are set, LSZC will be 0.
	hashvalue_SG for a the source address of an (S,G), instead of the RP		If the mask is zero, then LSZC will be 32 for IPv4, and 128 for IPv6.
	address,
	o For ASM groups, with a non-zero RP_Hash Mask, hash value is		The number of GDR Candidates is denoted as GDRC.
	calculated as:
	hashvalue_RP = (((RP_address & RP_hashmask) >> N) & 0xFFFF) % M		The idea behind the Modulo hash algorithm is in simple terms that the
			corresponding mask is applied to a value, then the result is shifted
			right LSZC(mask) bits so that the least significant bits that were
			masked out are not considered. Then this result is masked by 0xFFFF,
			keeping only the last 32 bits of the result (this only makes a
			difference for IPv6). Finally, the hash value is this result modulo
			the number of GDR Candidates (GDRC).
	RP_address is the address of the RP defined for the group. N		The Modulo hash algorithm for computing the values hashvalue_RP,
	is the number of zeroes, counted from the least significant bit		hashvalue_Group and hashvalue_SG is defined as follows.
	of the RP_hashmask. M is the number of GDR Candidates.
	For example, Router X with IPv4 address 203.0.113.1 receives a		hashvalue_RP is calculated as:
	DRLBGDR Hello Option from the DR, which announces RP Hash Mask
	0.0.255.0 and a list of GDR Candidates, sorted by IP addresses
	from high to low: 203.0.113.3, 203.0.113.2 and 203.0.113.1.
	The ordinal number assigned to those addresses would be:
	0 for 203.0.113.3; 1 for 203.0.113.2; 2 for 203.0.113.1 (Router		(((RP_address & RP_mask) >> LSZC(RP_mask)) & 0xFFFF) % GDRC
	X)
	Assume there are 2 RPs: RP1 192.0.2.1 for Group1 and RP2		RP_address is the address of the RP defined for the group and
	198.51.100.2 for Group2. Following the modulo hash algorithm:		RP_mask is the RP Hash Mask.
	N is 8 for 0.0.255.0, and M is 3 for the total number of GDR		hashvalue_Group is calculated as:
	Candidates. The hashvalue_RP for RP1 192.0.2.1 is:
	(((192.0.2.1 & 0.0.255.0) >> 8) & 0xFFFF % 3) = 2 % 3 = 2		(((Group_address & Group_mask) >> LSZC(Group_mask)) & 0xFFFF) %
			GDRC
	matches the ordinal number assigned to Router X. Router X will		Group_address is the group address and Group_mask is the Group
	be the GDR for Group1, which uses 192.0.2.1 as the RP.		Hash Mask.
	The hashvalue_RP for RP2 198.51.100.2 is:		hashvalue_SG is calculated as:
	(((198.51.100.2 & 0.0.255.0) >> 8) & 0xFFFF % 3) = 100 % 3 = 1		((((Source_address & Source_mask) >> LSZC(Source_mask)) & 0xFFFF)
	which is different from Router X's ordinal number(2) hence,		^ (((Group_address & Group_mask) >> LSZC(Group_mask)) & 0xFFFF)) %
	Router X will not be GDR for Group2.		GDRC
	o If RP_hashmask is 0, a hash value for an ASM group is calculated		Group_address is the group address and Group_mask is the Group
	using the Group Hash Mask:		Hash Mask.
	hashvalue_Group = (((Group_address & Group_hashmask) >> N) &		5.2.1. Modulo Hash Algorithm Example
	0xFFFF) % M
	Compare hashvalue_Group with Ordinal number assigned to Router		To help illustrate the algorithm, consider this example. Router X
	X, to decide if Router X is the GDR.		with IPv4 address 203.0.113.1 receives a DRLB-List Hello Option from
			the DR, which announces RP Hash Mask 0.0.255.0 and a list of GDR
			Candidates, sorted by IP addresses from high to low: 203.0.113.3,
			203.0.113.2 and 203.0.113.1. The ordinal number assigned to those
			addresses would be:
	o For SSM groups, a hash value is calculated using both the Source		0 for 203.0.113.3; 1 for 203.0.113.2; 2 for 203.0.113.1 (Router X)
	and Group Hash Mask:
	hashvalue_SG = ((((Source_address & Source_hashmask) >> N_S) &		Assume there are 2 RPs: RP1 192.0.2.1 for Group1 and RP2 198.51.100.2
	0xFFFF) ^ (((Group_address & Group_hashmask) >> N_G) & 0xFFFF))		for Group2. Following the modulo hash algorithm:
	% M
	4.3.1. Limitations		LSZC(0.0.255.0) is 8 and GDRC is 3. The hashvalue_RP for Group1 with
			RP RP1 is:
			(((192.0.2.1 & 0.0.255.0) >> 8) & 0xFFFF % 3) = 2 % 3 = 2
			which matches the ordinal number assigned to Router X. Router X will
			be the GDR for Group1.
			The hashvalue_RP for Group2 with RP RP2 is:
			(((198.51.100.2 & 0.0.255.0) >> 8) & 0xFFFF % 3) = 100 % 3 = 1
			which is different from the ordinal number of router X (2). Hence,
			Router X will not be GDR for Group2.
			5.2.2. Limitations
	The Modulo Hash Algorithm has poor failover characteristics when a		The Modulo Hash Algorithm has poor failover characteristics when a
	shared LAN has more than two GDRs. In the case of more than two GDRs		shared LAN has more than two GDRs. In the case of more than two GDRs
	on a LAN, when one GDR fails, all of the groups may be reassigned to		on a LAN, when one GDR fails, all of the groups may be reassigned to
	a new GDR, even if they were not assigned to the failed GDR.		a different GDR, even if they were not assigned to the failed GDR.
	However, many deployments use only two routers on a shared LAN for		However, many deployments use only two routers on a shared LAN for
	redundancy purposes. Future work may define new hash algorithms		redundancy purposes. Future work may define new hash algorithms
	where only groups assigned to the failed GDR get reassigned.		where only groups assigned to the failed GDR get reassigned.
	4.4. PIM Hello Options		5.3. PIM Hello Options
	When a last hop PIM router sends a PIM Hello for an interface with
	this specification enabled, it includes a new option, called "Load
	Balancing Capability (DRLBC)".
	Besides this DRLBC Hello Option, the elected PIM DR also includes a
	new "DR Load Balancing GDR (DRLBGDR) Hello Option". The DRLBGDR
	Hello Option consists of three Hash Masks as defined above and also a
	sorted list of GDR Candidate addresses on the last hop LAN.
	The elected PIM DR uses DRLBC Hello Option advertised by all routers		When a PIM router sends a PIM Hello on an interface with this
	on the last hop LAN to compose the DRLBGDR Option. The GDR		specification enabled, it includes a new option, called "Load
	Candidates use the DRLBGDR Hello Option advertised by the PIM DR to		Balancing Capability (DRLB-Cap)".
	calculate the hash value.
	5. Hello Option Formats		Besides this DRLB-Cap Hello Option, the elected PIM DR also includes
			a new "DR Load Balancing List (DRLB-List) Hello Option". The DRLB-
			List Hello Option consists of three Hash Masks as defined above and
			also a sorted list of GDR Candidate addresses on the LAN.
	5.1. PIM DR Load Balancing Capability (DRLBC) Hello Option		5.3.1. PIM DR Load Balancing Capability (DRLB-Cap) Hello Option
	0 1 2 3		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		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
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+		+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| Type = TBD | Length = 4 |		| Type = 34 | Length = 4 |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+		+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| Reserved |Hash Algorithm |		| Reserved |Hash Algorithm |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+		+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	Figure 3: Capability Hello Option		Figure 3: PIM DR Load Balancing Capability Hello Option
	Type: TBD		Type: 34
	Length: 4		Length: 4
	Hash Algorithm: 0 for Modulo		Reserved: Transmitted as zero, ignored on receipt.
	This DRLBC Hello Option MUST be advertised by last hop routers on		Hash Algorithm: Hash algorithm type. 0 for the Modulo algorithm
	interfaces with this specification enabled.		defined in this document.
	5.2. PIM DR Load Balancing GDR (DRLBGDR) Hello Option		This DRLB-Cap Hello Option MUST be advertised by routers on all
			interfaces where DR Load Balancing is enabled.
			5.3.2. PIM DR Load Balancing List (DRLB-List) Hello Option
	0 1 2 3		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		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
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+		+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| Type = TBD | Length |		| Type = 35 | Length |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+		+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| Group Mask |		| Group Mask |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+		+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| Source Mask |		| Source Mask |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+		+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| RP Mask |		| RP Mask |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+		+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| GDR Candidate Address(es) |		| GDR Candidate Address(es) |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+		+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	Figure 4: GDR Hello Option		Figure 4: PIM DR Load Balancing List Hello Option
	Type: TBD		Type: 35
	Length: (3 + n) x (4 or 16) where n is the number of GDR		Length: (3 + n) x (4 or 16), where n is the number of GDR
	candidates.		candidates.
	Group Mask (32/128 bits): Mask		Group Mask (32/128 bits): Mask applied to group addresses as part
			of hash computation.
	Source Mask (32/128 bits): Mask		Source Mask (32/128 bits): Mask applied to source addresses as
			part of hash computation.
	RP Mask (32/128 bits): Mask		RP Mask (32/128 bits): Mask applied to RP addresses as part of
			hash computation.
	All masks MUST be in the same address family as the Hello IP		All masks MUST have the same number of bits as the IP source
	header.		address in the PIM Hello IP header.
	GDR Address (32/128 bits): Address(es) of GDR Candidate(s)		GDR Address (32/128 bits): Address(es) of GDR Candidate(s)
	All addresses must be in the same address family as the Hello		All addresses MUST be in the same address family as the PIM
	IP header. The addresses are sorted in descending order. The		Hello IP header. It is RECOMMENDED that the addresses are
	order is converted to the ordinal number associated with each		sorted in descending order.
	GDR candidate in hash value calculation. For example, if
	addresses advertised are R3, R2, R1, the ordinal number
	assigned to R3 is 0, to R2 is 1 and to R1 is 2.
	If the "Interface ID" option, as specified in [RFC6395], is		If the "Interface ID" option, as specified in [RFC6395], is
	present in a GDR Candidate's PIM Hello message, and the "Router		present in a GDR Candidate's PIM Hello message, and the "Router
	ID" portion is non-zero:		ID" portion is non-zero:
	+ For IPv4, the "GDR Candidate Address" will be set directly		+ For IPv4, the "GDR Candidate Address" will be set directly
	to the "Router ID".		to the "Router ID".
	+ For IPv6, the "GDR Candidate Address" will be set to the		+ For IPv6, the "GDR Candidate Address" will be 96 bits of
	IPv4-IPv6 translated address of the "Router ID", as		zeroes followed by the 32 bit Router ID.
	described in [RFC4291], that is the "Router-ID" is appended
	to the prefix of 96 bits of zeroes.
	If the "Interface ID" option is not present in a GDR
	Candidate's PIM Hello message, or if the "Interface ID" option
	is present but the "Router ID" field is zero, the "GDR
	Candidate Address" will be the IPv4 or IPv6 source address of
	the PIM Hello message.
	This DRLBGDR Hello Option MUST only be advertised by the		If the "Interface ID" option is not present in a GDR Candidate'
	elected PIM DR.		PIM Hello message, or if the "Interface ID" option is present
			but the "Router ID" field is zero, the "GDR Candidate Address"
			will be the IPv4 or IPv6 source address of the PIM Hello
			message.
	6. Protocol Specification		This DRLB-List Hello Option MUST only be advertised by the
			elected PIM DR. It MUST be ignored if received from a non-DR.
	6.1. PIM DR Operation		5.4. PIM DR Operation
	The DR election process is still the same as defined in [RFC7761]. A		The DR election process is still the same as defined in [RFC7761]. A
	DR that has this specification enabled on an interface advertises the		DR that has this specification enabled on an interface advertises the
	new DRLBGDR Hello Option, which contains mask values from user		new DRLB-List Hello Option, which contains mask values from user
	configuration, followed by a sorted list of GDR Candidate Addresses,		configuration (or default values), followed by a list of GDR
	from the highest value to the lowest value. Moreover, same as non-DR		Candidate Addresses. It is RECOMMENDED that the list is sorted, from
	routers, the DR also advertises DRLBC Hello Option to indicate its		the highest value to the lowest value. The reason for sorting the
			list is to make the behavior deterministic, regardless of the order
			the DR learns of new candidates. Note that same as non-DR routers,
			the DR also advertises DRLB-Cap Hello Option to indicate its
	capability of supporting this specification and the type of its GDR		capability of supporting this specification and the type of its GDR
	election hash algorithm.		election hash algorithm.
	If a PIM DR receives a PIM Hello with the DRLBGDR Option, the PIM DR		If a PIM DR receives a neighbor DRLB-Cap Hello Option, which contains
	SHOULD ignore the TLV.
	If a PIM DR receives a neighbor DRLBC Hello Option, which contains
	the same hash algorithm as the DR, and the neighbor has the same DR		the same hash algorithm as the DR, and the neighbor has the same DR
	priority as the DR, PIM DR SHOULD consider the neighbor as a GDR		priority as the DR, PIM DR SHOULD consider the neighbor as a GDR
	Candidate and insert the GDR Candidate's Address into the sorted list		Candidate and insert the GDR Candidate' Address into the list of the
	of the DRLBGDR Option. However, the DR MAY have policies limiting		DRLB-List Option. However, the DR may have policies limiting which
	which GDR Candidates, or the number of GDR Candidates to include.		GDR Candidates, or the number of GDR Candidates to include. The DR
			would normally include itself in the list of GDR Candidates.
	6.2. PIM GDR Candidate Operation		If a PIM neighbor included in the list expires, stops announcing the
			DRLB-Cap Hello Option, changes DR priority, changes hash algorithm or
			otherwise becomes ineligibile as a candidate, the DR should
			immediately send a triggered hello with a new list in the DRLB-List
			option, excluding the neighbor.
			If a new router becomes eligible as a candidate, there is no urgency
			in sending out an updated list. An updated list SHOULD be included
			in the next hello.
			5.5. PIM GDR Candidate Operation
	When an IGMP/MLD report is received, without this specification, only		When an IGMP/MLD report is received, without this specification, only
	the PIM DR will handle the join and potentially run into the issues		the PIM DR will handle the join and potentially run into the issues
	described earlier. Using this specification, a hash algorithm is		described earlier. Using this specification, a hash algorithm is
	used by the GDR Candidates to determine which router is going to be		used by the GDR Candidates to determine which router is going to be
	responsible for building forwarding trees on behalf of the host.		responsible for building forwarding trees on behalf of the host.
	If this specification is enabled on an interface, the router MUST		If this specification is enabled on an interface, the router MUST
	include the DRLBC Hello Option in its PIM Hello on the interface.		include the DRLB-Cap Hello Option in all PIM Hello messages sent on
	Note that the presence of the DRLBC Option in PIM Hello does not		that interface. Note that the presence of the DRLB-Cap Option in PIM
	guarantee that this router would be considered as a GDR candidate.		Hello does not guarantee that this router would be considered as a
	Once DR election is done, the DRLBGDR Hello Option would be received		GDR candidate. Once DR election is done, the DRLB-List Hello Option
	from the current PIM DR on the link which would contain a list of		would be received from the current PIM DR on the link which would
	GDRs selected by the PIM DR.		contain a list of GDRs Candidates selected by the PIM DR.
	A router only acts as a GDR candidate if it is included in the GDR
	list of the DRLBGDR Hello Option.
	A GDR Candidate may receive a DRLBGDR Hello Option from the PIM DR
	with different Hash Masks from those the candidate was configured
	with. The GDR Candidate MUST use the Hash Masks advertised by the
	PIM DR to calculate the hash value.
	A GDR Candidate MUST ignore the DRLBGDR Hello Option if it is
	received from a PIM router which is not the DR.
	If the PIM DR does not support this specification, GDR election will		A router only acts as a GDR Candidate if it is included in the GDR
	not take place, and only the PIM DR joins the multicast tree.		Candidate list of the DRLB-List Hello Option. See next section for
			details.
	6.2.1. Router Receives New DRLBGDR		5.6. DRLB-List Hello Option Processing
	The first time a router receives a DRLBGDR option from the PIM DR, it		This section discusses processing of the DRLB-List Hello Option. All
	MUST process the option and check if it is in the GDR list.		routers MUST ignore the DRLB-List Hello Option if it is received from
			a PIM router which is not the DR. The option MUST only be processed
			by routers that are announcing the DRLB-Cap Option. Also, the
			algorithm announced in the DRLB-Cap Option, MUST be the same as what
			was announced by the DR. All GDR Candidates MUST use the Hash Masks
			advertised in the Option, even if they differ from those the
			candidate was configured with.
	1. If a router is not listed as a GDR candidate in DRLBGDR, no		A router stores the latest option contents that was announced, if
	action is needed.		any, and deletes the previous contents. The router MUST also compare
			the new contents with any previous contents, and if there are any
			changes, continue processing as below. Note that if the option does
			not pass the above checks, the below processing MUST be done as if
			the option was not announced.
	2. If a router is listed as a GDR candidate in DRLBGDR, then it MUST		If the contents of the DRLB-List Option, the masks or the candidate
	process each of the groups, or source and group pairs if SSM, in		list, differs from the previously saved copy, it is received for the
	the IGMP/MLD reports. The masks are announced in the PIM Hello		first time, or it is no longer being received or accepted, the option
	by the DR in the DRLBGDR Hello Option. For each group in the		MUST be processed as below.
	reports that is in ASM mode, and each source and group pair if
	the group is in SSM mode, it (PIM Router) needs to run the hash
	algorithm (described in section 4.3) based on the announced
	Source, Group or RP masks to determine if it is the GDR for
	specified group, or source and group pair. If the hash result is
	to be the GDR for the multicast flow, it does build the multicast
	forwarding tree. If it is not the GDR for the multicast flow, no
	action is needed.
	6.2.2. Router Receives Updated DRLBGDR		1. If the router was not included in the previous GDR list, or there
			was no previous GDR list, but it is included in the new GDR list,
			the router MUST for each of the groups, or source and group pairs
			if the group is in SSM mode, with local receiver interest, run
			the hash algorithm to determine which of them it is the GDR for.
	If a router (GDR or non GDR) receives an unchanged DRLBGDR from the		If it is not the GDR for a group, or source and group pair if
	current PIM DR, no action is needed.		SSM, no processing is required.
	If a router (GDR or non GDR) receives a new or modified DRLBGDR from		If it is hashed as the GDR, it needs to build a multicast
	the current PIM DR, it requires processing as described below:		forwarding tree.
	1. If it was included in the previous GDR list, and still is		2. If the router was included in the previous GDR list, and still is
	included in the new GDR list: It needs to process each of the		included in the new GDR list: The router MUST for each of the
	groups, or source and group pairs if the group is in SSM mode,		groups, or source and group pairs if the group is in SSM mode,
	and run the hash algorithm to check if it is still the GDR for		with local receiver interest, run the hash algorithm to determine
	the given group, or source and group pair if SSM.		which of them it is the GDR for.
	If it was the GDR for a group, or source and group pair if		If it was the GDR for a group, or source and group pair if
	SSM, and the new hash result chose it as the GDR, then no		SSM, and the new hash result chose it as the GDR, then no
	processing is required.		processing is required.
	If it was the GDR for a group, or source and group pair if		If it was the GDR for a group, or source and group pair if
	SSM, earlier and now it is no longer the GDR, then it sets its		SSM, earlier and now it is no longer the GDR, then it sets the
	assert metric for the multicast flow to be		assert metric preference to maximum (0x7FFFFFFF) and the
	(PIM_ASSERT_INFINITY - 1), as explained in Section 6.3.		assert metric to one less than maximum (0xFFFFFFFE), as
			explained in [Section 5.7].
	If it was not the GDR for a group, or source and group pair if		If it was not the GDR for a group, or source and group pair if
	SSM, earlier, and the new hash does not make it GDR, then no		SSM, earlier, and the new hash does not make it GDR, then no
	processing is required.		processing is required.
	If it was not the GDR for an earlier group, or source and		If it was not the GDR for an earlier group, or source and
	group pair if SSM, and now becomes the GDR, it starts building		group pair if SSM, and now becomes the GDR, it starts building
	multicast forwarding tree for this flow.		multicast forwarding tree for this flow.
	2. If it was included in the previous GDR list, but is not included		3. If the router was included in the previous GDR list, but is not
	in the new GDR list: It needs to process each of the groups, or		included in the new GDR list, or there is no new GDR list: The
	source and group pairs if the group is in SSM mode.		router MUST for each of the groups, or source and group pairs if
			the group is in SSM mode, with local receiver interest do as
			follows.
	If it was the GDR for a group, or source and group pair if		If it was the GDR for a group, or source and group pair if
	SSM, it sets its assert metric for the multicast flow to be		SSM, it sets the assert metric preference to maximum
	(PIM_ASSERT_INFINITY - 1), as explained in Section 6.3.		(0x7FFFFFFF) and the assert metric to one less than maximum
			(0xFFFFFFFE), as explained in [Section 5.7].
	If it was not the GDR, then no processing is required.		If it was not the GDR, then no processing is required.
	3. If it was not included in the previous GDR list, but is included		5.7. PIM Assert Modification
	in the new GDR list, the router MUST run the hash algorithm for
	each of the groups, source and group pairs if SSM.
	If it is not the GDR for a group, or source and group pair if
	SSM, no processing is required.
	If it is hashed as the GDR, it needs to build a multicast
	forwarding tree.
	6.3. PIM Assert Modification
	It is possible that the identity of the GDR might change in the
	middle of an active flow. Examples when this could happen include:
	When a new PIM router comes up		GDR changes may occur due to configuration change, due to GDR
			candidates going down, and also new routers coming up and becoming
			GDR candidates. This may occur while flows are being forwarded. If
			the GDR for an active flow changes, there is likely to be some
			disruption, such as packet loss or duplicates. By using asserts,
			packet loss is minimized, while allowing a small amount of
			duplicates.
	When a GDR restarts		When a router stops acting as the GDR for a group, or source and
			group pair if SSM, it MUST set the assert metric preference to
			maximum (0x7FFFFFFF) and the assert metric to one less than maximum
			(0xFFFFFFFE). This was also mentioned in the previous section. That
			is, whenever it sends or receives an assert for the group, it must
			use these values as the metric preference and metric rather than the
			values provided by routing. This is similar to what is done for
			AssertCancel Messages in [RFC7761], except that the metric value here
			is one less.
	When the GDR changes, existing traffic might be disrupted.		The rest of this section is just for illustration purposes and not
	Duplicates or packet loss might be observed. To illustrate the case,		part of the protocol definition.
	consider the following scenario where there are two flows G1 and G2.
	R1 is the GDR for G1, and R2 is the GDR for G2. When R3 comes up
	online, it is possible that R3 becomes GDR for both G1 and G2, hence
	R3 starts to build the forwarding tree for G1 and G2. If R1 and R2
	stop forwarding before R3 completes the process, packet loss might
	occur. On the other hand, if R1 and R2 continue forwarding while R3
	is building the forwarding trees, duplicates might occur.
	This is not a typical deployment scenario but might still happen.		To illustrate the behavior when there is a GDR change, consider the
	Here we describe a mechanism to minimize the impact. We essentially		following scenario where there are two flows G1 and G2. R1 is the
	want to minimize packet loss. Therefore, we would allow a small		GDR for G1, and R2 is the GDR for G2. When R3 comes up, it is
	amount of duplicates and depend on PIM Assert to minimize the		possible that R3 becomes GDR for both G1 and G2, hence R3 starts to
	duplication.		build the forwarding tree for G1 and G2. If R1 and R2 stop
			forwarding before R3 completes the process, packet loss might occur.
			On the other hand, if R1 and R2 continue forwarding while R3 is
			building the forwarding trees, duplicates might occur.
	When the role of GDR changes as above, instead of immediately		When the role of GDR changes as above, instead of immediately
	stopping forwarding, R1 and R2 continue forwarding to G1 and G2		stopping forwarding, R1 and R2 continue forwarding to G1 and G2
	respectively, while, at the same time, R3 build forwarding trees for		respectively, while, at the same time, R3 build forwarding trees for
	G1 and G2. This will lead to PIM Asserts.		G1 and G2. This will lead to PIM Asserts.
	With the introduction of GDR, the following modification to the
	Assert packet MUST be done: if a router enables this specification on
	its downstream interface, but it is not a GDR (before network event
	it was GDR), it would adjust its Assert metric to
	(PIM_ASSERT_INFINITY - 1).
	Using the above example, for G1, assume R1 and R3 agree on the new		Using the above example, for G1, assume R1 and R3 agree on the new
	GDR, which is R3. R1 will set its Assert metric as		GDR, which is R3. With the new assert behavior, R1 sets its assert
	(PIM_ASSERT_INFINITY - 1). That will make R3, which has normal		metric to the near maximum value discussed above. That will make R3,
	metric in its Assert as the Assert winner.		which has normal metric in its Assert as the Assert winner.
	For G2, assume it takes a slightly longer time for R2 to find out		For G2, assume it takes a slightly longer time for R2 to find out
	that R3 is the new GDR and still considers itself being the GDR while		that R3 is the new GDR and still considers itself being the GDR while
	R3 already has assumed the role of GDR. Since both R2 and R3 think		R3 already has assumed the role of GDR. Since both R2 and R3 think
	they are GDRs, they further compare their metric and IP addresses.		they are GDRs, they further compare their metric and IP addresses.
	If R3 has the better routing metric, or the same metric but a better		If R3 has the better routing metric, or the same metric but a better
	tie-breaker, the result will be consistent during GDR selection. If		tie-breaker, the result will be consistent during GDR selection. If
	unfortunately, R2 has the better metric or the same metric but a		unfortunately, R2 has the better metric or the same metric but a
	better tie-breaker, R2 will become the Assert winner and continues to		better tie-breaker, R2 will become the Assert winner and continues to
	forward traffic. This will continue until:		forward traffic. Shortly after when R2 finds out that it is no
			longer the GDR, R2 will change to using the near maximum assert
	The next PIM Hello Option from DR selects R3 as the GDR. R3 will		metric. Next time R2 sends an assert message, it will lose the
	then build the forwarding tree and send an Assert.		assert and stop forwarding. As assert winner, R2 would send periodic
			assert messages per [RFC7761].
	The process continues until R2 agrees to the selection of R3 as the
	GDR, and sets its own Assert metric to (PIM_ASSERT_INFINITY - 1),
	which will make R3 the Assert winner. During the process, we will
	see intermittent duplication of traffic but packet loss will be
	minimized. In the unlikely case that R2 never relinquishes its role
	as GDR (while every other router thinks otherwise), the proposed
	mechanism also helps to keep the duplication to a minimum until
	manual intervention takes place to remedy the situation.
	7. Compatibility		5.8. Backward Compatibility
	In the case of a hybrid Ethernet shared LAN (where some PIM routers		In the case of a hybrid Ethernet shared LAN (where some PIM routers
	enable the specification defined in this document, and some do not)		enable the specification defined in this document, and some do not).
	o If a router which does not support this specification becomes the		o If a router which does not support this specification becomes the
	DR on the LAN, then it is the only router acting as a DR, and		DR on the LAN, then it is the only router acting as a DR, and
	there will be no load-balancing.		there will be no load-balancing.
	o If a router which does not support this specification becomes a		o If a router which does not support this specification becomes a
	non-DR on link, then it acts as non-DR defined in [RFC7761], and		non-DR on link, then it acts as non-DR defined in [RFC7761], and
	it will not take part in any load-balancing.		it will not take part in any load-balancing. Load-balancing may
			still happen.
	8. Manageability Considerations		6. Manageability Considerations
			An administrator needs to consider what the total bandwidth
			requirements are and find a set of routers that together has enough
			total capacity, while making sure that each of the router can handle
			its part, assuming that the traffic is distributed roughly equally
			among the routers. Ideally, one should also have enough bandwidth to
			handle the case where at least one router fails. Ideally all the
			routers should have reachability to the sources, and RPs if
			applicable, that is not via the LAN.
			Care must be taken when choosing what hash masks to configure. One
			would typically configure the same masks on all the routers, so that
			they are the same, regardless of which router is elected as DR. The
			default masks are likely suitable for most deployment. The RP Hash
			Mask must be configured (the default is no bits set) if one wishes to
			hash based on the RP address rather than the group address for ASM.
			The default masks will use the entire group addresses, and source
			addresses if SSM, as part of the hash. An administrator may set
			other masks that masks out part of the addresses to ensure that
			certain flows always get hashed to the same router. How this is
			achieved depends on how the group addresses are allocated.
	Only the routers announcing the same Hash Algorithm as the DR would		Only the routers announcing the same Hash Algorithm as the DR would
	be considered as GDR candidates. Network administrators need to make		be considered as GDR candidates. Network administrators need to make
	sure that the desired set of routers announce the same algorithm.		sure that the desired set of routers announce the same algorithm.
	Migration between different algorithms is not considered in this		Migration between different algorithms is not considered in this
	document.		document.
	9. IANA Considerations		7. IANA Considerations
	IANA has temporarily assigned type 34 for the PIM DR Load Balancing		IANA has temporarily assigned type 34 for the PIM DR Load Balancing
	Capability (DRLBC) Hello Option, and type 35 for the PIM DR Load		Capability (DRLB-Cap) Hello Option, and type 35 for the PIM DR Load
	Balancing GDR (DRLBGDR) Hello Option in the PIM-Hello Options		Balancing List (DRLB-List) Hello Option in the PIM-Hello Options
	registry. IANA is requested to make these assignments permanent when		registry. IANA is requested to make these assignments permanent when
	this document is published as an RFC. The string TBD should be		this document is published as an RFC. Note that the option names
	replaced by the assigned values accordingly.		have changed slightly since the temporary assignments were made.
			Also, the length of option 34 is always 4, the registry currently
			says it is variable.
	This document requests IANA to create a registry called "Designated		This document requests IANA to create a registry called "Designated
	Router Load Balancing Hash Algorithms" in the "Protocol Independent		Router Load Balancing Hash Algorithms" in the "Protocol Independent
	Multicast (PIM)" branch of the registry tree. The registry lists		Multicast (PIM)" branch of the registry tree. The registry lists
	hash algorithms for use by PIM Designated Router Load Balancing.		hash algorithms for use by PIM Designated Router Load Balancing.
	9.1. Initial registry		7.1. Initial registry
	The initial content of the registry should be as follows.		The initial content of the registry should be as follows.
	Type Name Reference		Type Name Reference
	------ ---------------------------------------- --------------------		------ ---------------------------------------- --------------------
	0 Modulo This document		0 Modulo This document
	1-255 Unassigned		1-255 Unassigned
	9.2. Assignment of new hash algorithms		7.2. Assignment of new hash algorithms
	Assignment of new hash algorithms is done according to the "IETF		Assignment of new hash algorithms is done according to the "IETF
	Review" model, see [RFC5226].		Review" model, see [RFC8126].
	10. Security Considerations		8. Security Considerations
	Security of the new DR Load Balancing PIM Hello Options is only		Security of the new DR Load Balancing PIM Hello Options is only
	guaranteed by the security of PIM Hello messages, so the security		guaranteed by the security of PIM Hello messages, so the security
	considerations for PIM Hello messages as described in PIM-SM		considerations for PIM Hello messages as described in PIM-SM
	[RFC7761] apply here.		[RFC7761] apply here.
	11. Acknowledgement		If the DR is subverted it could omit or add certain GDRs or announce
			an unsupported algorithm. If another router is subverted, it could
			be made DR and cause similar issues. While these issues are specific
			to this specification, they are not that different from existing
			attacks such as subverting a DR and lowering the DR priority, causing
			a different router to become the DR.
	The authors would like to thank Steve Simlo, Taki Millonis for		If a GDR is subverted, it could potentially be made to stop
	helping with the original idea, Bill Atwood, Bharat Joshi for review		forwarding all the traffic it is expected to forward. This is also
	comments, Toerless Eckert and Rishabh Parekh for helpful conversation		similar today to if a DR is subverted.
	on the document.
	Special thanks to Anish Kachinthaya, Anvitha Kachinthaya and Jake		9. Acknowledgement
	Holland for reviewing the document and providing comments.
	12. References		The authors would like to thank Steve Simlo and Taki Millonis for
			helping with the original idea; Alia Atlas, Bill Atwood, Jake
			Holland, Bharat Joshi, Anish Kachinthaya, Anvitha Kachinthaya and
			Alvaro Retana for reviews and comments; and Toerless Eckert and
			Rishabh Parekh for helpful conversation on the document.
	12.1. Normative References		10. References
			10.1. Normative References
	[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate		[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
	Requirement Levels", BCP 14, RFC 2119,		Requirement Levels", BCP 14, RFC 2119,
	DOI 10.17487/RFC2119, March 1997,		DOI 10.17487/RFC2119, March 1997,
	<https://www.rfc-editor.org/info/rfc2119>.		<https://www.rfc-editor.org/info/rfc2119>.
	[RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing
	Architecture", RFC 4291, DOI 10.17487/RFC4291, February
	2006, <https://www.rfc-editor.org/info/rfc4291>.
	[RFC6395] Gulrajani, S. and S. Venaas, "An Interface Identifier (ID)		[RFC6395] Gulrajani, S. and S. Venaas, "An Interface Identifier (ID)
	Hello Option for PIM", RFC 6395, DOI 10.17487/RFC6395,		Hello Option for PIM", RFC 6395, DOI 10.17487/RFC6395,
	October 2011, <https://www.rfc-editor.org/info/rfc6395>.		October 2011, <https://www.rfc-editor.org/info/rfc6395>.
	[RFC7761] Fenner, B., Handley, M., Holbrook, H., Kouvelas, I.,		[RFC7761] Fenner, B., Handley, M., Holbrook, H., Kouvelas, I.,
	Parekh, R., Zhang, Z., and L. Zheng, "Protocol Independent		Parekh, R., Zhang, Z., and L. Zheng, "Protocol Independent
	Multicast - Sparse Mode (PIM-SM): Protocol Specification		Multicast - Sparse Mode (PIM-SM): Protocol Specification
	(Revised)", STD 83, RFC 7761, DOI 10.17487/RFC7761, March		(Revised)", STD 83, RFC 7761, DOI 10.17487/RFC7761, March
	2016, <https://www.rfc-editor.org/info/rfc7761>.		2016, <https://www.rfc-editor.org/info/rfc7761>.
	12.2. Informative References		[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>.
	[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an		10.2. Informative References
	IANA Considerations Section in RFCs", RFC 5226,
	DOI 10.17487/RFC5226, May 2008,		[RFC3376] Cain, B., Deering, S., Kouvelas, I., Fenner, B., and A.
	<https://www.rfc-editor.org/info/rfc5226>.		Thyagarajan, "Internet Group Management Protocol, Version
			3", RFC 3376, DOI 10.17487/RFC3376, October 2002,
			<https://www.rfc-editor.org/info/rfc3376>.
			[RFC3810] Vida, R., Ed. and L. Costa, Ed., "Multicast Listener
			Discovery Version 2 (MLDv2) for IPv6", RFC 3810,
			DOI 10.17487/RFC3810, June 2004,
			<https://www.rfc-editor.org/info/rfc3810>.
			[RFC4541] Christensen, M., Kimball, K., and F. Solensky,
			"Considerations for Internet Group Management Protocol
			(IGMP) and Multicast Listener Discovery (MLD) Snooping
			Switches", RFC 4541, DOI 10.17487/RFC4541, May 2006,
			<https://www.rfc-editor.org/info/rfc4541>.
			[RFC4607] Holbrook, H. and B. Cain, "Source-Specific Multicast for
			IP", RFC 4607, DOI 10.17487/RFC4607, August 2006,
			<https://www.rfc-editor.org/info/rfc4607>.
			[RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for
			Writing an IANA Considerations Section in RFCs", BCP 26,
			RFC 8126, DOI 10.17487/RFC8126, June 2017,
			<https://www.rfc-editor.org/info/rfc8126>.
	Authors' Addresses		Authors' Addresses
	Yiqun Cai		Yiqun Cai
	Alibaba Group		Alibaba Group
	Email: yiqun.cai@alibaba-inc.com		Email: yiqun.cai@alibaba-inc.com
	Heidi Ou		Heidi Ou
	Alibaba Group		Alibaba Group
			Email: heidi.ou@alibaba-inc.com
	Sri Vallepalli		Sri Vallepalli
	Cisco Systems, Inc.		Cisco Systems, Inc.
	3625 Cisco Way		3625 Cisco Way
	San Jose CA 95134		San Jose CA 95134
	USA		USA
	Email: svallepa@cisco.com		Email: svallepa@cisco.com
	Mankamana Mishra		Mankamana Mishra
	Cisco Systems, Inc.		Cisco Systems, Inc.
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