Internet Engineering Task Force (IETF) IJ. Wijnands

Internet Engineering Task Force (IETF) IJ. Wijnands
Request for Comments: 8364 S. Venaas
Category: Experimental Cisco Systems, Inc.
ISSN: 2070-1721 M. Brig
Aegis BMD Program Office
A. Jonasson
FMV
March 2018

PIM Flooding Mechanism (PFM) and Source Discovery (SD)

Abstract

Protocol Independent Multicast - Sparse Mode (PIM-SM) uses a
Rendezvous Point (RP) and shared trees to forward multicast packets
from new sources. Once Last-Hop Routers (LHRs) receive packets from
a new source, they may join the Shortest Path Tree (SPT) for the
source for optimal forwarding. This document defines a new mechanism
that provides a way to support PIM-SM without the need for PIM
registers, RPs, or shared trees. Multicast source information is
flooded throughout the multicast domain using a new generic PIM
Flooding Mechanism (PFM). This allows LHRs to learn about new
sources without receiving initial data packets.

Status of This Memo

This document is not an Internet Standards Track specification; it is
published for examination, experimental implementation, and
evaluation.

This document defines an Experimental Protocol for the Internet
community. This document is a product of the Internet Engineering
Task Force (IETF). It represents the consensus of the IETF
community. It has received public review and has been approved for
publication by the Internet Engineering Steering Group (IESG). Not
all documents approved by the IESG are candidates for any level of
Internet Standard; see Section 2 of RFC 7841.

Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc8364.

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Copyright Notice

Copyright (c) 2018 IETF Trust and the persons identified as the
document authors. All rights reserved.

This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.

Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Conventions Used in This Document . . . . . . . . . . . . 4
1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4
2. Testing and Deployment Experiences . . . . . . . . . . . . . 5
3. A Generic PIM Flooding Mechanism . . . . . . . . . . . . . . 5
3.1. PFM Message Format . . . . . . . . . . . . . . . . . . . 6
3.2. Administrative Boundaries . . . . . . . . . . . . . . . . 7
3.3. Originating PFM Messages . . . . . . . . . . . . . . . . 7
3.4. Processing PFM Messages . . . . . . . . . . . . . . . . . 9
3.4.1. Initial Checks . . . . . . . . . . . . . . . . . . . 9
3.4.2. Processing and Forwarding of PFM Messages . . . . . . 10
4. Distributing SG Mappings . . . . . . . . . . . . . . . . . . 11
4.1. Group Source Holdtime TLV . . . . . . . . . . . . . . . . 11
4.2. Originating Group Source Holdtime TLVs . . . . . . . . . 12
4.3. Processing GSH TLVs . . . . . . . . . . . . . . . . . . . 13
4.4. The First Packets and Bursty Sources . . . . . . . . . . 13
4.5. Resiliency to Network Partitioning . . . . . . . . . . . 14
5. Configurable Parameters . . . . . . . . . . . . . . . . . . . 15
6. Security Considerations . . . . . . . . . . . . . . . . . . . 15
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 16
8.1. Normative References . . . . . . . . . . . . . . . . . . 16
8.2. Informative References . . . . . . . . . . . . . . . . . 17
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 18
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 18

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1. Introduction

Protocol Independent Multicast - Sparse Mode (PIM-SM) [RFC7761] uses
a Rendezvous Point (RP) and shared trees to forward multicast packets
to Last-Hop Routers (LHRs). After the first packet is received by an
LHR, the source of the multicast stream is learned and the Shortest
Path Tree (SPT) can be joined. This document defines a new mechanism
that provides a way to support PIM-SM without the need for PIM
registers, RPs, or shared trees. Multicast source information is
flooded throughout the multicast domain using a new generic PIM
flooding mechanism. By removing the need for RPs and shared trees,
the PIM-SM procedures are simplified, thus improving router
operations and management, and making the protocol more robust.
Also, the data packets are only sent on the SPTs, providing optimal
forwarding.

This mechanism has some similarities to Protocol Independent
Multicast - Dense Mode (PIM-DM) with its State-Refresh signaling
[RFC3973], except that there is no initial flooding of data packets
for new sources. It provides the traffic efficiency of PIM-SM, while
being as easy to deploy as PIM-DM. The downside is that it cannot
provide forwarding of initial packets from a new source, see
Section 4.4. PIM-DM is very different from PIM-SM; it's not as
mature, it is categorized as Experimental not an Internet Standard,
and there are only a few implementations of it. The solution in this
document consists of a lightweight source discovery mechanism on top
of the Source-Specific Multicast (SSM) [RFC4607] parts of PIM-SM. It
is feasible to implement only a subset of PIM-SM to provide SSM
support and, in addition, implement the mechanism in this document to
offer a source discovery mechanism for applications that do not
provide their own source discovery.

This document defines a generic flooding mechanism for distributing
information throughout a PIM domain. While the forwarding rules are
largely similar to the Bootstrap Router (BSR) mechanism [RFC5059],
any router can originate information; this allows for flooding of any
kind of information. Each message contains one or more pieces of
information encoded as TLVs. This document defines one TLV used for
distributing information about active multicast sources. Other
documents may define additional TLVs.

Note that this document is an Experimental RFC. While the flooding
mechanism is largely similar to BSR, there are some concerns about
scale as there can be multiple routers distributing information, and
potentially a larger amount of data that needs to be processed and
stored. Distributing knowledge of active sources in this way is new;
there are some concerns, mainly regarding potentially large amounts
of source states that need to be distributed. While there has been

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some testing in the field, we need to learn more about the forwarding
efficiency, both the amount of processing per router, propagation
delay, and the amount of state that can be distributed. In
particular, how many active sources one can support without consuming
too many resources. There are also parameters, see Section 5, that
can be tuned regarding how frequently information is distributed. It
is not clear what parameters are useful for different types of
networks.

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

BSR: Bootstrap Router

RPF: Reverse Path Forwarding

SPT: Shortest Path Tree

FHR: First-Hop Router, directly connected to the source

LHR: Last-Hop Router, directly connected to the receiver

PFM: PIM Flooding Mechanism

PFM-SD: PFM Source Discovery

SG Mapping: Multicast source group (SG) mapping

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2. Testing and Deployment Experiences

A prototype of this specification has been implemented, and there has
been some limited testing in the field. The prototype was tested in
a network with low-bandwidth radio links. The network has frequent
topology changes, including frequent link or router failures.
Previously existing mechanisms were tested (for example, PIM-SM and
PIM-DM).

With PIM-SM, the existing RP election mechanisms were found to be too
slow. With PIM-DM, issues were observed with new multicast sources
starving low-bandwidth links even when there were no receivers; in
some cases, so much so that there was no bandwidth left for prune
messages.

For the PFM-SD prototype tests, all routers were configured to send
PFM-SD for the directly connected source and to cache received
announcements. Applications such as SIP with multicast subscriber
discovery, multicast voice conferencing, position tracking, and NTP
were successfully tested. The tests went quite well. Packets were
rerouted as needed; there was no unnecessary forwarding of packets.
Ease of configuration was seen as a plus.

3. A Generic PIM Flooding Mechanism

The Bootstrap Router (BSR) mechanism [RFC5059] is a commonly used
mechanism for distributing dynamic Group-to-RP mappings in PIM. It
is responsible for flooding information about such mappings
throughout a PIM domain so that all routers in the domain can have
the same information. BSR, as defined, is only able to distribute
Group-to-RP mappings. This document defines a more generic mechanism
that can flood any kind of information. Administrative boundaries,
see Section 3.2, may be configured to limit to which parts of a
network the information is flooded.

The forwarding rules are identical to BSR, except that one can
control whether routers should forward unsupported data types. For
some types of information, it is quite useful that it can be
distributed without all routers having to support the particular
type, while there may also be types where it is necessary for every
single router to support it. The mechanism includes an originator
address that is used for RPF checking to restrict the flooding and
prevent loops, just like BSR. Like BSR, messages are forwarded hop-
by-hop; the messages are link-local, and each router will process and
resend the messages. Note that there is no equivalent to the BSR
election mechanism; there can be multiple originators. This
mechanism is named the PIM Flooding Mechanism (PFM).

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3.1. PFM Message Format

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|PIM Ver| Type |N| Reserved | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Originator Address (Encoded-Unicast format) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|T| Type 1 | Length 1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Value 1 |
| . |
| . |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| . |
| . |
|T| Type n | Length n |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Value n |
| . |
| . |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

PIM Version, Reserved, and Checksum: As specified in [RFC7761].

Type: PIM Message Type. Value 12 for a PFM message.

[N]o-Forward bit: When set, this bit means that the PFM message is
not to be forwarded. This bit is defined to prevent Bootstrap
message forwarding in [RFC5059].

Originator Address: The address of the router that originated the
message. This can be any address assigned to the originating
router, but it MUST be routable in the domain to allow successful
forwarding. The format for this address is given in the Encoded-
Unicast address in [RFC7761].

[T]ransitive bit: Each TLV in the message includes a bit called the
"Transitive" bit that controls whether the TLV is forwarded by
routers that do not support the given type. See Section 3.4.2.

Type 1..n: A message contains one or more TLVs, in this case n TLVs.
The Type specifies what kind of information is in the Value. The
Type range is from 0 to 32767 (15 bits).

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Length 1..n: The length of the Value field in octets.

Value 1..n: The value associated with the type and of the specified
length.

3.2. Administrative Boundaries

PFM messages are generally forwarded hop-by-hop to all PIM routers.
However, similar to BSR, one may configure administrative boundaries
to limit the information to certain domains or parts of the network.
Implementations MUST have a way of defining a set of interfaces on a
router as administrative boundaries for all PFM messages or,
optionally, for certain TLVs, allowing for different boundaries for
different TLVs. Usually, one wants boundaries to be bidirectional,
but an implementation MAY also provide unidirectional boundaries.
When forwarding a message, a router MUST NOT send it out on an
interface that is an outgoing boundary, including a bidirectional
boundary, for all PFM messages. If an interface is an outgoing
boundary for certain TLVs, the message MUST NOT be sent out on the
interface if it is a boundary for all the TLVs in the message.
Otherwise, the router MUST remove all the boundary TLVs from the
message and send the message with the remaining TLVs. Also, when
receiving a PFM message on an interface, the message MUST be
discarded if the interface is an incoming boundary, including a
bidirectional boundary, for all PFM messages. If the interface is an
incoming boundary for certain TLVs, the router MUST ignore all
boundary TLVs. If all the TLVs in the message are boundary TLVs,
then the message is effectively ignored. Note that when forwarding
an incoming message, the boundary is applied before forwarding. If
the message was discarded or all the TLVs were ignored, then no
message is forwarded. When a message is forwarded, it MUST NOT
contain any TLVs for which the incoming interface is an incoming or
bidirectional boundary.

3.3. Originating PFM Messages

A router originates a PFM message when it needs to distribute
information using a PFM message to other routers in the network.
When a message is originated depends on what information is
distributed. For instance, this document defines a TLV to distribute
information about active sources. When a router has a new active
source, a PFM message should be sent as soon as possible. Hence, a
PFM message should be sent every time there is a new active source.
However, the TLV also contains a holdtime and PFM messages need to be
sent periodically. Generally speaking, a PFM message would typically
be sent when there is a local state change, causing information to be
distributed with the PFM to change. Also, some information may need
to be sent periodically. These messages are called "triggered" and

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"periodic" messages, respectively. Each TLV definition will need to
define when a triggered PFM message needs to be originated, whether
or not to send periodic messages, and how frequently to send them.

A router MUST NOT originate more than Max_PFM_Message_Rate messages
per minute. This document does not mandate how this should be
implemented; some possible ways could be having a minimal time
between each message, counting the number of messages originated and
resetting the count every minute, or using a leaky bucket algorithm.
One benefit of using a leaky bucket algorithm is that it can handle
bursts better. The default value of Max_PFM_Message_Rate is 6. The
value MUST be configurable. Depending on the network, one may want
to use a larger value of Max_PFM_Message_Rate to favor propagation of
new information, but with a large number of routers and many updates,
the total number of messages might become too large and require too
much processing.

There MUST be a minimum of Min_PFM_Message_Gap milliseconds between
each originated message. The default value of Min_PFM_Message_Gap is
1000 (1 second). The value MUST be configurable.

Unless otherwise specified by the TLV definitions, there is no
relationship between different TLVs, and an implementation can choose
whether to combine TLVs in one message or across separate messages.
It is RECOMMENDED to combine multiple TLVs in one message to reduce
the number of messages, but it is also RECOMMENDED that the message
be small enough to avoid fragmentation at the IP layer. When a
triggered PFM message needs to be sent due to a state change, a
router MAY send a message containing only the information that
changed. If there are many changes occurring at about the same time,
it might be possible to combine multiple changes in one message. In
the case where periodic messages are also needed, an implementation
MAY include periodic PFM information in a triggered PFM. For
example, if some information needs to be sent every 60 seconds and a
triggered PFM message is about to be sent 20 seconds before the next
periodic PFM message was scheduled, the triggered PFM message might
include the periodic information and the next periodic PFM message
can then be scheduled 60 seconds after that rather than 20 seconds
later.

When a router originates a PFM message, it puts one of its own
addresses in the originator field. An implementation MUST allow an
administrator to configure which address is used. For a message to
be received by all routers in a domain, all the routers need to have
a route for this address due to the RPF-based forwarding. Hence, an
administrator needs to be careful about which address to choose.
When this is not configured, an implementation MUST NOT use a link-

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local address. It is RECOMMENDED to use an address of a virtual
interface such that the originator can remain unchanged and routable
independent of which physical interfaces or links may go down.

The No-Forward bit MUST NOT be set, except for the case when a router
receives a PIM Hello from a new neighbor or a PIM Hello with a new
Generation Identifier (GenID), defined in [RFC7761], is received from
an existing neighbor. In that case, an implementation MAY send PFM
messages containing relevant information so that the neighbor can
quickly get the correct state. The definition of the different PFM
message TLVs needs to specify what, if anything, needs to be sent in
this case. If such a PFM message is sent, the No-Forward bit MUST be
set, and the message must be sent within 60 seconds after the
neighbor state change. The processing rules for PFM messages will
ensure that any other neighbors on the same link ignore the message.
This behavior (and the choice of 60 seconds) is similar to what is
defined for the No-Forward bit in [RFC5059].

3.4. Processing PFM Messages

A router that receives a PFM message MUST perform the initial checks
specified here. If the checks fail, the message MUST be dropped. An
error MAY be logged; otherwise, the message MUST be dropped silently.
If the checks pass, the contents are processed according to the
processing rules of the included TLVs.

3.4.1. Initial Checks

In order to do further processing, a message MUST meet the following
requirements. The message MUST be from a directly connected PIM
neighbor and the destination address MUST be ALL-PIM-ROUTERS. Also,
the interface MUST NOT be an incoming, nor a bidirectional,
administrative boundary for PFM messages, see Section 3.2. If the
No-Forward bit is not set, the message MUST be from the RPF neighbor
of the originator address. If the No-Forward bit is set, this
system, the router doing these checks, MUST have enabled the PIM
protocol within the last 60 seconds. See Section 3.3 for details.
In pseudocode, the algorithm is as follows:

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if ((DirectlyConnected(PFM.src_ip_address) == FALSE) OR
(PFM.src_ip_address is not a PIM neighbor) OR
(PFM.dst_ip_address != ALL-PIM-ROUTERS) OR
(Incoming interface is admin boundary for PFM)) {
drop the message silently, optionally log error.
}
if (PFM.no_forward_bit == 0) {
if (PFM.src_ip_address !=
RPF_neighbor(PFM.originator_ip_address)) {
drop the message silently, optionally log error.
}
} else if (more than 60 seconds elapsed since PIM enabled)) {
drop the message silently, optionally log error.
}

Note that "src_ip_address" is the source address in the IP header of
the PFM message. "Originator" is the originator field inside the PFM
message and is the router that originated the message. When the
message is forwarded hop-by-hop, the originator address never
changes, while the source address will be an address belonging to the
router that last forwarded the message.

3.4.2. Processing and Forwarding of PFM Messages

When the message is received, the initial checks above must be
performed. If it passes the checks, then for each included TLV,
perform processing according to the specification for that TLV.

After processing, the message is forwarded. Some TLVs may be omitted
or modified in the forwarded message. This depends on administrative
boundaries (see Section 3.2), the type specification, and the setting
of the Transitive bit for the TLV. If a router supports the type,
then the TLV is forwarded with no changes unless otherwise specified
by the type specification. A router not supporting the given type
MUST include the TLV in the forwarded message if and only if the
Transitive bit is set. Whether or not a router supports the type,
the value of the Transitive bit MUST be preserved if the TLV is
included in the forwarded message. The message is forwarded out of
all interfaces with PIM neighbors (including the interface it was
received on). As specified in Section 3.2, if an interface is an
outgoing boundary for any TLVs, the message MUST NOT be sent out on
the interface if it is an outgoing boundary for all the TLVs in the
message. Otherwise, the router MUST remove any outgoing boundary
TLVs of the interface from the message and send the message out that
interface with the remaining TLVs.

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4. Distributing SG Mappings

The generic PFM defined in the previous section can be used for
distributing SG mappings about active multicast sources throughout a
PIM domain. A Group Source Holdtime (GSH) TLV is defined for this
purpose.

4.1. Group Source Holdtime TLV

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1| Type = 1 | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Group Address (Encoded-Group format) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Src Count | Src Holdtime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Src Address 1 (Encoded-Unicast format) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Src Address 2 (Encoded-Unicast format) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| . |
| . |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Src Address m (Encoded-Unicast format) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

1: The Transitive bit is set to 1. This means that this type will
be forwarded even if a router does not support it. See
Section 3.4.2.

Type: This TLV has type 1.

Length: The length of the value in octets.

Group Address: The group that sources are to be announced for. The
format for this address is given in the Encoded-Group format in
[RFC7761].

Src Count: The number of source addresses that are included.

Src Holdtime: The holdtime (in seconds) for the included source(s).

Src Address: The source address for the corresponding group. The
format for these addresses is given in the Encoded-Unicast address
in [RFC7761].

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4.2. Originating Group Source Holdtime TLVs

A PFM message MAY contain one or more Group Source Holdtime (GSH)
TLVs. This is used to flood information about active multicast
sources. Each FHR that is directly connected to an active multicast
source originates PFM messages containing GSH TLVs. How a multicast
router discovers the source of the multicast packet, and when it
considers itself the FHR, follows the same procedures as the
registering process described in [RFC7761]. When an FHR has decided
that a register needs to be sent per [RFC7761], the SG is not
registered via the PIM-SM register procedures, but the SG mapping is
included in a GSH TLV in a PFM message. Note that only the SG
mapping is distributed in the message: not the entire packet as would
have been done with a PIM register.

The PFM messages containing the GSH TLV are sent periodically for as
long as the multicast source is active, similar to how PIM registers
are sent periodically. This means that as long as the source is
active, it is included in a PFM message originated every
Group_Source_Holdtime_Period seconds, within the general PFM timing
requirements in Section 3.3. The default value of
Group_Source_Holdtime_Period is 60. The value MUST be configurable.
The holdtime for the source MUST be set to either zero or
Group_Source_Holdtime_Holdtime. The value of the
Group_Source_Holdtime_Holdtime parameter MUST be larger than
Group_Source_Holdtime_Period. It is RECOMMENDED to be 3.5 times the
Group_Source_Holdtime_Period. The default value is 210 (seconds).
The value MUST be configurable. A source MAY be announced with a
holdtime of zero to indicate that the source is no longer active.

If an implementation supports originating GSH TLVs with different
holdtimes for different sources, it can (if needed) send multiple
TLVs with the same group address. Due to the format, all the sources
in the same TLV have the same holdtime.

When a new source is detected, an implementation MAY send a PFM
message containing just that particular source. However, it MAY also
include information about other sources that were just detected,
sources that are scheduled for periodic announcement later, or other
types of information. See Section 3.3 for details. Note that when a
new source is detected, one should trigger the sending of a PFM
message as soon as possible; whereas if a source becomes inactive,
there is no reason to trigger a message. There is no urgency in
removing state for inactive sources. Note that the message timing
requirements in Section 3.3 apply. This means that one cannot always
send a triggered message immediately when a new source is detected.
In order to meet the timing requirements, the sending of the message
may have to be delayed for a small amount of time.

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When a new PIM neighbor is detected or an existing neighbor changes
GenID, an implementation MAY send a triggered PFM message containing
GSH TLVs for any SG mappings it has learned by receiving PFM GSH TLVs
as well as any active directly connected sources. See Section 3.3
for further details.

4.3. Processing GSH TLVs

A router that receives a PFM message containing GSH TLVs MUST parse
the GSH TLVs and store each of them as SG mappings with an Expiry
Timer started with the advertised holdtime, that is, unless the
implementation specifically does not support GSH TLVs, the router is
configured to ignore GSH TLVs in general, or it is configured to
ignore GSH TLVs for certain sources or groups. In particular, an
administrator might configure a router not to process GSH TLVs if the
router is known never to have any directly connected receivers.

For each group that has directly connected receivers, this router
SHOULD send PIM (S,G) joins for all the SG mappings advertised in the
message for the group. Generally, joins are sent, but there could
be, for instance, an administrative policy limiting which sources and
groups to join. The SG mappings are kept alive for as long as the
Expiry Timer for the source is running. Once the Expiry Timer
expires, a PIM router MAY send a PIM (S,G) prune to remove itself
from the tree. However, when this happens, there should be no more
packets sent by the source, so it may be desirable to allow the state
to time out rather than sending a prune.

Note that a holdtime of zero has a special meaning. It is to be
treated as if the source just expired, and then the state should be
removed. Source information MUST NOT be removed due to the source
being omitted in a message. For instance, if there are a large
number of sources for a group, there may be multiple PFM messages,
each message containing a different list of sources for the group.

4.4. The First Packets and Bursty Sources

The PIM register procedure is designed to deliver multicast packets
to the RP in the absence of an SPT from the RP to the source. The
register packets received on the RP are decapsulated and forwarded
down the shared tree to the LHRs. As soon as an SPT is built,
multicast packets would flow natively over the SPT to the RP or LHR
and the register process would stop. The PIM register process
ensures packet delivery until an SPT is in place reaching the FHR.
If the packets were not unicast encapsulated to the RP, they would be
dropped by the FHR until the SPT is set up. This functionality is
important for applications where the initial packet(s) must be
received for the application to work correctly. Another reason would

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be for bursty sources. If the application sends out a multicast
packet every 4 minutes (or longer), the SPT is torn down (typically
after 3:30 minutes of inactivity) before the next packet is forwarded
down the tree. This will prevent multicast packets from ever being
forwarded. A well-behaved application should be able to deal with
packet loss since IP is a best-effort-based packet delivery system.
But in reality, this is not always the case.

With the procedures defined in this document, the packet(s) received
by the FHR will be dropped until the LHR has learned about the source
and the SPT is built. For bursty sources or applications sensitive
for the delivery of the first packet, that means this solution would
not be very applicable. This solution is mostly useful for
applications that don't have a strong dependency on the initial
packet(s) and have a fairly constant data rate, like video
distribution, for example. For applications with strong dependency
on the initial packet(s), using BIDIR-PIM [RFC5015] or SSM [RFC4607]
is recommended. The protocol operations are much simpler compared to
PIM-SM; they will cause less churn in the network. Both guarantee
best-effort delivery for the initial packet(s).

4.5. Resiliency to Network Partitioning

In a PIM-SM deployment where the network becomes partitioned due to
link or node failure, it is possible that the RP becomes unreachable
to a certain part of the network. New sources that become active in
that partition will not be able to register to the RP and receivers
within that partition will not be able to receive the traffic.
Ideally, having a candidate RP in each partition is desirable, but
which routers will form a partitioned network is something unknown in
advance. In order to be fully resilient, each router in the network
may end up being a candidate RP. This would increase the operational
complexity of the network.

The solution described in this document does not suffer from that
problem. If a network becomes partitioned and new sources become
active, the receivers in that partition will receive the SG mappings
and join the source tree. Each partition works independently of the
other partitions and will continue to have access to sources within
that partition. Once the network has healed, the periodic flooding
of SG mappings ensures that they are reflooded into the other
partitions and then other receivers can join the newly learned
sources.

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5. Configurable Parameters

This document contains a number of configurable parameters. These
parameters are formally defined in Sections 3.3 and 4.2, but they are
repeated here for ease of reference. These parameters all have
default values as noted below.

Max_PFM_Message_Rate: The maximum number of PFM messages a router is
allowed to originate per minute; see Section 3.3 for details. The
default value is 6.

Min_PFM_Message_Gap: The minimum amount of time between each PFM
message originated by a router in milliseconds; see Section 3.3
for details. The default is 1000.

Group_Source_Holdtime_Period: The announcement period for Group
Source Holdtime TLVs in seconds; see Section 4.2 for details. The
default value is 60.

Group_Source_Holdtime_Holdtime: The holdtime for Group Source
Holdtime TLVs in seconds; see Section 4.2 for details. The
default value is 210.

6. Security Considerations

For general PIM message security, see [RFC7761]. PFM messages MUST
only be accepted from a PIM neighbor, but as discussed in [RFC7761],
any router can become a PIM neighbor by sending a Hello message. To
control from where to accept PFM packets, one can limit on which
interfaces PIM is enabled. Also, one can configure interfaces as
administrative boundaries for PFM messages, see Section 3.2. The
implications of forged PFM messages depend on which TLVs they
contain. Documents defining new TLVs will need to discuss the
security considerations for the specific TLVs. In general though,
the PFM messages are flooded within the network; by forging a large
number of PFM messages, one might stress all the routers in the
network.

If an attacker can forge PFM messages, then such messages may contain
arbitrary GSH TLVs. An issue here is that an attacker might send
such TLVs for a huge amount of sources, potentially causing every
router in the network to store huge amounts of source state. Also,
if there is receiver interest for the groups specified in the GSH
TLVs, routers with directly connected receivers will build SPTs for
the announced sources, even if the sources are not actually active.
Building such trees will consume additional resources on routers that
the trees pass through.

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PIM-SM link-local messages can be authenticated using IPsec, see
Section 6.3 of [RFC7761] and [RFC5796]. Since PFM messages are link-
local messages sent hop-by-hop, a link-local PFM message can be
authenticated using IPsec such that a router can verify that a
message was sent by a trusted neighbor and has not been modified.
However, to verify that a received message contains correct
information announced by the originator specified in the message, one
will have to trust every router on the path from the originator and
that each router has authenticated the received message.

7. IANA Considerations

This document registers a new PIM message type for the PIM Flooding
Mechanism (PFM) with the name "PIM Flooding Mechanism" in the "PIM
Message Types" registry with the value of 12.

IANA has also created a registry for PFM TLVs called "PIM Flooding
Mechanism Message Types". Assignments for the registry are to be
made according to the policy "IETF Review" as defined in [RFC8126].
The initial content of the registry is as follows:

Type Name Reference
---------------------------------------------
0 Reserved [RFC8364]
1 Source Group Holdtime [RFC8364]
2-32767 Unassigned

8. References

8.1. 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>.

[RFC5059] Bhaskar, N., Gall, A., Lingard, J., and S. Venaas,
"Bootstrap Router (BSR) Mechanism for Protocol Independent
Multicast (PIM)", RFC 5059, DOI 10.17487/RFC5059, January
2008, <https://www.rfc-editor.org/info/rfc5059>.

[RFC5796] Atwood, W., Islam, S., and M. Siami, "Authentication and
Confidentiality in Protocol Independent Multicast Sparse
Mode (PIM-SM) Link-Local Messages", RFC 5796,
DOI 10.17487/RFC5796, March 2010,
<https://www.rfc-editor.org/info/rfc5796>.

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[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>.

[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>.

[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>.

8.2. Informative References

[RFC3973] Adams, A., Nicholas, J., and W. Siadak, "Protocol
Independent Multicast - Dense Mode (PIM-DM): Protocol
Specification (Revised)", RFC 3973, DOI 10.17487/RFC3973,
January 2005, <https://www.rfc-editor.org/info/rfc3973>.

[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>.

[RFC5015] Handley, M., Kouvelas, I., Speakman, T., and L. Vicisano,
"Bidirectional Protocol Independent Multicast (BIDIR-
PIM)", RFC 5015, DOI 10.17487/RFC5015, October 2007,
<https://www.rfc-editor.org/info/rfc5015>.

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Acknowledgments

The authors would like to thank Arjen Boers for contributing to the
initial idea, and David Black, Stewart Bryant, Yiqun Cai,
Papadimitriou Dimitri, Toerless Eckert, Dino Farinacci, Alvaro
Retana, and Liang Xia for their very helpful comments on the
document.

Authors' Addresses

IJsbrand Wijnands
Cisco Systems, Inc.
De kleetlaan 6a
Diegem 1831
Belgium

Email: [email protected]

Stig Venaas
Cisco Systems, Inc.
Tasman Drive
San Jose CA 95134
United States of America

Email: [email protected]

Michael Brig
Aegis BMD Program Office
17211 Avenue D, Suite 160
Dahlgren VA 22448-5148
United States of America

Email: [email protected]

Anders Jonasson
Swedish Defence Material Administration (FMV)
Loennvaegen 4
Vaexjoe 35243
Sweden

Email: [email protected]

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