Internet DRAFT - draft-ietf-pim-source-discovery-bsr
draft-ietf-pim-source-discovery-bsr
Network Working Group IJ. Wijnands
Internet-Draft S. Venaas
Intended status: Experimental Cisco Systems, Inc.
Expires: August 4, 2018 M. Brig
Aegis BMD Program Office
A. Jonasson
Swedish Defence Material Administration (FMV)
January 31, 2018
PIM Flooding Mechanism and Source Discovery
draft-ietf-pim-source-discovery-bsr-12
Abstract
PIM Sparse-Mode (PIM-SM) uses a Rendezvous Point (RP) and shared
trees to forward multicast packets from new sources. Once last hop
routers receive packets from a new source, they may join the Shortest
Path Tree for the source for optimal forwarding. This draft 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. This allows last hop routers to
learn about new sources without receiving initial data packets.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
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Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on August 4, 2018.
Copyright Notice
Copyright (c) 2018 IETF Trust and the persons identified as the
document authors. All rights reserved.
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This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(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
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Conventions Used in This Document . . . . . . . . . . . . 4
1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4
2. Testing and Deployment Experiences . . . . . . . . . . . . . 4
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 Source Group Mappings . . . . . . . . . . . . . 10
4.1. Group Source Holdtime TLV . . . . . . . . . . . . . . . . 10
4.2. Originating Group Source Holdtime TLVs . . . . . . . . . 11
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 . . . . . . . . . . . . . . . . . . . 14
6. Security Considerations . . . . . . . . . . . . . . . . . . . 15
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16
8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 16
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 16
9.1. Normative References . . . . . . . . . . . . . . . . . . 16
9.2. Informative References . . . . . . . . . . . . . . . . . 17
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 17
1. Introduction
PIM Sparse-Mode (PIM-SM) [RFC7761] uses a Rendezvous Point (RP) and
shared trees to forward multicast packets to Last Hop Routers (LHR).
After the first packet is received by a LHR, the source of the
multicast stream is learned and the Shortest Path Tree (SPT) can be
joined. This draft 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
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removing the need for RPs and shared trees, the PIM-SM procedures are
simplified, improving router operations, 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 PIM 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 and not as mature, Experimental vs Internet Standard, and
there are only a few implementations. 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
feasable to implement only a subset of PIM-SM to provide SSM support,
and in addition implement the mechanism in this draft 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 Bootstrap Router mechanism (BSR) [RFC5059], any
router can originate information, and it allows for flooding of any
kind of information. Each message contains one or more pieces of
information encoded as TLVs (type, length and value). This document
defines one TLV used for distributing information about active
multicast sources. Other documents may define additional TLVs.
Note that this document is experimental. 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 larger amount of data that needs to be processed and
stored. Distributing knowledge of active sources in this way is new,
and there are some concerns, mainly regarding potentially large
amounts of source states that need to be distributed. While there
has been some testing in the field, we need to learn more about the
forwarding efficiency, both the amount of processing per router, and
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, and it is not clear what parameters are useful for
different types of networks.
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1.1. Conventions Used in This Document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
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 mapping
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 like PIM-SM and PIM-DM were tested.
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 are no receivers, in
some cases such that there was no bandwidth left for prune messages.
For the PFM-SD prototype tests, all routers were configured to send
PFM-SD for 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 and there were no unnecessary forwarding of
packets. Ease of configuration was seen as a plus.
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3. A Generic PIM Flooding Mechanism
The Bootstrap Router mechanism (BSR) [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 which 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 (pending IANA) 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 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).
Length 1..n: The length of the the value field in octets.
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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 an interface
that is an outgoing boundary, including bidirectional boundary, for
all PFM messages. If an interface is an outgoing boundary for
certain TLVs, the message MUST NOT be sent out 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 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 PFM to change. Also, some information may need to
be sent periodically. These messages are called triggered and
periodic messages, respectively. Each TLV definition will need to
define when a triggered PFM message needs to be originated, and also
whether to send periodic messages, and how frequent.
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A router MUST NOT originate more than Max_PFM_Message_Rate messages
per minute. This document does not mandate how this should be
implemented, but 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
is 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 occuring 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. E.g., if
some information needs to be sent every 60 seconds and a triggered
PFM is about to be sent 20 seconds before the next periodic PFM was
scheduled, the triggered PFM might include the periodic information
and the next periodic PFM 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 which address to choose. When this
is not configured, an implementation MUST NOT use a link-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
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Generation Identifier, 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 need 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 ignores 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, but otherwise the message MUST be dropped
silently. If the checks pass, the contents is 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, the destination address MUST be ALL-PIM-ROUTERS. Also, the
interface MUST NOT be an incoming, nor bidirectional, administrative
boundary for PFM messages, see Section 3.2. If No-Forward is not
set, the message MUST be from the RPF neighbor of the originator
address. If No-Forward 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 pseudo-code the algorithm
is as follows:
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.
}
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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 messsage 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 a router
supports the type or not, 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 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.
4. Distributing Source Group Mappings
The generic flooding mechanism (PFM) defined in the previous section
can be used for distributing source group 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
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|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].
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 a FHR has decided
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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 an GSH TLV in a PFM message. Note, 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 sending of a PFM message
as soon as possible, while 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, sending of the message may
have to be delayed a small amount of time.
When a new PIM neighbor is detected, or an existing neighbor changes
Generation Identifier, an implementation MAY send a triggered PFM
message containing GSH TLVs for any Source Group mappings it has
learned by receiving PFM GSH TLVs as well as any active directly
connected sources. See Section 3.3 for further details.
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4.3. Processing GSH TLVs
A router that receives a PFM message containing GSH TLVs MUST parse
the GSH TLVs and store each of the GSH TLVs as SG mappings with a
holdtimer started with the advertised holdtime, unless the
implementation specifically does not support GSH TLVs, the router is
configured to ignore GSH TLVs in general, or to ignore GSH TLVs for
certain sources or groups. In particular, an administrator might
configure a router to not process GSH TLVs if the router is known to
never 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 for
instance be administrative policy limiting which sources and groups
to join. The SG mappings are kept alive for as long as the holdtimer
for the source is running. Once the holdtimer 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 state to be removed.
Source information MUST NOT be removed due to the source being
omitted in a message. For instance, if there is 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 a Shortest Path Tree (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 setup. This functionality is
important for applications where the initial packet(s) must be
received for the application to work correctly. Another reason would
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 cause no multicast packet to ever be
forwarded. A well behaved application should be able to deal with
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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. That means for bursty sources or applications
sensitive for the delivery of the first packet this solution would
not be very applicable. This solution is mostly useful for
applications that don't have 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 PIM Bidir [RFC5015] or SSM [RFC4607] is
recommended. The protocol operations are much simpler compared to
PIM SM, it will cause less churn in the network and 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 are not able to receive the traffic. Ideally
you would want to have a candidate RP in each partition, but you
never know in advance which routers will form a partitioned network.
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 partitioned will receive the SG
Mappings and join the source tree. Each partition works
independently of the other partition(s) 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
re-flooded into the other partition(s) and other receivers can join
to the newly learned sources.
5. Configurable Parameters
This document contains a number of configurable parameters. These
parameters are formally defined in Section 3.3 and Section 4.2, but
they are repeated here for ease of reference. These parameters all
have default values as noted below.
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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
When it comes to 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
which interfaces PIM is enabled, and 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, and 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 Shortest
Path Trees 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.
PIM-SM link-local messages can be authenticated using IPsec, see
[RFC7761] section 6.3 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
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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 requires the assignment of a new PIM message type for
the PIM Flooding Mechanism (PFM) with the name "PIM Flooding
Mechanism". IANA is also requested to create 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 should be:
Type Name Reference
--------------------------------------------------
0 Reserved [this document]
1 Source Group Holdtime [this document]
2-32767 Unassigned
8. 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 draft.
9. References
9.1. 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>.
9.2. 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>.
Authors' Addresses
IJsbrand Wijnands
Cisco Systems, Inc.
De kleetlaan 6a
Diegem 1831
Belgium
Email: ice@cisco.com
Stig Venaas
Cisco Systems, Inc.
Tasman Drive
San Jose CA 95134
USA
Email: stig@cisco.com
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Michael Brig
Aegis BMD Program Office
17211 Avenue D, Suite 160
Dahlgren VA 22448-5148
USA
Email: michael.brig@mda.mil
Anders Jonasson
Swedish Defence Material Administration (FMV)
Loennvaegen 4
Vaexjoe 35243
Sweden
Email: anders@jomac.se
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