One document matched: draft-ietf-pim-source-discovery-bsr-03.xml
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE rfc SYSTEM "rfc2629.dtd" [
]>
<rfc category="exp" ipr="trust200902" docName="draft-ietf-pim-source-discovery-bsr-03">
<?rfc toc="yes"?>
<?rfc compact="yes"?>
<?rfc subcompact="no"?>
<?rfc symrefs="yes"?>
<front>
<title abbrev="PIM flooding mechanism and source discovery">PIM flooding mechanism and source discovery</title>
<author initials="IJ." surname="Wijnands" fullname="IJsbrand Wijnands">
<organization>Cisco Systems, Inc.</organization>
<address>
<postal>
<street>De kleetlaan 6a</street>
<city>Diegem</city>
<code>1831</code>
<country>Belgium</country>
</postal>
<email>ice@cisco.com</email>
</address>
</author>
<author initials="S." surname="Venaas" fullname="Stig Venaas">
<organization>Cisco Systems, Inc.</organization>
<address>
<postal>
<street>Tasman Drive</street>
<city>San Jose</city>
<code>CA 95134</code>
<country>USA</country>
</postal>
<email>stig@cisco.com</email>
</address>
</author>
<author initials="M." surname="Brig" fullname="Michael Brig">
<organization>Aegis BMD Program Office</organization>
<address>
<postal>
<street>17211 Avenue D, Suite 160</street>
<city>Dahlgren</city>
<code>VA 22448-5148</code>
<country>USA</country>
</postal>
<email>michael.brig@mda.mil</email>
</address>
</author>
<author initials="A." surname="Jonasson" fullname="Anders Jonasson">
<organization>Swedish Defence Material Administration (FMV)</organization>
<address>
<postal>
<street>Lönnvägen 4</street>
<city>Växjö</city>
<code>35243</code>
<country>Sweden</country>
</postal>
<email>anders@jomac.se</email>
</address>
</author>
<date/>
<area>Routing</area>
<keyword>Multicast</keyword>
<abstract>
<t>
PIM Sparse-Mode uses a Rendezvous Point (RP) and shared trees to forward
multicast packets to Last Hop Routers (LHR). After the first packet is
received by the LHR, the source of the multicast stream is learned and the
Shortest Path Tree (SPT) can be joined. This draft proposes a solution to
support PIM Sparse Mode (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 mechanism is
defined in this document, and is modeled after the PIM Bootstrap Router
protocol. By removing the need for RPs and shared trees, the PIM-SM procedures
are simplified, improving router operations, management and making the
protocol more robust.
</t>
</abstract>
</front>
<middle>
<section title="Introduction">
<t>
PIM Sparse-Mode uses a Rendezvous Point (RP) and shared trees to forward
multicast packets to Last Hop Routers (LHR). After the first packet is
received by the LHR, the source of the multicast stream is learned and the
Shortest Path Tree (SPT) can be joined. This draft proposes a solution to
support PIM Sparse Mode (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 mechanism is
defined in this document, and is modeled after the Bootstrap Router
protocol <xref target='RFC5059'/>. By removing the need for RPs and shared
trees, the PIM-SM procedures are simplified, improving router operations,
management and making the protocol more robust.
</t>
<section title="Conventions used in this document">
<t>
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 <xref target='RFC2119'/>.
</t>
</section>
<section title="Terminology">
<t>
<list style='hanging'>
<t hangText='RP:'>
Rendezvous Point.
</t>
<t hangText='BSR:'>
Bootstrap Router.
</t>
<t hangText='RPF:'>
Reverse Path Forwarding.
</t>
<t hangText='SPT:'>
Shortest Path Tree.
</t>
<t hangText='FHR:'>
First Hop Router, directly connected to the source.
</t>
<t hangText='LHR:'>
Last Hop Router, directly connected to the receiver.
</t>
<t hangText='SG Mapping:'>
Multicast source to group mapping.
</t>
<t hangText='SG Message:'>
A PIM message containing SG Mappings.
</t>
</list>
</t>
</section>
</section>
<section title="Testing and deployment experiences">
<t>
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.
In this network with frequent topology changes and link or router
failures PIM-SM with RP election is 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 were no bandwidth left for prune message.
For the tests, all routers were configured to send PFP-SA 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.
</t>
</section>
<section title="A generic PIM flooding mechanism">
<t>
The Bootstrap Router protocol (BSR) <xref target='RFC5059'/> is a commonly used
protocol 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. We are
defining a more generic mechanism that can flood any kind of information
throughout a PIM domain. It is not necessarily a domain though, it depends
on the administrative boundaries being configured. The forwarding rules are
identical to BSR, except that there is no BSR election and that one can
control whether routers should forward messages of unsupported 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 protocol includes
an originator address which is used for RPF checking to restrict the flooding,
just like BSR. Just like BSR it is also sent hop by hop. Note that there
is no built in election mechanism as in BSR, so there can be multiple
originators. It is still possible to add such an election mechanism on a type
by type bases if this protocol is used in scenarios where this is desirable.
We include a type field, which can allow boundaries to be defined, and
election to take place, independently per type. We call this protocol the PIM
Flooding Protocol (PFP).
</t>
<section title="PFP message format">
<t>
<figure>
<artwork>
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) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PFP Type | Reserved |U|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type 1 | Length 1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Value 1 |
| . |
| . |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| . |
| . |
| Type n | Length n |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Value n |
| . |
| . |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
</artwork>
</figure>
</t>
<t>
<list style='hanging'>
<t hangText='PIM Version: '>
Reserved, Checksum Described in <xref target='RFC4601'/>.
</t>
<t hangText='Type: '>
PIM Message Type. Value (pending IANA) for a PFP message.
</t>
<t hangText='[N]o-Forward bit: '>
When set, this bit means that the PFP message is not to be forwarded.
</t>
<t hangText='Originator Address: '>
The address of the router that originated the message. This can be any
address assigned to this router, but MUST be routable in the domain to allow
successful forwarding (just like BSR address). The format for this address
is given in the Encoded-Unicast address in <xref target='RFC4601'/>.
</t>
<t hangText='PFP Type: '>
There may be different sub protocols or different uses for this generic
protocol. The PFP Type specifies which sub protocol it is used for.
</t>
<t hangText='[U]nknown-No-Forwarding bit: '>
Some sub protocols may require that each router do some processing of the
contents and not simply forwarding. This bit controls how a router should
treat an unknown PFP Type. When set, a router MUST NOT forward the message
when the PFP Type is unknown. When clear, a router MUST forward the message
when possible. If the PFP Type is known, then the specification of that type
will specify how to handle
the message, including whether it should be forwarded.
</t>
<t hangText='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. Note that the
Type space is shared between all PFP types. Not all
types make sense for all PFP types though.
</t>
<t hangText='Length 1..n: '>
The length of the the value field.
</t>
<t><vspace blankLines='1'/></t>
<t hangText='Value 1..n: '>
The value associated with the type and of the specified length.
</t>
</list>
</t>
</section>
<section title="Processing PFP messages">
<t>
A router that receives an PFP message must perform the initial checks
specified here. If it passes, the contents is processed according to the
PFP type if known. If the type is unknown it may still be forwarded.
</t>
<section title="Initial checks">
<t>
The initial checks performed are largely similar to what is
done for BSR messages. The message MUST be from a directly connected
neighbor for which we have active Hello state. It MUST have been
sent to the ALL-PIM-ROUTERS group, and unless No-Forward is set,
it MUST have been sent by the RPF neighbor towards the router
that originated the message; or, if it is a No-Forward BSM,
we must have restarted within 60 seconds.
</t>
</section>
<section title="Processing messages with known PFP type">
<t>
If the PFP type is known, as in supported by the implementation,
the processing and potential forwarding is done according to the
specification for that PFP type. If the PFP type specification
does not specify any particular forwarding rules, the message
is forwarded out of all interfaces with PIM neighbors (including
the interface it is received on).
</t>
</section>
<section title="Processing messages with unknown PFP type">
<t>
If the PFP type is unknown, the message MUST be dropped if the
Unknown-No-Forwarding bit is set. If the bit is not set, the
message is forwarded out of all interfaces with PIM neighbors
(including the interface it is received on).
</t>
</section>
</section>
</section>
<section title="Distributing Source to Group Mappings">
<t>
We want to provide information about active multicast sources throughout a
PIM domain by making use of the generic flooding mechanism defined in the
previous section. We request PFP Type 0 to be assigned for
this purpose. We call a message with PFP Type 0 an SG Message.
We also define a PFP TLV which we request to
be type 0.
How this TLV is used with PFP Type 0 is defined in the next section.
Other PFP Types may specify the use of this TLV for other purposes.
For PFP Type 0 the U-bit MUST NOT be set. This means that routers not
supporting PFP Type 0 would still forward the message.
</t>
<section title="Group Source Holdtime TLV">
<t>
<figure>
<artwork>
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 0 | 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) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
</artwork>
</figure>
</t>
<t>
<list style='hanging'>
<t hangText='Type: '>
This TLV has type 0.
</t>
<t hangText='Length: '>
The length of the value.
</t>
<t hangText='Group Address: '>
The group we are announcing sources for. The format
for this address is given in the Encoded-Group format in
<xref target='RFC4601'/>.
</t>
<t hangText='Src Count: '>
How many unicast encoded sources address encodings follow.
</t>
<t hangText='Src Holdtime: '>
The Holdtime (in seconds) for the corresponding source(s).
</t>
<t hangText='Src Address: '>
The source address for the corresponding group.
The format for these addresses is given in the Encoded-Unicast address in <xref target='RFC4601'/>.
</t>
</list>
</t>
</section>
<section title="Originating SG messages">
<t>
An SG Mesage, that is a PFP message of Type 0, may contain one or more
Group Source Holdtime TLVs. This is used to flood information about
active multicast sources. Each FHR that is directly connected to an active
multicast source originates SG BSR messages. 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
<xref target='RFC4601'/>. After it is decided that a register needs to be sent,
the SG is not registered via the PIM SM register procedures, but the SG mapping
is included in an SG 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 router originating the SG messages includes one of its own addresses in
the originator field. Note that this address must be routeable due to RPF
checking. The SG messages are periodically sent for as long as the
multicast source is active, similar to how PIM registers are periodically sent.
The default announcement period is 60 seconds, which means that as long as
the source is active, it is included in an SG message originated every 60
seconds. The holdtime for the source is by default 210 seconds. Other values
can be configured, but the holdtime must be larger than the announcement
period. It is RECOMMENDED to be 3.5 times the announcement period. Note that
as a special case a source MAY be announced with a holdtime of 0 to indicate
that the source is no longer active.
</t>
</section>
<section title="Processing SG messages">
<t>
A router that receives an SG message should parse the message and store the
SG mappings with a holdtimer started with the advertised holdtime for that
group. If there are directly connected receivers for that group this router
should send PIM (S,G) joins for all the SG mappings advertised in the message.
The SG mappings are kept alive for as long as the holdtimer for the source
is running. Once the holdtimer expires a PIM router SHOULD send a PIM (S,G)
prune to remove itself from the tree. Note that a holdtime of 0 has a
special meaning. It is to be treated as if the source just expired, causing
a prune to be sent and state to be removed. Source information MUST not
be removed due to it being omitted in a message. For instance, if there
are a large number of sources for a group, there may be multiple SG messages
for the same group, each message containing a different list of sources.
</t>
</section>
<section title="The first packets and bursty sources">
<t>
The PIM register procedure is designed to deliver Multicast packets to the RP
in the absence of a native SPT tree 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 tree 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 tree 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 really 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.
</t>
<t>
With the procedures proposed in this draft the packet(s) received by the FHR
will be dropped until the LHR has learned about the source and the SPT tree is
built. That means for bursty sources or applications sensitive for the delivery
of the first packet this proposal would not be very applicable. This proposal
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)
we recommend using PIM Bidir <xref target='RFC5015'/> or SSM
<xref target='RFC4607'/>. 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).
</t>
<t>
Another solution to address the problems described above is documented
in <xref target='I-D.ietf-magma-msnip'/>. This proposal allows for a host to
tell the FHR its willingness to act as Source for a certain Group before
sending the data packets. LHRs have time to join the SPT tree before the host
starts sending which would avoid packet loss. The SG mappings announced by
<xref target='I-D.ietf-magma-msnip'/> can be advertised directly in SG
messages, allowing a very nice integration of both proposals.
The life time of the SPT is not driven by the liveliness of Multicast data
packets (which is the case with PIM SM), but by the announcements driven via
<xref target='I-D.ietf-magma-msnip'/>. This will also prevent packet loss due
to bursty sources.
</t>
</section>
<section title="Resiliency to network partitioning">
<t>
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.
</t>
<t>
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. As soon as the network heals,
the SG Mappings are re-flooded into the other partition(s) and other receivers
can join to the newly learned sources.
</t>
</section>
</section>
<section title="Security Considerations">
<t>
The security considerations are mainly similar to what is documented in
<xref target='RFC5059'/>. It may be a concern that rogue devices can inject
packets that are flooded throughout a domain. PFP packets SHOULD only be
accepted from a PIM neighbor. Deployments may use mechanisms for
authenticating PIM neighbors.
</t>
</section> <!-- end of security Considerations -->
<section title="IANA considerations"> <!-- RFC 2434 -->
<t>
This document requires the assignment of a new PIM Protocol type for the
PIM Flooding Protocol (PFP). IANA is also requested to create a registry for
PFP Types with type 0 allocated to "Source-Group Message". IANA is also
requested to create a registry for PFP TLVs, with type 0 allocated to the
"Source Group Holdtime" TLV. The allocation procedures are yet to be
determined.
</t>
</section>
<section title="Acknowledgments">
<t>
The authors would like to thank Arjen Boers for contributing to the
initial idea and Yiqun Cai for his comments on the draft.
</t>
</section> <!-- end of acknowledgements -->
</middle>
<back>
<references title='Normative References'>
<?rfc include='reference.RFC.2119' ?>
<?rfc include='reference.RFC.4601' ?>
<?rfc include='reference.RFC.5059' ?>
</references>
<references title='Informative References'>
<?rfc include='reference.RFC.4607' ?>
<?rfc include='reference.RFC.5015' ?>
<?rfc include='reference.I-D.ietf-magma-msnip' ?>
</references>
</back>
</rfc>
| PAFTECH AB 2003-2026 | 2026-04-22 05:13:43 |