Internet DRAFT - draft-wijnands-pim-source-discovery-bsr
draft-wijnands-pim-source-discovery-bsr
Network Working Group IJ. Wijnands
Internet-Draft S. Venaas
Intended status: Experimental Cisco Systems, Inc.
Expires: January 13, 2014 M. Brig
Aegis BMD Program Office
July 12, 2013
PIM flooding mechanism and source discovery
draft-wijnands-pim-source-discovery-bsr-03
Abstract
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.
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
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on January 13, 2014.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Conventions used in this document . . . . . . . . . . . . 3
1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
2. A generic PIM flooding mechanism . . . . . . . . . . . . . . 3
2.1. PFP message format . . . . . . . . . . . . . . . . . . . 4
3. Distributing Source to Group Mappings . . . . . . . . . . . . 5
3.1. Group Source Holdtime TLV . . . . . . . . . . . . . . . . 5
4. Originating SG messages . . . . . . . . . . . . . . . . . . . 6
5. Processing SG messages . . . . . . . . . . . . . . . . . . . 6
6. The first packets and bursty sources . . . . . . . . . . . . 7
7. Resiliency to network partitioning . . . . . . . . . . . . . 8
8. Security Considerations . . . . . . . . . . . . . . . . . . . 8
9. IANA considerations . . . . . . . . . . . . . . . . . . . . . 8
10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 8
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 9
11.1. Normative References . . . . . . . . . . . . . . . . . . 9
11.2. Informative References . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 9
1. Introduction
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 [RFC5059]. By
removing the need for RPs and shared trees, the PIM-SM procedures are
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simplified, improving router operations, management and making the
protocol more robust.
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.
SG Mapping: Multicast source to group mapping.
SG Message: A PIM message containing SG Mappings.
2. A generic PIM flooding mechanism
The Bootstrap Router protocol (BSR) [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 administrative boundaries
being configured. The forwarding rules are identical to BSR, except
that there is no BSR election. 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
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).
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2.1. PFP 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) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PFP Type | Reserved |U|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type 1 | Length 1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Value 1 |
| . |
| . |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| . |
| . |
| Type n | Length n |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Value n |
| . |
| . |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
PIM Version: Reserved, Checksum Described in [RFC4601].
Type: PIM Message Type. Value (pending IANA) for a PFP message.
[N]o-Forward bit: When set, this bit means that the PFP message is
not to be forwarded.
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 [RFC4601].
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.
[U]nknown-No-Forwarding bit: Some sub protocols may require each
router to 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
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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.
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. Not
all types make sense for all protocol types though.
Length 1..n: The length of the the value field.
Value 1..n: The value associated with the type and of the specified
length.
3. Distributing Source to Group Mappings
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.
3.1. 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 0 | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Group Address (Encoded-Group format) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Src Count | Src Holdtime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Src Address 1 (Encoded-Unicast format) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Src Address 2 (Encoded-Unicast format) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| . |
| . |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| Src Address m (Encoded-Unicast format) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type: This TLV has type 0.
Length: The length of the value.
Group Address: The group we are announcing sources for. The format
for this address is given in the Encoded-Group format in
[RFC4601].
Src Count: How many unicast encoded sources address encodings
follow.
Src Holdtime: The Holdtime (in seconds) for the corresponding
source(s).
Src Address: The source address for the corresponding group. The
format for these addresses is given in the Encoded-Unicast address
in [RFC4601].
4. Originating SG messages
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 [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.
5. Processing SG messages
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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 (S,G) prune must be sent to remove itself
from the tree.
6. The first packets and bursty sources
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 bridges the gap between how long it takes to build the SPT
tree to the FHR. If the packets would not be 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.
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 [RFC5015] or SSM
[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).
Another solution to address the problems described above is
documented in [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
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SPT tree before the host starts sending which would avoid packet
loss. The SG mappings announced by [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 [I-D.ietf-magma-msnip]. This
will also prevent packet loss due to bursty sources.
7. 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. As soon as the network
heals, the SG Mappings are re-flooded into the other partition(s) and
other receives can join to the newly learned sources.
8. Security Considerations
The security considerations are no different from what is documented
in [RFC5059].
9. IANA considerations
This document requires the assignment of a new PIM Protocol type for
the PIM Flooding Protocol (PFP). IANA also needs to create a
registry for PFP Types with type 0 allocated to "Source-Group
Message". IANA also needs 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.
10. Acknowledgments
The authors would like to thank Arjen Boers for contributing to the
initial idea and Yiqun Cai for his comments on the draft.
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11. References
11.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4601] Fenner, B., Handley, M., Holbrook, H., and I. Kouvelas,
"Protocol Independent Multicast - Sparse Mode (PIM-SM):
Protocol Specification (Revised)", RFC 4601, August 2006.
[RFC5059] Bhaskar, N., Gall, A., Lingard, J., and S. Venaas,
"Bootstrap Router (BSR) Mechanism for Protocol Independent
Multicast (PIM)", RFC 5059, January 2008.
11.2. Informative References
[RFC4607] Holbrook, H. and B. Cain, "Source-Specific Multicast for
IP", RFC 4607, August 2006.
[RFC5015] Handley, M., Kouvelas, I., Speakman, T., and L. Vicisano,
"Bidirectional Protocol Independent Multicast (BIDIR-
PIM)", RFC 5015, October 2007.
[I-D.ietf-magma-msnip]
Fenner, B., Haberman, B., Holbrook, H., Kouvelas, I., and
S. Venaas, "Multicast Source Notification of Interest
Protocol (MSNIP)", draft-ietf-magma-msnip-06 (work in
progress), March 2011.
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
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