Network Working Group IJ. Wijnands
Internet-Draft S. Venaas
Intended status: Experimental Cisco Systems, Inc.
Expires: September 11, 2017 M. Brig
Aegis BMD Program Office
A. Jonasson
Swedish Defence Material Administration (FMV)
March 10, 2017
PIM flooding mechanism and source discovery
draft-ietf-pim-source-discovery-bsr-06
Abstract
PIM Sparse-Mode uses a Rendezvous Point 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 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 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.
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This Internet-Draft will expire on September 11, 2017.
Copyright Notice
Copyright (c) 2017 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Conventions used in this document . . . . . . . . . . . . 3
1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
2. Testing and deployment experiences . . . . . . . . . . . . . 3
3. A generic PIM flooding mechanism . . . . . . . . . . . . . . 4
3.1. PFM message format . . . . . . . . . . . . . . . . . . . 5
3.2. Processing PFM messages . . . . . . . . . . . . . . . . . 6
3.2.1. Initial checks . . . . . . . . . . . . . . . . . . . 6
3.2.2. Processing and forwarding of PFM messages . . . . . . 6
4. Distributing Source to Group Mappings . . . . . . . . . . . . 7
4.1. Group Source Holdtime TLV . . . . . . . . . . . . . . . . 7
4.2. Originating PFM messages . . . . . . . . . . . . . . . . 8
4.3. Processing GSH TLVs . . . . . . . . . . . . . . . . . . . 8
4.4. The first packets and bursty sources . . . . . . . . . . 9
4.5. Resiliency to network partitioning . . . . . . . . . . . 10
5. Security Considerations . . . . . . . . . . . . . . . . . . . 10
6. IANA considerations . . . . . . . . . . . . . . . . . . . . . 10
7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 10
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 11
8.1. Normative References . . . . . . . . . . . . . . . . . . 11
8.2. Informative References . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 11
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 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 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. By removing the
need for RPs and shared trees, the PIM-SM procedures are simplified,
improving router operations, management and making the protocol more
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robust. Also the data packets are only sent on the SPTs, providing
optimal forwarding.
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-SA: PFM Source Announcement
SG Mapping: Multicast source to 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 frequest 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 message.
For the PFM-SA prototype tests, all routers were configured to send
PFM-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
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rerouted as needed and there were no unnecessary forwarding of
packets. Ease of configuration was seen as a plus.
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. 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 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. Note that there is no equivalent
to the BSR election mechanism;, there can be multiple originators.
We call this mechanism 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) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type 1 | Length 1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Value 1 |
| . |
| . |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| . |
| . |
| Type n | Length n |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Value n |
| . |
| . |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
PIM Version: Reserved, Checksum Described 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.
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].
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.
Length 1..n: The length of the the value field.
Value 1..n: The value associated with the type and of the specified
length.
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3.2. 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.
3.2.1. Initial checks
In order to do further processing, a message MUST meet the following
requirements. The message MUST be from a directly connected neighbor
for which we have active Hello state, and it MUST have been sent to
the ALL-PIM-ROUTERS group. Also, the interface MUST NOT be an
administrative boundary for PFM. If No-Forward is not set, it MUST
have been sent by the RPF neighbor for the originator address. If
No-Forward is set, we MUST have restarted within 60 seconds. In
pseudo-code the algorithm is as follows:
if ((DirectlyConnected(PFM.src_ip_address) == FALSE) OR
(we have no Hello state for PFM.src_ip_address) 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 startup)) {
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.
3.2.2. Processing and forwarding of PFM messages
When the message is received, the initial checks above must be
performed. If it passes the checks, we then for each included TLV
perform processing according to the specification for that TLV.
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After processing, we forward the message. Unless otherwise specified
by the type specification, the TLVs in the forwarded message are
identical to the TLVs in the received message. However, if the most
significant bit in the type field is set (the type value is larger
than 32767) and we do not support the type, then that particular type
should be omitted from the forwarded messages. The message is
forwarded out of all interfaces with PIM neighbors (including the
interface it was received on).
4. Distributing Source to Group Mappings
The generic flooding mechanism (PFM) defined in the previous section
can be used for distributing source to group mappings about active
multicast sources throughout a PIM domain. A Group Source Holtime
(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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 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) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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
[RFC7761].
Src Count: How many unicast encoded sources address encodings
follow.
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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 [RFC7761].
4.2. Originating PFM messages
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
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 router originating the PFM
messages includes one of its own addresses in the originator field.
Note that this address SHOULD be routeable due to RPF checking. The
PFM messages containing the GSH TLV 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 a
PFM message originated every 60 seconds. The holdtime for the source
is by default 210 seconds. Other values MAY be configured, but the
holdtime MUST be either zero, or larger than the announcement period.
It is RECOMMENDED to be 3.5 times the announcement period. 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.
4.3. Processing GSH TLVs
A router that receives a PFM message containing GSH TLVs SHOULD parse
the message and store each of the GSH TLVs as SG mappings with a
holdtimer started with the advertised holdtime. 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. The SG mappings are kept alive for as long as the holdtimer
for the source is running. Once the holdtimer expires a PIM router
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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
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) 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).
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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. 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.
5. Security Considerations
The security considerations are mainly similar to what is documented
in [RFC5059]. It is a concern that rogue devices can inject packets
that are flooded throughout a domain. PFM packets must only be
accepted from a PIM neighbor. Deployments may use mechanisms for
authenticating PIM neighbors. For PFM-SA it is an issue that
injected packets from a rogue device could send SG mappings for a
large number of source addresses, causing routers to use memory
storing these mappings, and also if they have interest in the groups,
build Shortest Path Trees for sources that are not actually active.
6. IANA considerations
This document requires the assignment of a new PIM message type for
the PIM Flooding Mechanism (PFM). IANA is also requested to create a
registry for PFM TLVs, with type 0 assigned to the "Source Group
Holdtime" TLV. Values in the range 1-65535 are "Unassigned".
Assignments for the registry are to be made according to the policy
"IETF Review" as defined in [RFC5226].
7. Acknowledgments
The authors would like to thank Arjen Boers for contributing to the
initial idea, and Yiqun Cai and Dino Farinacci for their comments on
the draft.
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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,
<http://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, <http://www.rfc-editor.org/info/rfc5059>.
[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, <http://www.rfc-editor.org/info/rfc7761>.
8.2. Informative References
[RFC4607] Holbrook, H. and B. Cain, "Source-Specific Multicast for
IP", RFC 4607, DOI 10.17487/RFC4607, August 2006,
<http://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,
<http://www.rfc-editor.org/info/rfc5015>.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
DOI 10.17487/RFC5226, May 2008,
<http://www.rfc-editor.org/info/rfc5226>.
Authors' Addresses
IJsbrand Wijnands
Cisco Systems, Inc.
De kleetlaan 6a
Diegem 1831
Belgium
Email: ice@cisco.com
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Stig Venaas
Cisco Systems, Inc.
Tasman Drive
San Jose CA 95134
USA
Email: stig@cisco.com
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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