Internet Engineering Task Force (IETF) A. Karan
Request for Comments: 7431 C. Filsfils
Category: Informational IJ. Wijnands, Ed.
ISSN: 2070-1721 Cisco Systems, Inc.
B. Decraene
Orange
August 2015
Multicast-Only Fast Reroute
Abstract
As IPTV deployments grow in number and size, service providers are
looking for solutions that minimize the service disruption due to
faults in the IP network carrying the packets for these services.
This document describes a mechanism for minimizing packet loss in a
network when node or link failures occur. Multicast-only Fast
Reroute (MoFRR) works by making simple enhancements to multicast
routing protocols such as Protocol Independent Multicast (PIM) and
Multipoint LDP (mLDP).
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for informational purposes.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Not all documents
approved by the IESG are a candidate for any level of Internet
Standard; see Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc7431.
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Table of Contents
1. Introduction ....................................................3
1.1. Conventions Used in This Document ..........................3
1.2. Terminology ................................................3
2. Basic Overview ..................................................4
3. Determination of the Secondary UMH ..............................5
3.1. ECMP-Mode MoFRR ............................................5
3.2. Non-ECMP-Mode MoFRR ........................................5
4. Upstream Multicast Hop Selection ................................6
4.1. PIM ........................................................6
4.2. mLDP .......................................................6
5. Detecting Failures ..............................................6
6. MoFRR Applicability to Dual-Plane Topology ......................7
7. Other Topologies ...............................................10
8. Capacity Planning for MoFRR ....................................11
9. PE Nodes .......................................................11
10. Other Applications ............................................11
11. Security Considerations .......................................12
12. References ....................................................12
12.1. Normative References .....................................12
12.2. Informative References ...................................12
Acknowledgments ...................................................13
Contributors ......................................................13
Authors' Addresses ................................................14
1. Introduction
Different solutions have been developed and deployed to improve
service guarantees, both for multicast video traffic and Video on
Demand traffic. Most of these solutions are geared towards finding
an alternate path around one or more failed network elements (link,
node, or path failures).
This document describes a mechanism for minimizing packet loss in a
network when node or link failures occur. Multicast-only Fast
Reroute (MoFRR) works by making simple changes to the way selected
routers use multicast protocols such as PIM and mLDP. No changes to
the protocols themselves are required. With MoFRR, in many cases,
multicast routing protocols don't necessarily have to depend on or
have to wait on unicast routing protocols to detect network failures;
see Section 5.
On a Merge Point, MoFRR logic determines a primary Upstream Multicast
Hop (UMH) and a secondary UMH and joins the tree via both
simultaneously. Data packets are received over the primary and
secondary paths. Only the packets from the primary UMH are accepted
and forwarded down the tree; the packets from the secondary UMH are
discarded. The UMH determination is different for PIM and mLDP and
explained in Section 4. When a failure is detected on the path to
the primary UMH, the repair occurs by changing the secondary UMH into
the primary and the primary into the secondary. Since the repair is
local, it is fast -- greatly improving convergence times in the event
of node or link failures on the path to the primary UMH.
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
MoFRR: Multicast-only Fast Reroute.
ECMP: Equal-Cost Multipath.
mLDP: Multipoint Label Distribution Protocol.
PIM: Protocol Independent Multicast.
UMH: Upstream Multicast Hop. A candidate next-hop that can be used
to reach the root of the tree.
tree: Either a PIM (S,G)/(*,G) tree or an mLDP Point-to-Multipoint
(P2MP) or Multipoint-to-Multipoint (MP2MP) LSP.
OIF: Outgoing interface. An interface used to forward multicast
packets down the tree towards the receivers. Either a PIM
(S,G)/(*,G) tree or an mLDP P2MP or MP2MP LSP.
LFA: Loop-Free Alternate as defined in [RFC5286]. In unicast Fast
Reroute, this is an alternate next-hop that can be used to reach a
unicast destination without using the protected link or node.
Merge Point: A router that joins a multicast stream via two divergent
upstream paths.
RPF: Reverse Path Forwarding.
RP: Rendezvous Point.
LSP: Label Switched Path.
LSR: Label Switching Router.
BFD: Bidirectional Forwarding Detection.
IGP: Interior Gateway Protocol.
MVPN: Multicast Virtual Private Network.
POP: Point Of Presence, an access point into the network.
2. Basic Overview
The basic idea of MoFRR is for a Merge Point router to join a
multicast tree via two divergent upstream paths in order to get
maximum redundancy. The determination of this alternate upstream is
defined in Section 3.
In order to maximize robustness against any failure, the two paths
should be as diverse as possible. Ideally, they should not merge
upstream. Sometimes the topology guarantees maximal redundancy;
other times additional configuration or techniques are needed to
enforce it. See Section 6 for more discussion on the applicability
of MoFRR depending on the network topology.
A Merge Point router should only accept and forward on one of the
upstream paths at a time in order to avoid duplicate packet
forwarding. The selection of the primary and secondary UMH is done
by the MoFRR logic and normally based on unicast routing to find
loop-free candidates. This is described in Section 4.
Note, the impact of an additional amount of data on the network is
mitigated when tree membership is densely populated. When a part of
the network has redundant data flowing, join latency for new joining
members is reduced because it's likely a tree Merge Point is not far
away.
3. Determination of the Secondary UMH
The secondary UMH is a Loop-Free Alternate (LFA) as per [RFC5286].
3.1. ECMP-Mode MoFRR
If the IGP installs two ECMP paths to the source, then as per
[RFC5286] the LFA is a primary next-hop. If the multicast tree is
enabled for ECMP-mode MoFRR, the router installs the paths as primary
and secondary UMHs. Before the failure, only packets received from
the primary UMH path are processed, while packets received from the
secondary UMH are dropped.
The selected primary UMH SHOULD be the same as if the MoFRR extension
were not enabled.
If more than two ECMP paths exist, one is selected as primary and
another as secondary UMH. The selection of the primary and secondary
is a local decision. Information from the IGP link-state topology
could be leveraged to optimize this selection such that the primary
and secondary paths are maximal divergent and don't lead to the same
upstream node. Note that MoFRR does not restrict the number of UMH
paths that are joined. Implementations may use as many paths as are
configured.
3.2. Non-ECMP-Mode MoFRR
A router X configured for non-ECMP-mode MoFRR for a multicast tree
joins a primary path to its primary UMH and a secondary path to its
LFA UMH. In order to prevent control-plane loops, a router MUST stop
joining the secondary UMH if this UMH is the only member in the OIF
list.
To illustrate the reason for this rule, let's consider the example in
Figure 3. If two Provider Edge routers, PE1 and PE2, have received
an IGMP request for a multicast tree, they will both join the primary
path on their plane and a secondary path to the neighbor PE. If
their receivers leave at the same time, it's possible for the
multicast tree on PE1 and PE2 to never get deleted, as the PEs
refresh each other via the secondary path joins (remember that a
secondary path join is not distinguishable from a primary join).
4. Upstream Multicast Hop Selection
An Upstream Multicast Hop (UMH) is a candidate next-hop that can be
used to reach the root of the tree. This is normally based on
unicast routing to find loop-free candidate(s). With MoFRR
procedures, we select a primary and a backup UMH. The procedures for
determining the UMH are different for PIM and mLDP.
4.1. PIM
The UMH selection in PIM is also known as the Reverse Path Forwarding
(RPF) procedure. Based on a unicast route lookup on either the
source address or Rendezvous Point (RP) [RFC4601], an upstream
interface is selected for sending the PIM Joins/Prunes AND accepting
the multicast packets. The interface the packets are received on is
used to pass or fail the RPF check. If packets are received on an
interface that was not selected as the primary by the RPF procedure,
the packets are discarded.
4.2. mLDP
The UMH selection in mLDP also depends on unicast routing, but the
difference from PIM is that the acceptance of multicast packets is
based on MPLS labels and is independent of the interface on which the
packet is received. Using the procedures as defined in [RFC6388], an
upstream Label Switching Router (LSR) is elected. The upstream LSR
that was elected for a Label Switched Path (LSP) gets a unique local
MPLS label allocated. Multicast packets are only forwarded if the
MPLS label matches the MPLS label that was allocated for that LSP's
(primary) upstream LSR.
5. Detecting Failures
Once the two paths are established, the next step is detecting a
failure on the primary path to know when to switch to the backup
path. This is a local issue, but this section explores some
possibilities.
The first (and simplest) option is to detect the failure of the local
interface as it's done for unicast Fast Reroute. Detection can be
performed using the loss of signal or the loss of probing packets
(e.g., BFD). This option can be used in combination with the other
options as documented below. Just like for unicast fast reroute,
50 msec switchover is possible.
A second option consists of comparing the packets received on the
primary and secondary streams but only forwarding one of them -- the
first one received, no matter which interface it is received on.
Zero packet loss is possible for RTP-based streams.
A third option assumes a minimum known packet rate for a given data
stream. If a packet is not received on the primary RPF within this
time frame, the router assumes primary path failure and switches to
the secondary RPF interface. 50 msec switchover may be possible for
high-rate streams (e.g., IPTV where SD video has a continuous inter-
packet gap of about 3 msec), but in general the delay is dependent on
the rate of the multicast stream.
A fourth option leverages the significant improvements of the IGP
convergence speed. When the primary path to the source is withdrawn
by the IGP, the MoFRR-enabled router switches over to the backup
path, and the UMH is changed to the secondary UMH. Since the
secondary path is already in place, and assuming it is disjoint from
the primary path, convergence times would not include the time
required to build a new tree and hence are smaller. Sub-second to
sub-200 msec switchover should be possible.
6. MoFRR Applicability to Dual-Plane Topology
MoFRR applicability is topology dependent. The applicability is the
same as LFA FRR, which is discussed in [RFC6571].
The following section will discuss MoFRR applicability to dual-plane
network topologies.
MoFRR works best in dual-planes topologies as illustrated in the
figures below. MoFRR may be enabled on any router in the network.
In the figures below, MoFRR is shown enabled on the Provider Edge
(PE) routers to illustrate one way in which the technology may be
deployed.
S
P / \ P
/ \
^ G1 R1 ^
P / \ P
/ \
G2----------R2 ^
| \ | \ P
^ | \ | \
P | G3----------R3
| | | |
| | | | ^
G4---|------R4 | P
^ \ | \ |
P \ | \ |
G5----------R5
^ | | ^
P | | P
| |
Gi Ri
\ \__ ^ /|
\ \ S1/ | ^
^ \ ^\ / |P2
P1 \ S2\_/__ |
\ / \|
PE1 PE2
P = Primary path
S = Secondary path
Figure 1: Two-Plane Network Design
The topology has two planes, a primary plane and a secondary plane
that are fully disjoint from each other all the way into the POPs.
This two-plane design is common in service provider networks as it
eliminates single point of failures in their core network. The links
marked P indicate the normal (primary) path of how the PIM Joins flow
from the POPs towards the source of the network. Multicast streams,
especially for the densely watched channels, typically flow along
both the planes in the network anyway.
The only change MoFRR adds to this is on the links marked S where the
PE routers join a secondary path to their secondary ECMP UMH. As a
result of this, each PE router receives two copies of the same
stream, one from the primary plane and the other from the secondary
plane. As a result of normal UMH behavior, the multicast stream
received over the primary path is accepted and forwarded to the
downstream receivers. The copy of the stream received from the
secondary UMH is discarded.
When a router detects a routing failure on the path to its primary
UMH, it will switch to the secondary UMH and accept packets for that
stream. If the failure is repaired, the router may switch back. The
primary and secondary UMHs have only local context and not end-to-end
context.
As one can see, MoFRR achieves the faster convergence by pre-building
the secondary multicast tree and receiving the traffic on that
secondary path. The example discussed above is a simple case where
there are two ECMP paths from each PE device towards the source, one
along the primary plane and one along the secondary. In cases where
the topology is asymmetric or is a ring, this ECMP nature does not
hold, and additional rules have to be taken into account to choose
when and where to join the secondary path.
MoFRR is appealing in such topologies for the following reasons:
1. Ease of deployment and simplicity: the functionality is only
required on the PE devices, although it may be configured on all
routers in the topology. Furthermore, each PE device can be
enabled separately; there is no need for network-wide
coordination in order to deploy MoFRR. Interoperability testing
is not required as there are no PIM or mLDP protocol changes.
2. End-to-end failure detection and recovery: any failure along the
path from the source to the PE can be detected and repaired with
the secondary disjoint stream. (See the second, third, and
fourth options in Section 5.)
3. Capacity efficiency: as illustrated in the previous example, the
multicast trees corresponding to IPTV channels cover the backbone
and distribution topology in a very dense manner. As a
consequence, the secondary path grafts onto the normal multicast
trees (i.e., trees signaled by PIM or mLDP without the MoFRR
extension) at the aggregation level and hence does not demand any
extra capacity either on the distribution links or in the
backbone. The secondary path simply uses the capacity that is
normally used, without any duplication. This is different from
conventional FRR mechanisms that often duplicate the capacity
requirements when the backup path crosses links/nodes that
already carry the primary/normal tree, and thus twice as much
capacity is required.
4. Loop-free: the secondary path join is sent on an ECMP disjoint
path. By definition, the neighbor receiving this request is
closer to the source and hence will not cause a loop.
The topology we just analyzed is very frequent and can be modeled as
per Figure 2. The PE has two ECMP disjoint paths to the source.
Each ECMP path uses a disjoint plane of the network.
Source
/ \
Plane1 Plane2
| |
A1 A2
\ /
PE
Figure 2: PE is Dual-Homed to Dual-Plane Backbone
Another frequent topology is described in Figure 3. PEs are grouped
by pairs. In each pair, each PE is connected to a different plane.
Each PE has one single shortest-path to a source (via its connected
plane). There is no ECMP like in Figure 2. However, there is
clearly a way to provide MoFRR benefits as each PE can offer a
disjoint secondary path to the PE in the other plane (via the
disjoint path).
The MoFRR secondary neighbor selection process needs to be extended
in this case as one cannot simply rely on using an ECMP path as
secondary neighbor. This extension is referred to as non-ECMP-mode
MoFRR and is described in Section 3.2.
Source
/ \
Plane1 Plane2
| |
A1 A2
| |
PE1----PE2
Figure 3: PEs Are Connected in Pairs to Dual-Plane Backbone
7. Other Topologies
As mentioned in Section 6, MoFRR works best in dual-plane topologies.
If MoFRR is applied to non-dual-plane networks, it's possible that
the secondary path is affected by the same failure that affected the
primary path. In that case, there is no guarantee that the backup
path will provide an uninterrupted traffic flow of packets without
loss or duplication.
8. Capacity Planning for MoFRR
The previous section has described two very frequent designs (Figures
2 and 3) which provide maximum MoFRR benefits.
Designers with topologies different than Figures 2 and 3 can still
benefit from MoFRR, thanks to the use of capacity planning tools.
Such tools are able to simulate the ability of each PE to build two
disjoint branches of the same tree. This simulation could be for
hundreds of PEs and hundreds of sources.
This allows an assessment of the MoFRR protection coverage of a given
network, for a set of sources.
If the protection coverage is deemed insufficient, the designer can
use such a tool to optimize the topology (add links, change IGP
metrics).
9. PE Nodes
Many Service Providers devise their topology such that PEs have
disjoint paths to the multicast sources. MoFRR leverages the
existence of these disjoint paths without any PIM or mLDP protocol
modification. Interoperability testing is thus not required. In
such topologies, MoFRR only needs to be deployed on the PE devices.
Each PE device can be enabled one by one.
10. Other Applications
While all the examples in this document show the MoFRR applicability
on PE devices, it is clear that MoFRR could be enabled on aggregation
or core routers.
MoFRR can be popular in data center network configurations. With the
advent of lower-cost Ethernet and increasing port density in routers,
there is more meshed connectivity than ever before. When using a
three-level access, distribution, and core layers in a data center,
there is a lot of inexpensive bandwidth connecting the layers. This
will lend itself to more opportunities for ECMP paths at multiple
layers. This allows for multiple layers of redundancy protecting
link and node failure at each layer with minimal redundancy cost.
Redundancy costs are reduced because only one packet is forwarded at
every link along the primary and secondary data paths so there is no
duplication of data on any link thereby providing make-before-break
protection at a very small cost.
A MoFRR router only accepts packets from the primary path and
discards packets from the secondary path. For that reason,
management applications (like ping and mtrace) will not work when
verifying the secondary path.
The MoFRR principle may be applied to MVPNs.
11. Security Considerations
There are no security considerations for this design other than what
is already in the main PIM specification [RFC4601] and mLDP
specification [RFC6388].
12. References
12.1. 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>.
[RFC5286] Atlas, A., Ed., and A. Zinin, Ed., "Basic Specification
for IP Fast Reroute: Loop-Free Alternates", RFC 5286,
DOI 10.17487/RFC5286, September 2008,
<http://www.rfc-editor.org/info/rfc5286>.
12.2. Informative References
[RFC4601] Fenner, B., Handley, M., Holbrook, H., and I. Kouvelas,
"Protocol Independent Multicast - Sparse Mode (PIM-SM):
Protocol Specification (Revised)", RFC 4601,
DOI 10.17487/RFC4601, August 2006,
<http://www.rfc-editor.org/info/rfc4601>.
[RFC6388] Wijnands, IJ., Ed., Minei, I., Ed., Kompella, K., and B.
Thomas, "Label Distribution Protocol Extensions for Point-
to-Multipoint and Multipoint-to-Multipoint Label Switched
Paths", RFC 6388, DOI 10.17487/RFC6388, November 2011,
<http://www.rfc-editor.org/info/rfc6388>.
[RFC6571] Filsfils, C., Ed., Francois, P., Ed., Shand, M., Decraene,
B., Uttaro, J., Leymann, N., and M. Horneffer, "Loop-Free
Alternate (LFA) Applicability in Service Provider (SP)
Networks", RFC 6571, DOI 10.17487/RFC6571, June 2012,
<http://www.rfc-editor.org/info/rfc6571>.
Acknowledgments
Thanks to Dave Oran and Alvaro Retana for their review and comments
on this document.
The authors would like to especially acknowledge Dino Farinacci, John
Zwiebel, and Greg Shepherd for the genesis of the MoFRR concept.
Contributors
Below is a list of the contributors in alphabetical order:
Dino Farinacci
Email: farinacci@gmail.com
Wim Henderickx
Alcatel-Lucent
Copernicuslaan 50
Antwerp 2018
Belgium
Email: wim.henderickx@alcatel-lucent.com
Uwe Joorde
Deutsche Telekom
Dahlweg 100
D-48153 Muenster
Germany
Email: Uwe.Joorde@telekom.de
Nicolai Leymann
Deutsche Telekom
Winterfeldtstrasse 21
Berlin 10781
Germany
Email: N.Leymann@telekom.de
Jeff Tantsura
Ericsson
300 Holger Way
San Jose, CA 95134
United States
Email: jeff.tantsura@ericsson.com
Authors' Addresses
Apoorva Karan
Cisco Systems, Inc.
3750 Cisco Way
San Jose, CA 95134
United States
Email: apoorva@cisco.com
Clarence Filsfils
Cisco Systems, Inc.
De kleetlaan 6a
Diegem BRABANT 1831
Belgium
Email: cfilsfil@cisco.com
IJsbrand Wijnands (editor)
Cisco Systems, Inc.
De Kleetlaan 6a
Diegem 1831
Belgium
Email: ice@cisco.com
Bruno Decraene
Orange
38-40 rue du General Leclerc
Issy Moulineaux Cedex 9, 92794
France
Email: bruno.decraene@orange.com
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