Multicast only Fast Re-Route
draft-ietf-rtgwg-mofrr-00
The information below is for an old version of the document.
| Document | Type |
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|---|---|---|---|
| Authors | Apoorva Karan , Clarence Filsfils , Dino Farinacci , IJsbrand Wijnands , Bruno Decraene , Uwe Joorde , Wim Henderickx | ||
| Last updated | 2013-04-05 (Latest revision 2012-10-02) | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
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draft-ietf-rtgwg-mofrr-00
Network Working Group A. Karan
Internet-Draft C. Filsfils
Intended status: Informational D. Farinacci
Expires: April 5, 2013 IJ. Wijnands, Ed.
Cisco Systems, Inc.
B. Decraene
France Telecom
U. Joorde
Deutsche Telekom
W. Henderickx
Alcatel-Lucent
October 02, 2012
Multicast only Fast Re-Route
draft-ietf-rtgwg-mofrr-00
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 draft describes a mechanism for minimizing packet loss in a
network when node or link failures occur. Multicast only Fast Re-
Route (MoFRR) works by making simple enhancements to multicast
routing protocols such as PIM and mLDP.
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
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
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 April 5, 2013.
Copyright Notice
Copyright (c) 2012 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
(http://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
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Conventions used in this document . . . . . . . . . . . . 3
1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
2. Basic Overview . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Upstream Multicast Hop Selection . . . . . . . . . . . . . . . 4
3.1. PIM . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3.2. mLDP . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
4. Topologies for MoFRR . . . . . . . . . . . . . . . . . . . . . 5
4.1. Dual-Plane Topology . . . . . . . . . . . . . . . . . . . 5
5. Detecting Failures . . . . . . . . . . . . . . . . . . . . . . 8
6. ECMP-mode MoFRR . . . . . . . . . . . . . . . . . . . . . . . 9
7. Non-ECMP-mode MoFRR . . . . . . . . . . . . . . . . . . . . . 9
7.1. Variation . . . . . . . . . . . . . . . . . . . . . . . . 11
8. Keep It Simple Principle . . . . . . . . . . . . . . . . . . . 11
9. Capacity Planning for MoFRR . . . . . . . . . . . . . . . . . 11
10. Other Applications . . . . . . . . . . . . . . . . . . . . . . 12
11. Security Considerations . . . . . . . . . . . . . . . . . . . 12
12. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 13
13. Contributing authors . . . . . . . . . . . . . . . . . . . . . 13
14. References . . . . . . . . . . . . . . . . . . . . . . . . . . 13
14.1. Normative References . . . . . . . . . . . . . . . . . . . 13
14.2. Informative References . . . . . . . . . . . . . . . . . . 13
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 13
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1. Introduction
Multiple techniques have been developed and deployed to improve
service guarantees, both for multicast video traffic and Video on
Demand traffic. Most existing solutions are geared towards finding
an alternate path around one or more failed network elements (link,
node, path failures).
This draft describes a mechanism for minimizing packet loss in a
network when node or link failures occur. Multicast only Fast Re-
Route (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.
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 later in this document. 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 Re-Route.
ECMP : Equal Cost Multi-Path.
mLDP : Multi-point 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.
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tree : Either a PIM (S,G)/(*,G) tree or a mLDP P2MP or 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 a mLDP P2MP or MP2MP LSP.
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 two divergent paths SHOULD never merge
upstream, otherwise the maximal redundancy is compromised. Sometimes
the topology guarantees maximal redundancy, other times additional
configuration or techniques are needed to enforce it. See later in
this document.
A merge point router should only accept and forward on one of the
upstream paths at the 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.
Note, the impact of 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 its likely a tree merge point is not far
away.
3. 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. See below;
3.1. 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 by the RPF procedure, or not the
primary, the packets are discarded.
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3.2. mLDP
The UMH selection in mLDP also depends on unicast routing, but the
difference with PIM is that the acceptance of multicast packets is
based on MPLS labels and independent on the interface the packet is
received on. Using the procedures as defined in [RFC6388] an
upstream Label Switched 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 LSPs
(primary) upstream LSR.
4. Topologies for MoFRR
MoFRR works best in topologies illustrated in the figure 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.
4.1. Dual-Plane Topology
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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
FIG1. 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 PJ indicate the normal 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 anyways.
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
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secondary UNH is discarded.
When a router detects a routing failure on the path to its 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. PEs not enabled for MoFRR do not see any
change or degradation. Inter-operability testing is not required
as there are no PIM or mLDP protocol change.
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.
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 graft into the normal Multicast
trees (ie. trees signaled by PIM or mLDP without MoFRR extension)
at the aggregation level and hence do not demand any extra
capacity either on the distribution links or in the backbone.
They simply use the capacity that is normally used, without any
duplication. This is different from conventional FRR mechanisms
which often duplicate the capacity requirements (the backup path
crosses links/nodes which already carry the primary/normal tree
and hence 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
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per Fig2. 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
FIG2. PE is dual-homed to Dual-Plane Backbone
Another frequent topology is described in Fig 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 Fig 2. However, there is clearly a
way to provide MoFRR benefits as each PE can offer a disjoint
secondary path to the other plane PE (via the disjoint path).
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
extension and is described later in the document.
Source
/ \
Plane1 Plane2
| |
A1 A2
| |
PE1----PE2
FIG3. PEs are connected in pairs to Dual-Plane Backbone
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.
A first option consists of comparing the packets received on the
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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 second 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. 50msec switchover is possible.
A third 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, 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. Realistic availability
requirements (sub-second to sub-200msec) should be possible.
A fourth option consists in leveraging connected link failure. This
option makes sense when MoFRR is deployed across the network (not
only at PE).
6. ECMP-mode MoFRR
If the IGP installs two ECMP paths to the source and if the Multicast
tree is enabled for ECMP-Mode MoFRR, the router installs them as
primary and secondary UMH. Only packets received from the primary
UMH path are processed. Packets received from the secondary UMH are
dropped.
The selected primary UMH should be the same as if MoFRR extension was
not enabled.
If more than two ECMP paths exist, two are selected as primary and
secondary UMH. Information from the IGP link-state topology could be
leveraged to optimize this selection.
Note, MoFRR does not restrict the number of UMH paths that are
joined. Implementations may use as many paths as are configured.
7. Non-ECMP-mode MoFRR
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SourceS
/ \
/ \
Backbone
| |
| |
| |
X--------N
Fig5. Non-ECMP-Mode MoFRR
X is configured for MoFRR for a Multicast tree
R(X) is Xs UMH to S
N is a neighbor of X
R(N) is Ns UMH to S
xs represents the IGP metric from X to S
ns represents the IGP metric from N to S
xn represents the IGP metric from X to N
A router X configured for non-ECMP-mode MoFRR for a Multicast tree
joins a primary path to its primary UMH R(X) and a secondary path to
UMH N if the following three conditions are met.
C1: xs < xn + ns
C2: ns < nx + xs
C3: X cannot join the secondary path N if N is the only member of the OIF list
The first condition ensures that N is not on the primary branch from
X to S.
The second condition ensures that X is not on the primary branch from
N to S.
These two conditions ensure that at least locally the two paths are
disjoint.
The third condition is required to break control-plane loops which
could occur in some scenarios.
For example in FIG3, if 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 would
leave at the same time, it could be possible for the Multicast tree
on PE1 and PE2 to never get deleted as each PE refresh each other via
the secondary path joins (remember that a secondary path join is not
distinguishable from a primary join. MoFRR does not require any PIM
or mLDP protocol modification).
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A control-plane loop occurs when two nodes keep a state forever due
to joining the secondary path to each other. This forever condition
is not acceptable as no real receiver is connected to the nodes
(directly via IGMP or indirectly via PIM). Rule 3 prevents this case
as it prevents the mutual refresh of secondary joins and it applies
it in the specific case where there is no real receiver connected.
7.1. Variation
Rule R3 can be removed if Rule 2 is restricted as follows:
R2p: ns < xs
This ensures that X will only join the secondary path to a neighbor N
who is strictly closer to the source than X is. By reciprocity, N
will thus never join the secondary path for the same Multicast tree
via X. The strictly smaller than is key here.
Note that this non-ECMP-mode MoFRR variation does not support the
square topology and hence is less preferred.
8. Keep It Simple Principle
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. PEs not enabled for MoFRR
do not see any change or degradation.
Multicast streams with Tight SLA requirements are often characterized
by a continuous high packet rate (SD video has a continuous
interpacket gap of ~ 3msec). MoFRR simply leverages the stream
characteristic to detect any failures along the primary branch and
switch-over on the secondary branch in a few 10s of msec.
9. Capacity Planning for MoFRR
As for LFA FRR (draft-ietf-rtgwg-lfa-applicability-00), MoFRR
applicability is topology dependent.
In this document, we have described two very frequent designs (Fig 2
and Fig 3) which provide maximum MoFRR benefits.
Designers with topologies different than Fig2 and 3 can still benefit
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from MoFRR benefits 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 for hundreds of PEs and
hundreds of sources.
This allows to assess 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 tool to optimize the topology (add links, change igp
metrics).
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
3-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.
Alternate methods to detect failures such as MPLS-OAM or BFD may be
considered.
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] .
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12. Acknowledgments
The authors would like to thank John Zwiebel, Greg Shepherd and Dave
Oran for their review of the draft.
13. Contributing authors
Below is a list of other contributing authors in alphabetical order:
Nicolai Leymann
Deutsche Telekom
Winterfeldtstrasse 21
Berlin 10781
DE
Email: N.Leymann@telekom.de
14. References
14.1. Normative References
[RFC5036] Andersson, L., Minei, I., and B. Thomas, "LDP
Specification", RFC 5036, October 2007.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
14.2. Informative References
[RFC4601] Fenner, B., Handley, M., Holbrook, H., and I. Kouvelas,
"Protocol Independent Multicast - Sparse Mode (PIM-SM):
Protocol Specification (Revised)", RFC 4601, August 2006.
[RFC6388] Wijnands, IJ., Minei, I., Kompella, K., and B. Thomas,
"Label Distribution Protocol Extensions for Point-to-
Multipoint and Multipoint-to-Multipoint Label Switched
Paths", RFC 6388, November 2011.
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Authors' Addresses
Apoorva Karan
Cisco Systems, Inc.
3750 Cisco Way
San Jose CA, 95134
USA
Email: apoorva@cisco.com
Clarence Filsfils
Cisco Systems, Inc.
De kleetlaan 6a
Diegem BRABANT 1831
Belgium
Email: cfilsfil@cisco.com
Dino Farinacci
Cisco Systems, Inc.
425 East Tasman Drive
San Jose CA, 95134
USA
Email: dino@cisco.com
IJsbrand Wijnands (editor)
Cisco Systems, Inc.
De Kleetlaan 6a
Diegem 1831
BE
Email: ice@cisco.com
Bruno Decraene
France Telecom
38-40 rue du General Leclerc
Issy Moulineaux cedex 9, 92794
FR
Email: bruno.decraene@orange-ftgroup.com
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Uwe Joorde
Deutsche Telekom
Hammer Str. 216-226
Muenster D-48153
DE
Email: Uwe.Joorde@telekom.de
Wim Henderickx
Alcatel-Lucent
Copernicuslaan 50
Antwerp 2018
Belgium
Email: wim.henderickx@alcatel-lucent.com
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