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draft-ietf-mpls-tp-ring-protection
Network Working Group S. Bryant, Ed.
Internet-Draft Cisco
Intended status: Informational N. Sprecher, Ed.
Expires: April 28, 2010 Y. Weingarten
Nokia Siemens Networks
October 25, 2009
MPLS-TP Ring Protection
draft-weingarten-mpls-tp-ring-protection-01.txt
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Abstract
This document describes mechanisms to address the requirements for
protection of ring topologies for Multi-Protocol Label Switching
Transport Profile (MPLS-TP) Label Switched Paths (LSP) and
Pseudowires (PW) on multiple layers. Ring topologies offer the
possibility of reducing the OAM overhead while providing a simplified
protection mechanism. The document analyzes two basic ring
protection schemes and explains how ring protection can be viewed as
an application of linear protection.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Contributing Authors . . . . . . . . . . . . . . . . . . . 4
2. Ring Topologies . . . . . . . . . . . . . . . . . . . . . . . 4
2.1. Interconnected rings . . . . . . . . . . . . . . . . . . . 5
3. Ring protection schemes . . . . . . . . . . . . . . . . . . . 7
3.1. Wrapping . . . . . . . . . . . . . . . . . . . . . . . . . 7
3.1.1. Optimization of wrapping . . . . . . . . . . . . . . . 9
3.2. Steering . . . . . . . . . . . . . . . . . . . . . . . . . 9
3.2.1. Point-to-Multipoint paths . . . . . . . . . . . . . . 10
4. Conclusions and Recommendations . . . . . . . . . . . . . . . 11
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
6. Security Considerations . . . . . . . . . . . . . . . . . . . 11
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 12
8. Informative References . . . . . . . . . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 12
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1. Introduction
Multi-Protocol Label Switching Transport Profile (MPLS-TP) is being
standardized as part of a joint effort between the Internet
Engineering Task Force (IETF) and the International Telecommunication
Union Standardization (ITU-T). These specifications are based on the
requirements that were generated from this joint effort.
The requirements for MPLS-TP [MPLS-TP Reqs] inidcates that there is
requirement to support a network that may include sections that
constitute a MPLS-TP ring (either logical or physical). The support
for ring topologies as stated in the requirements is based on the
ability to demonstrate that this topology allows the network to
optimize either the protection or the number of Operations,
Administration & Maintenance (OAM) entities needed to maintain the
network.
This document will examine different proposed mechanisms for
protection of a ring in the context of MPLS-TP and try and determine
how they may optimize the protection and the OAM procedures for a
ring topology. Finally, we plan to show how the generic protection
mechanisms can be used to address the requirements in an optimized
manner.
1.1. Contributing Authors
Akira Sakurai (NEC), Rolf Winter (NEC)
2. Ring Topologies
The MPLS-TP Requirements [MPLS-TP Reqs] defines a ring as a topology
in which each LSR is connected to exactly two neighboring LSRs, each
via a single point-to-point birectional MPLS-TP capable link. A ring
provides certain advantages in transport networks, including:
o Configuration of point-to-multipoint paths around a ring are
easily accomplished.
o There are always two paths between any two LSRs on a ring that can
be easily identified and associated.
o It is believed that the number of OAM entities needed, in order to
detect faults and perform recovery actions, may be minimized in a
ring topology.
The following figure shows a MPLS-TP ring that is a segment that may
be traversed by numerous LSPs or PWs. In particular, the figure
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shows that for all LSP that connect to the ring through LSR-B and
exit the ring from LSR-F we can define two paths through the ring
(the first path along B-A-F, and the second B-C-D-E-F).
____
=========>/ LSR\
* \__B_/ *
* @ # *
* @ # *
__* @ # *___
/LSR\ @ #/LSR\
\_C_/ @ #\_A_/
* @ # *
* @ #*
_*_ @ #*_
/LSR\@ /LSR\========>
\_D_/@ \_F_/
* @ @*
* @ @*
* @@____@@*
*/ LSR\*
\__E_/
===> connected LSP *** physical link
### logical path @@@ secondary logical path
Figure 1: A MPLS-TP ring
2.1. Interconnected rings
The Requirements document [MPLS-TP Reqs] states that the ring
protection must support a single ring that may be interconnected to
other rings. In addition, traffic that traverses a number of rings
within a network of interconnected rings must be protected even if
the interconnection nodes and links fail.
When interconnecting rings in a network there are two common
interconnection schemes:
o Dual-node interconnect - when the interconnected rings are
interconnected by two nodes from each ring (see Figure 2)
o Single-node interconnect - when the connection between the
interconnected rings are through a single node (see Figure 3)
The protection schemes presented in Section 3 are capable of
protecting each interconnected ring as a separate entity independent
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of the other rings in the network. This protects the traffic that
traverses the entire network, as each ring will continue to transfer
the traffic to the interconnection points, and from there to the next
ring.
When the interconnection nodes or links fail, there is the need to
protect these connection points. Therefore, it should be noted that
in the case of single-node interconnect the interconnection node
(LSR-A in Figure 3) is a single-point of failure and such an
interconnection scheme should be avoided. The protection of the
dual-node interconnect is essentially a linear-protection situation
and should be protected using appropriate protection mechanisms.
____ ___
/ LSR\ /LSR\
* \__B_/ * *\_1_/*
* * * *
* * * *
__* *___ _*_ * ___
/LSR\ /LSR\****/LSR\ /LSR\
\_C_/ \_A_/ \_6_/ \_2_/
* Ring #1 * * Ring #2 *
* * * *
_*_ *_ _*_ _*_
/LSR\ /LSR\ /LSR\ /LSR\
\_D_/ \_F_/****\_5_/ \_3_/
* * * *
* * * *
* ____ * * ____ *
*/ LSR\* */LSR \*
\__E_/ \__4_/
*** physical link
Figure 2: Dual-node interconnected rings
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____ ___
/ LSR\ /LSR\
* \__B_/ * *\_1_/*
* * * *
* * * *
__* *___ * * ___
/LSR\ /LSR\* /LSR\
\_C_/ \_A_/ \_2_/
* Ring #1 * * Ring #2 *
* * * *
_*_ _*_ * ___ _*_
/LSR\ /LSR\ */LSR\ /LSR\
\_D_/ \_F_/ \_5_/ \_3_/
* * * *
* * * *
* ____ * *___ *
*/ LSR\* /LSR\*
\__E_/ \_4_/
*** physical link
Figure 3: Single-node interconnected rings
3. Ring protection schemes
There are two classic mechanisms that have been proposed in various
forums to perform recovery of a topological ring network - "wrapping"
and "steering". The following sub-sections will examine these two
mechanisms.
3.1. Wrapping
The "easier" recovery architecture is "wrapping". This mechanism is
local to the LSRs that are neighbors to the detected fault. When a
fault is detected, the neighboring LSR "wrap" all data traffic around
the ring until arriving at the LSR that is on the opposite side of
the fault, at which point the traffic continues on the normal working
path until the egress from the ring segment.
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____
=========>/ LSR\
* \__B_/ *
* @@@@@@@# *
* @ @# *
___* @ @# *___
/LSR\ @ @#/LSR\
\_C_/ @ #\_A_/
* @ # *
* @ XX
_*_ @ #*_
/LSR\@ /LSR\
\_D_/@ @\_F_/
* @ @#*
* @ @@#*
* @@____@##*
*/ LSR\*
\__E_/========>
===> connected LSP *** physical link
### logical path @@@ Bypass tunnel
Figure 4: Wrapping protection
In this figure we have a ring with a LSP that enters the ring at
LSR-B and exits at LSR-E. The normal working path follows through
B-A-F-E. If a signal fault is detected on the link A<-->F, then
there is the need for configuring a bypass tunnel [FRR] between A &
F. The traffic will be transmitted over this bypass tunnel from A to
F, and then will continue on the normal working path from F->E.
Essentially, in this protection scheme, the traffic will follow the
path - B-A-B-C-D-E-F-E.
This protection scheme is simple in the sense that there is no need
for coordination between the different LSR in the ring - only the
LSRs that detect the fault must wrap the traffic, either via the
bypass tunnel (at the near-end) or back to the normal path (at the
far-end).
When applying this scheme to a MPLS-TP ring topology segment there
are the following considerations:
o The OAM should be performed at either the link level (by defining
a TCME between each adjacent pair of LSR) and/or per LSR (by
defining a TCME between the LSR that are neighbors of the
protected LSR) when using node-level protection.
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o For each protected TCME there is a need to define a bypass tunnel
that traverses the alternate path around the ring to connect
between the two ends of the TCME. If protecting both the links
and the nodes, then, for a ring with N nodes, there is a need for
O(2N) bypass tunnels.
o Protection of point-to-multipoint paths is similar to the simple
protection since the data continues along the original path after
wrapping around the ring. The one exception is the case where the
failed node was one of the egress points for the data.
o When wrapping the data is transmitted over some of the links
twice, once in each direction. For example, in the figure above
the traffic is transmitted both B-->A and then A-->B, later it is
transmitted E-->F and F-->E. This means that there is additional
bandwith needed for this protection.
o The wrapping also involves greater latency in delivering the
packets, as a result of traversing the entire ring.
o The resource allocation for the bypass tunnels could be
problematic, since most of the tunnels will not be used
simultaneously. One possibility could be to allocate '0'
resources and depend on the NMS to allocate the proper resources
around the ring.
3.1.1. Optimization of wrapping
It may be possible to optimize the basic performance, in terms of
bandwidth utilization, of the Fast-reroute mechanism when protecting
particular LSPs. However, it is important to consider the basic
criteria for ring protection, i.e. an appreciable minimization of the
number of OAM sessions necessarily or the protection resources
needed.
One suggestion for optimization is to define the bypass tunnel to
wrap only until the furthest egress point of the working LSP. This
method reduces the number of links that are traversed twice, in most
cases, by not wrapping back to the working path at the far-end of the
signal fault. However, the method requires defining a particular
bypass tunnel for each LSP and cannot use the optimization of the OAM
sessions presented above to protect all LSPs with a single set of
bypass LSPs.
3.2. Steering
The second common scheme for ring protection redirects the traffic
from the ingress point to the alternate route around the ring to the
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egress point. This is illustrated in Figure 1 above, where if a
Signal Fault is detected on the working path (B-A-F), then the
traffic is redirected by B to the secondary path (i.e. B-C-D-E-F).
When considering this mechanism it is almost identical to linear 1:1
protection. The two paths around the ring act as the working and
recovery paths. There is need to communicate to the ingress node the
need to switch over to the protection path and there is a need to
coordinate the switchover between the two end-points of the protected
path.
There is one aspect that this diverts from the basic linear
protection scheme - in the number of OAM sessions that would be
neccesary to detect faults in the protected domain. Whereas, using
generic linear protection would neccesitate a separate OAM session
per LSP that traverses the ring, when using ring protection there is
a possiblity of taking proper advantage of the realization that we
are dealing with a ring, and reduce the number of OAM sessions. This
is done by defining an OAM session on the basis of a Path Segment
Tunnel (PST), i.e. between any two nodes of the ring. This would
lead to the number of OAM sessions for a ring with N nodes to be
O(N*N/2), which could be very large. However, taking into
consideration that the required support of rings, is for rings with
up-to 16 nodes - this implies that the number of OAM sessions should
be on the order of (16*16/2) or 128.
This form of OAM would allow the ingress LSR to directly detect any
faulty situations and redirect traffic to the secondary path without
the need for any additional communication to the LSR.
The following observations can be drawn from using this protection
mechanism for MPLS-TP ring topologies:
o Steering can be based on linear protection for the protection of a
single ring. For cases of interconnected rings further study is
necessary.
o The number of OAM sessions can be greatly minimized, relative to
using plain linear protection, by running the OAM sessions on the
ring segments for all LSPs that traverse the ring, rather than
running a OAM session per LSP. This fulfills the objective
presented in [MPLS-TP Reqs].
3.2.1. Point-to-Multipoint paths
[MPLS-TP Reqs] requires that ring protection must provide protection
for unidirectional point-to-multipoint paths through the ring. As
was pointed out in section 1, ring topologies provide a ready
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platform for supporting such data paths.
It is possible to create a protection architecture for p2mp within a
MPLS-TP ring topology, based on 1+1 linear protection by monitoring
two p2mp uni-directional PST from each ingress node on the ring with
egress points at each node. The two PST traverse the ring in
opposite directions, i.e. one checks the clockwise path and the
second the counter-clockwise path.
The data for a particular p2mp LSP is transmitted on both the working
and protecting LSP, using a permanent bridge. While each node
detects that there is connectivity from the ingress point, it
continues to select the data that is coming from the working path.
If a particular node stops receiving the connectivity messages from
the working path PST, it identifies that it must switch its selector
to read the data from the protecting path.
This architecture has the added advantages that there is no need for
the ingress node to identify the existence of the misconnectivity,
and there is no need for a return path from the egress points to the
ingress.
4. Conclusions and Recommendations
In order to fulfill the requirements for protection of ring
topologies for MPLS-TP networks, according to the conditions stated
in [MPLS-TP Reqs], the protection should be based on MPLS-TP 1:1
linear protection. This mechanism will cover the cases of a single
fault in a single ring topology.
When defining the OAM behavior of the ring nodes, they should define
a segment of all the LSPs that traverse a path within the ring. The
OAM should be executed for each ring path, i.e. PST, to detect
faults and trigger the protection switching within the ring.
5. IANA Considerations
This document makes no request of IANA.
Note to RFC Editor: this section may be removed on publication as an
RFC.
6. Security Considerations
This document does not by itself raise any particular security
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considerations.
7. Acknowledgements
The authors would like to thank all members of the teams (the Joint
Working Team, the MPLS Interoperability Design Team in IETF and the
T-MPLS Ad Hoc Group in ITU-T) involved in the definition and
specification of MPLS Transport Profile.
8. Informative References
[FRR] Pan, P., Swallow, G., and A. Atlas, "Fast Reroute
Exensions to RSVP-TE for LSP Tunnels", RFC 4090, May 2005.
[MPLS-TP Reqs]
Niven-Jenkins, B., Nadeau, T., and C. Pignataro,
"Requirements for the Trasport Profile of MPLS", RFC 5654,
April 2009.
Authors' Addresses
Stewart Bryant (editor)
Cisco
United Kingdom
Email: stbryant@cisco.com
Nurit Sprecher (editor)
Nokia Siemens Networks
3 Hanagar St. Neve Ne'eman B
Hod Hasharon, 45241
Israel
Email: nurit.sprecher@nsn.com
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Yaacov Weingarten
Nokia Siemens Networks
3 Hanagar St. Neve Ne'eman B
Hod Hasharon, 45241
Israel
Phone: +972-9-775 1827
Email: yaacov.weingarten@nsn.com
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