IETF Draft Changcheng Huang
Multi-Protocol Label Switching Vishal Sharma
Expires: January 2001 Srinivas Makam
Ken Owens
Tellabs Operations, Inc.
July 2000
A Path Protection/Restoration Mechanism for MPLS Networks
<draft-chang-mpls-path-protection-01.txt>
Status of this memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet-
Drafts.
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."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html
Abstract
It is expected that MPLS-based recovery could become a viable option
for obtaining faster restoration than layer 3 rerouting. To deliver
reliable service, however, multi-protocol label switching (MPLS)[1],
[2] requires a set of procedures to provide protection of the
traffic carried on the label switched paths (LSPs). This imposes
certain requirements on the path recovery process [3], and requires
procedures for the configuration of working and protection paths,
for the communication of fault information to appropriate switching
elements, and for the activation of appropriate switchover actions.
This document specifies a mechanism for path protection switching
and restoration in MPLS networks.
Table of Contents Page
1. Introduction 2
Chang et al [Page 1]
IETF Draft A Path Protection Mechanism for MPLS Networks July 2000
2.0 Purpose and Motivation 3
3.0 Key Features of the Proposed Mechanism 4
4.0 Core MPLS Path Protection Components 7
4.1 Reverse Notification Tree (RNT) 9
4.2 Protection Domain 11
4.3 Multiple Faults 12
4.4 Timers and Thresholds 13
5.0 Configuration 14
5.1 Establishing a Recovery/Protection Path 14
5.2 Creating an RNT 15
5.3 Engineering a Protection Domain 17
5.4 Configuring Timers 18
6.0 Fault Detection 19
7.0 Fault Notification 21
8.0 Switch Over 22
9.0 Switchback or Restoration 22
10.0 Protocol Specific Extensions 22
11.0 Security Considerations 22
12.0 Acknowledgements 22
13.0 Intellectual Property Considerations 22
14.0 AuthorsÆ Addresses 22
15.0 References 23
1.0 Introduction
With the migration of real-time and high-priority traffic to IP
networks, and with the need for IP networks to increasingly carry
mission-critical business data, network survivability has become
critical for future IP networks. Current routing algorithms, despite
being robust and survivable, can take a substantial amount of time, to
recover from a failure, on the order of several seconds to minutes,
which can cause serious disruption of service in the interim. This is
unacceptable for many applications that require a highly reliable
service, and has motivated network providers to give serious
consideration to the issue of network survivability.
Path-oriented technologies, such as MPLS, can be used to support
advanced survivability requirements and enhance the reliability of
IP networks. Different from legacy IP networks, MPLS networks
establish label switched paths (LSPs), where packets with the same
label follow the same path. This potentially allows MPLS networks to
pre-establish protection LSPs for working LSPs, and achieve better
protection switching times than those in legacy IP networks. With
this objective in mind, the present contribution describes a
mechanism to protect paths (or path segments) in MPLS networks.
Before discussing the specifics of this contribution, we first
outline the major components of a path protection solution, and
point out those that are within the scope of this document. A
complete solution for path protection requires the following
components:
(i) A method for selecting the working and protection paths.
(ii) A method for signaling the setup of the working and o practical cases of interest: (a)
when only the end node of a path is responsible for detecting
faults, and (b) when all the nodes along the path are responsible
for detecting faults. The main focus of this draft is the
specification of an efficient fault notification mechanism that
takes LSP merging into account (irrespective of whether they are
physically or virtually merged). Switchover and switchback
mechanisms also are also within the scope of the draft, but
component (vi) is outside the scope of the draft, so the draft does
not specify the details of the mechanisms used to detect that a
fault has been repaired.
2.0 Motivation and Purpose
The framework document [3] lays out the various options for MPLS-
based restoration/recovery. However, candidate approaches
corresponding to various viable recovery options are still needed.
Our work on proposing a path protection mechanism for MPLS networks
is motivated by the belief that path protection (in conjunction with
local repair) will be needed for truly reliable network operation.
The purpose of this contribution is to propose a path protection
mechanism that is:
(i) fast (compared to Layer 3, with the goal of being comparable to
SONET),
(ii) scalable,
(iii)bandwidth efficient,
Chang et al. Expires January 2001 [Page 3]
IETF Draft A Path Protection Mechanism for MPLS Networks July 2000
(iv)allows for path merging (i.e., is merging compatible), and
(v) works at the MPLS layer (that is, only uses knowledge that is
commonly available to MPLS routing and signaling modules).
3.0 Key Features of the Proposed Mechanism
This contribution describes an MPLS-based path recovery mechanism
that can facilitate fast protection switching. The mechanism
currently supports 1:1 protection [3].
Bypass tunneling is for further study. First, because tunnel setup
itself is not adequately defined yet, and second, because even
assuming a tunnel could be setup, in the presence of tunnels (or
tunneled segments) the mechanism still requires the ability to setup
bi-directional tunnels, which is not defined yet. The mechanism has
several timers to enable it to inter-work with protection
mechanisms at other layers. Some of the key features of the
protection mechanism are:
-- Special tree structure to efficiently distribute fault and/or
recovery information.
Existing published proposals for MPLS recovery have not addressed
the issue of fault notification in detail. Specifically, none of
these proposals has discussed how to perform fault notification for
the label merging case. In this draft, we propose a new fault
notification structure called the reverse notification tree (RNT),
which makes fault notification efficient and scalable (we provide
details of the RNT in subsequent sections).
-- Lightweight notification mechanism.
Reliable transport mechanisms, such as TCP, are typically state-
oriented and therefore difficult to scale. It is also very difficult
to support point-to-multipoint communications based on reliable
transport mechanisms. In our scheme, therefore, we use a stateless
notification mechanism to achieve scalability. The notification is
based on the transmission of UDP encapsulated IP packets that are
sent periodically until the nodes responsible for switchover learn
of the fault. Since no acknowledgements or handshaking between
adjacent nodes is needed, the mechanism works only with timers and
does not require the maintenance of state.
--Minimize delays of a recovery cycle.
An objective of the mechanism proposed in this draft is to minimize
the duration of the recovery cycle. Thus a stateless transport
mechanism together with high priority for control traffic minimizes
notification delay. Likewise, a simple label merging approach to
handle the traffic arriving on the working and protection paths
Chang et al. Expires January 2001 [Page 4]
IETF Draft A Path Protection Mechanism for MPLS Networks July 2000
eliminates the need for synchronization (or handshaking) between the
LSRs at the two ends of a recovery path. .
-- Work at the MPLS layer (that is, use information available to the
MPLS signaling and routing modules at the nodes)
The mechanism is designed to operate using only MPLS constructs and
the knowledge available to the MPLS modules at the nodes. Therefore,
the mechanism assumes, by default, that the working and protection
paths merge at a path merge LSR (PML) within the domain under
consideration. However, since the mechanism does not depend on the
path selection method, it also works in settings where a PML does
not exist, and a path selection algorithm (outside the scope of this
I-D) determines that the working and protection paths must terminate
at different egress LSRs. Note, however, that for the path selection
mechanism to be able to make this determination, it may need
knowledge beyond that which is commonly available to the MPLS
modules at a node. This is because determining whether a working
path can be protected by another path with a different egress LSR
requires Layer 3 knowledge to ascertain whether the LSR terminating
the recovery path is acceptable. In the remainder of this document,
we will focus on the PML case, with the understanding that the non-
PML case is also supported.
-- Ability to permit recovery mechanisms at different layers to
coexist.
In the evolving IP network infrastructure, recovery will
increasingly be possible at different layers, and interworking of
recovery mechanisms at different layers will be needed to ensure
smooth network operation in the presence of faults. For example,
optical layer or SONET layer recovery mechanisms could be used to
recover optical paths or SONET channels. However, these mechanisms
may initially be limited to ring topologies and may not provide the
right level of granularity at which recovery might be desired,
making MPLS-based protection desirable.
While MPLS-based protection can be faster than IP layer rerouting it
may be more costly to use (it may need to reserve extra bandwidth
for example). In certain cases it may become too complicated for
MPLS protection to be effective (for example, when there are
multiple faults, or faults on both the working and the protection
paths), making it necessary for IP layer rerouting to take over. In
this draft, we assume that MPLS layer protection is used first, and,
if necessary, recovery may be handed to IP layer rerouting following
that.
In addition to the key features outlined above, some other
characteristics of the mechanism are:
-- A liveness message to detect faults.
Although fault detection is outside the scope of this draft, we will
allow the existence of a generic æælivenessÆÆ message that can
complement any fault detection mechanism. This liveness message may,
Chang et al. Expires January 2001 [Page 5]
IETF Draft A Path Protection Mechanism for MPLS Networks July 2000
for example, be provided as part of an user/control plane OAM
function, or by an existing Hello message (as the RSVP ôHelloö
message) with an appropriately set timer value. We do not define
specific liveness mechanisms in this draft, deferring instead to
work on OAM in MPLS, which is where we expect such a liveness
message to be defined.
Our assumption is that faults fall into different classes, and that
different faults may be detected and corrected by different layers.
Some faults (for example, the loss of signal or transmitter faults)
may be detected and corrected by lower layer mechanisms (such as
SONET), while others (for example, failure of the reverse link) may
be detected (but may not be corrected) by lower layers and may be
communicated to the MPLS layer. Still other faults (such as node
failures or faults on the reverse link) may not be detected by lower
layers, and will have to be detected and corrected at the MPLS
layer. Therefore, we adopt the liveness message as a complementary
fault detection mechanism.
We note that in this draft we confine our discussion of protection
to a single MPLS domain, and do not consider protection/recovery
across multiple MPLS domains or across multiple administrative
boundaries. We note, however, that protection mechanisms in
different domains may be concatenated, and that (at least initially)
these mechanisms may work autonomously, across the (possibly)
multiple points of attachment between two adjacent domains. However,
coordination of protection mechanisms across multiple domains or
across multiple transport technologies is currently out of the scope
of this document.
4.0 Core MPLS Path Protection Components
This document assumes the terminology given in[1], [2], [3] , and
introduces some additional terms. For the convenience of the reader,
we repeat here some of the terminology from earlier documents.
Working Path
The protected path that carries traffic before the occurrence of a
fault. The working path is the part of the LSP between the PSL and
the PML (if any) or, in the absence of a PML, between the PSL and an
egress LSR. A working path is denoted by the sequence of LSRs
through which it passes. For example, in Fig. 1, the working path
that starts at LSR 1 and terminates at LSR 7 is denoted by (1-2-3-4-
6-7).
Recovery Path
The path by which traffic is restored after the occurrence of a
fault. In other words, the path along which traffic is directed by
the recovery mechanism. The recovery path can either be an
equivalent recovery path and ensure no reduction in quality of
service or be a limited recovery path and thereby not guarantee the
same quality of service (or some other criteria of performance) as
the working path. A recovery path is also denoted by the sequence of
Chang et al. Expires January 2001 [Page 6]
IETF Draft A Path Protection Mechanism for MPLS Networks July 2000
LSRs through which it passes. Again, in Fig. 1, the recovery path
that starts at LSR 1 and terminates at LSR 7 is denoted by (1-5-7).
Path Switch LSR (PSL)
An LSR that is the transmitter of both the working path traffic and
its corresponding recovery path traffic. The PSL is responsible for
switching of the traffic between the working path and the recovery
path. The PSL is the origin of the recovery traffic, but may or may
not be the origin of the working traffic (that is the working path
may be transiting the PSL).
Path Merge LSR (PML)
An LSR that receives both working path traffic and its corresponding
recovery path traffic, and either merges their traffic into a single
outgoing path, or, it is itself the destination, passes the traffic
on to the higher layer protocols. The PML is the destination of the
recovery path but may or may not be the destination of the working
path.
Intermediate LSR
An LSR on a working or recovery path that is neither a PSL nor a PML
for that path.
FIS (Fault Indication Signal)
A signal that indicates that a fault along a path has occurred. It
is relayed by each intermediate LSR to its upstream or downstream
neighbor, until it reaches an LSR that is set up to perform MPLS
recovery.
FRS (Fault Recovery Signal)
A signal that indicates that a fault along a path has been repaired.
Like the FIS, it is relayed by each intermediate LSR to its upstream
or downstream neighbor, until it reaches an LSR that performs a
switchback to the path for which the FIS was received.
Liveness Message
A generic name for any message exchanged periodically between two
adjacent LSRs that serves as a link probing mechanism. It provides
an integrity check of the forward and backward directions of the
link between the two LSRs as well as a check of neighbor liveness.
Path Continuity Test
A test that verifies the integrity and continuity of a path or a
path segment. The details of such a test are beyond the scope of
this draft. (This could be accomplished, for example, by sending a
control message along the same links and nodes as those traversed by
the data traffic.)
4.1 Reverse Notification Tree
Since LSPs are unidirectional entities and recovery requires the
notification of faults to the LSR(s) responsible for switchover to
Chang et al. Expires January 2001 [Page 7]
IETF Draft A Path Protection Mechanism for MPLS Networks July 2000
the recovery path, a mechanism must be provided for the fault
indication and the fault repair notification to travel from the
point of occurrence of the fault back to the PSL(s). The situation
is complicated in the following two cases:
(i) Physically merged LSPs: With label mergingmultiple working paths
may converge to form a multipoint-to-point tree, with the PSLs as
the leaves. In this case, therefore, the fault indication and -
repair notification should be able to travel along a reverse path of
the working path to all the PSLs affected by the fault. For example,
in Fig. 1, for a fault along link 34 the affected PSLs are 1 and 9,
where as for a fault along link 23, the only affected PSL is 1.
(ii) Virtually merged LSPs: When several LSPs originating at
different LSRs share a common segment beyond some node, and share a
common identifier (such as the SESSION ID in RSVP-TE), we call such
LSPs virtually merged. In this case also, savings in notification
can be realized by sending a single notification towards the
affected PSLs along segments shared by the LSPs emanating from these
PSLs, and allowing the notification to branch out at the merge
node(s). For example, in Fig. 1, for a failure along link 67 a
single notification could be sent for working paths 1-2-3-4-6-7 and
8-9-3-4-6-7 along their common segment 7-6-4-3. The notification
would branch out at node 3, which is the node where the LSP from
node 1 to node 7 and the LSP from node 8 to node 7 merge.
In both the cases above, an appropriate notification path can be
provided by the reverse notification tree (RNT which is a point-to-
multipoint tree that is an exact mirror image of the converged
working paths, along which the FIS and the FRS travel. (see Fig.
1). There are several advantages to using an RNT:
-- The RNT can be established in association with the working
path(s), simply by making each LSR along a working path remember its
upstream neighbor (or the collection of upstream neighbors whose
working paths converge at the LSR and exit as one). Thus, no
multicast routing is required. We elaborate more on the RNT in
Section 3.
-- Only one RNT is required for all the working paths that merge
(either physically or virtually) to form the multipoint-to-point
forward path. The RNT is rooted at an appropriately chosen LSR along
the common segment of the merged working LSPs and is terminated at
the PSLs. All intermediate LSRs on the converged working paths share
the same RNT.
Therefore, the RNT enables a reduction in the signaling overhead
associated with recovery. Unlike schemes that treat each LSP
independently, and require signaling between a PSL and the PML
individually for each LSP, the RNT allows for only one (or a small
number of) signaling messages on the shared segments of the LSPs.
-- The RNT can be implemented either at Layer 3 or at Layer 2. In
either case, the delay along the RNT needs to be carefully
controlled. This may be ensured by giving the highest priority to
Chang et al. Expires January 2001 [Page 8]
IETF Draft A Path Protection Mechanism for MPLS Networks July 2000
the fault and repair notification packets, which travel along the
RNT.
PML
+----+ L[11,13] +----+ +----+
| 11 |------+ +======| 14 |=========================| 15 |
| | | || | | P[14,15] | |
+----+ | || +----+ +----+
| || | :
+----+ ||P[13,14] | |
| 13 |======+ | :
PSL | |-------+ | |
+----+<-..-: | | :
| | | | |
L[12,13]| : |L[13,5] | :
+----+ | +----+ L[5,15] | |
| 12 |------+ | |-----------------------------------+ :
| | +===| 5 |<-.-..-..-..-..-..-..-..-..-..-..-..-+
+----+ || | |======================================+
P[1,5] || +----+ P[5,7] ||
+============+ ||
|| ||
|| ||
+----+ +----+ L[2,3] L[4,6] +----+ L[6,7] +----+
| 1 |----| 2 |--------+ +-------| 6 |----------| 7 |
| |<.-.| |<-..-+ | | +-..-<| |<-..-..-..| |
+----+ +----+ N32 : |I23 | : +----+ +----+
PSL | | | | PML ||
: | | : ||
| | | | ||
: | L[3,4] | : ||
+----+ I34 +----+ ||
| 3 |--------| 4 | P[10,7]||
| |<-..-..-| | ||
+----+ N43 +----+ ||
I93 | | ||
| : ||
| |N39 ||
| : ||
+----+ +----+ | | +----+ ||
| 8 |-----| 9 |----+ : | 10 |=============+
| | | |<-..-.+ P[9,10] | |
+----+ +----+==========================+----+
PSL
Legend:
--- = Working path
=== = Protection path
-..- = Reverse Notification Tree
---- = Working path
L[x,y] = Working path link between nodes x and y.
P[x,y] = Protection path link between nodes x and y.
Lxy = Label used by the LSP traversing link L[x,y] from x to y.
Nxy = Label used for RNT traffic sent from node x to node y.
Ixy = Interface between nodes x and y.
Chang et al. Expires January 2001 [Page 9]
IETF Draft A Path Protection Mechanism for MPLS Networks July 2000
Figure 1: Illustration of MPLS protection configuration
4.2 Protection Domain
A protection domain is defined as the set of LSRs over which a
working path and its corresponding recovery path are routed. Thus,
a protection domain is bounded by the LSRs that provide the
switching and (if needed) the merging functions for MPLS protection,
namely, the PSL and the PML (if present), respectively.
In other words, a protection domain in bounded by the PSL at one
end, and by the LSRs that form the end of the working or protection
path at the other. In general, if the working and protection paths
do not merge within the MPLS domain, the LSRs at the end of the
working and protection paths may be egress LSRs. The PSL and the PML
(alternatively, the end points of the working and protection paths
within the MPLS domain under consideration) are identified during
the setting up of an LSP, either via an offline algorithm or by an
algorithm that runs at the head-end of an LSP to decide the specific
nodes that the LSP must pass through. (Note that segments of the LSP
between the PSL and the PML may be loosely routed, as long as the
PSL and PML are known). Recovery should ideally be performed between
the source and destination (end-to-end), but in some cases segment
recovery may be desired (for example, when certain segments are more
unreliable than others) or may be the only option (due to the
topology of the network, see Fig. 1). For example, in Fig. 1, the
working path 8-9-3-4-6-7, can only have protection on the segment 9-
3-4-6-7.
Note that when multiple LSPs merge into a single LSP or when
multiple LSPs that share a common identifier follow the same path
beyond some node, the working paths corresponding to these LSPs also
converge. As explained in Section 2.4, an RNT can be used in this
case for propagating the failure and repair notification back to the
concerned PSL(s). We can therefore have a situation where different
protection domains share a common RNT. A protection domain is
denoted by specifying the working path and the recovery path. For
example, in Fig. 1, the protection domain bounded by LSR 1 and LSR
7, is denoted by (1-2-3-4-6-7, 1-5-7).
4.2.1 Relationship between protection domains with different RNTs
When protection domains have different RNTs, two cases may arise,
depending on whether or not any portions of the two domains overlap,
that is, have nodes or links in common. If the protection domains do
not overlap, the protection domains are independent (note that by
virtue of the RNTs in the two domains being different, neither the
working paths nor the RNTs in the two domains can overlap). In other
words, failures in one domain do not interact with failures in the
other domain. For example, the protection domain defined by (9-3-4-
6-7, 9-10-7) is completely independent of the domain defined by (11-
13-5-15, 11-13-14-15). As a result, as long as faults occur in
independent domains, the network shown in Fig. 1 can tolerate
Chang et al. Expires January 2001 [Page 10]
IETF Draft A Path Protection Mechanism for MPLS Networks July 2000
multiple -faults (for example, simultaneous failures on the working
path in each domain).
If protection domains with disjoint RNTs overlap, it implies that
the protection path of one intersects the working path of the other.
Therefore, although failures on the working paths of the two domains
do not affect oneanother, failures on the protection path of one may
affect the working path of the other and visa versa. For example,
the protection domain defined by (1-2-3-4-6-7, 1-5-7) is not
independent of the domain defined by (11-13-5-15, 11-13-14-15) since
LSR 5 lies on the protection path in the former domain and on the
working path in the latter domain.
4.2.2 Relationship between protection domains with the same RNT
When protection domains have the same RNT, different failures along
the working paths may affect both paths differently. As shown in
Fig. 1, for example, working paths 1-2-3-4-5-7 and 9-3-4-6-7 share
the same RNT. As a result, for a failure on some segments of the
working path, both domains will be affected, resulting in a
protection switch in both (for example, the segment 3-4-6-7 in Fig.
1). Likewise, for failures on other segments of the working path,
only one domain may be affected (for example, failure on segment 2-3
affects only the first working path 1-2-3-4-6-7, where as failure on
the segment 9-3 affects only the second working path 9-3-4-6-7).
4.3 Multiple Faults
We note that transferring the working traffic to the recovery path
is enough to take care of multiple faults on the working path.
However, if multiple faults happen such that there is at least one
failure on both the working and recovery paths, MPLS layer recovery
may no longer suffice. In this case, the network will either have to
allow for Layer 3 rerouting or have the PSL inform the administrator
via an alarm, thus enabling the manual reconfiguration of a
different working and backup path. (An OAM functionality could
greatly simplify such communication.) Note that for a PSL to be able
to generate an alarm, it must also have a mechanism for detecting
faults on the recovery path, such as a RNT for the recovery path (to
allow for the fault notification on the recovery path to be
propagated to the PSL).
4.4 Timers and Thresholds
For its proper operation, the protection mechanism described in this
contribution uses the following timers and thresholds:
Timer or Sym Function
Threshold bol
Chang et al. Expires January 2001 [Page 11]
IETF Draft A Path Protection Mechanism for MPLS Networks July 2000
Inter FIS t1 Interval at which successive FIS packets are
packet timer transmitted by a LSR to its upstream
neighbor.
Max. FIS t2 Max. time for which FIS packets are
duration timer transmitted by an LSR to its upstream peer.
Inter FRS t1Æ Interval at which successive FRS packets are
packet timer sent by a LSR to its upstream neighbor.
Max. FRS t2Æ Max. time for which the FRS packets are sent
duration timer by an LSR to its upstream neighbor.
Liveness msg. t4 Interval at which successive liveness
sender timer messages are sent by an LSR to peer LSRs that
have a working path (and RNT) through this
LSR.
Liveness msg. t4' A timer set to count down the interval at the
receiver end of which a liveness message should be
timeout timer received.
Hold-off Timer t5 Interval between the detection of a failure
[3] at an LSR, and the generation of the first
FIS message, to allow time for lower layer
protection to take effect.
Wait-to-Restore T6 Interval between the detection of a
Timer [3] recovery/failure at an LSR, and the
generation of the first FRS message, to allow
time for the stability of restoration.
Lost liveness K No. of liveness messages that can be lost
message before an LSR will declare a fault and
threshold generate the first FIS.
Table 1: Illustrating the various timers.
5.0 Configuration
Chang et al. Expires January 2001 [Page 12]
IETF Draft A Path Protection Mechanism for MPLS Networks July 2000
In the following sections, we describe the operation of the path
protection mechanism, and explain the various steps involved with
reference to Fig. 1.
Protection configuration consists of two aspects: establishing the
protection path and creating the reverse notification tree.
5.1 Establishing a Recovery/Protection Path
The establishment of the recovery path requires the identification
of the working path, and hence the protection domain. In most cases,
the working path and its corresponding recovery path would be
specified during LSP setup, either via a path selection algorithm
(running at a centralized location or at an ingress LSR) or via
administrative configuration. Observe that the specification of the
path, does not, strictly speaking, require the entire path to be
explicitly specified. Rather, it requires only that the PSL and PML
(or in the absence of a PML, the path egress points out of the MPLS
domain) be specified, with the segments between them being - loosely
routed, if required. In other words, the path would be established
between the two nodes at the boundaries of the protection domain via
(possibly loose) explicit (or source) routing using LDP [4], [5]
/RSVP [6], [7]signaling (alternatively, via constraint-based
routing, or using manual configuration).
Ingress Ingress Egress Egress Egress Egress
Label of Interface Label of Interface Label of Interface
RNT of RNT RNT of RNT RNT of RNT
N43 I34 N32 I23 N39 I93
Table 2. An example inverse cross-connect table for LSR 3 using MPLS
(Layer 2) RNT
Egress Egress Next Hop Egress Ingress
Label of Interface IP Interface Label of
Working of Address of RNT working
Path Working of RNT path
Path
L34 I34 I9 I93 L93
I2 I23 L23
Table 3. An example inverse cross-connect table for LSR 3 using a
hop-by-hop (Layer 3) RNT
The roles of the various core protection/recovery components are:
Chang et al. Expires January 2001 [Page 13]
IETF Draft A Path Protection Mechanism for MPLS Networks July 2000
PSL: The PSL initiates the working LSP and the recovery LSP. It is
also responsible for storing information about which LSPs (or
portions thereof) have protection enabled, and for maintaining a
binding between outgoing labels specifying the working path and the
protection/recovery path. The latter enables the switchover to the
recovery path upon the receipt of a protection switch trigger. The
PSL also maintains the timers, t4, t4Æ, t5, t6, and the threshold K.
PML: The PML participates in the setting up of a recovery path as a
merging LSR. . Therefore, it learns during signaling (or
configuration) about which working and protection paths are merged
to the same outgoing LSP. The PML also maintains timers t1, t1',t2,
t2Æ, t4, t4', t5, t6, and the threshold K.
Intermediate LSR: An intermediate LSR participates in the setup of
the recovery path, either as a normal LSR or as a merging LSR. It
also maintains timers t1, t1', t2, t2Æ, t4, t4Æ, t5, t6, and the
threshold K.
5.2 Creating the RNT
The RNT is used for propagating the FIS and the FRS, and can be
created by a simple extension to the LSP setup process. During the
establishment of the working path, the signaling message carries
with it the identity (address) of the upstream node that sent it
(for example, via the path attribute in RSVP). Each LSR along the
path simply remembers the identity of its immediately prior upstream
neighbor on each incoming link. Through the neighbor discovery
mechanism of the routing protocol, each LSR finds the interface
connecting it to the upstream LSRs. (It is assumed in this draft
that there is a bi-directional connection between two neighboring
LSRs, such as a bi-directional SONET link, a bi-directional lower
layer network link (e.g., an ATM VP), or a pair of bi-directional
tunnels over an IP subnetwork.) The node then creates an ææinverseÆÆ
cross-connect table that for each protected outgoing LSP maintains a
list of the incoming LSPs that merge into that outgoing LSP,
together with the identity of the upstream node and incoming
interface that each incoming LSP comes through. Upon receiving an
FIS, an LSR extracts the labels contained in it (which are the
labels of the protected LSPs that use the outgoing link that the FIS
was received on) and checks whether the current LSR is the PSL for
that LSP. If it is it terminates the FIS. Otherwise, it consults
its inverse cross-connect table to determine the identity of the
upstream nodes that the protected LSPs come from, and creates and
transmits an FIS to each of them.
Therefore, based on whether the RNT is implemented at Layer 3 or
Layer 2, two cases arise:
If the RNT is implemented by a point-to-multipoint LSP, then the
working path can be bound to the ingress label and interface of the
RNT LSP at a LSR. The ingress label and interface can then be used
as an index into the "inverse" cross-connect table to find the
egress labels and interfaces of the RNT LSP as shown in Table 2.
Chang et al. Expires January 2001 [Page 14]
IETF Draft A Path Protection Mechanism for MPLS Networks July 2000
Upon receiving an FIS, an LSR extracts the labels and checks whether
it is the PSL for that LSP. If it is, it terminates the FIS.
Otherwise, it consults its inverse cross-connect table to determine
the outgoing labels and interfaces, performs a label swap and
forwards the FIS to the appropriate upstream node(s). For example,
consider Figure 1, and assume that a Layer 2 point-to-multipoint
RNT, rooted at LSR 7 and extending to LSRs 1 and 9, is bound to the
multipoint-to-point forward paths starting at LSRs 1 and 8 and
terminating at LSR 7. Now in case of a fault on link L[4,6], LSR 3
receives an FIS on the RNT in a labeled packet with label N43. It
uses this label as an index into its inverse cross-connect table,
and learns that there are two previous nodes (namely those reachable
via interfaces I23 and I93 respectively) that the FIS needs to be
forwarded to. It encapsulates the received FIS into a labeled
packets with labels N32 and N39, and dispatches them along
interfaces I23 and I93 respectively.
If the RNT is implemented by a hop-by-hop Layer 3 mechanism, using,
for example, UDP packets (with a specific port number to identify
notification message type), then the egress label and interface of
the working path can be used as an index into the inverse cross-
connect table to obtain the IP addresses of the previous hop(s) and
the associated outgoing interface(s), as illustrated in Table 3. On
each hop, the FIS carried in the UDP packet carries the label and
interface of the working path for that hop. Thus, if the receiving
node is not a PSL, the label and interface in the FIS can be
extracted and can be used to access the inverse cross-connect table.
The label and interface used by the working LSP on the hop(s) to the
upstream node(s) are then inserted into FIS packet(s), and the FIS
packet(s) transmitted to the appropriate upstream node(s) along the
interface specified the inverse cross-connect table. Therefore, in
the hop-by-hop mechanism the FIS packets are not forwarded by a node
to its previous hops using its default layer 3 forwarding table.
Rather, these packets are forwarded via the outgoing interface
extracted from the nodeÆs inverse cross-connect table. As in the
example above, in case of a fault on link L[4,6], LSR 3 receives an
FIS from LSR 4 that contains the outgoing label L34 and the outgoing
interface I34 of the LSP affected by the fault. LSR3 uses these to
index its inverse cross-connect table (see Table 3), and learns, as
before, that there are two previous nodes (those reachable via
interfaces I23 and I93, respectively) that must receive an FIS. It
then creates two FIS packets as follows. The first, for transmission
along interface I23, contains the label L23 used by LSR 2 to
transmit data to LSR 3 along the working LSP. The second, for
transmission along interface I93, contains the label L93 used by LSR
9 to transmit data to LSR 3 along the working LSP.
The roles of the various core protection/recovery components are:
PSL: The PSL must be able to correlate the RNT with the working and
recovery paths. To this end, it maintains a table with a list of
working LSPs protected by an RNT, and the identity of the recovery
LSPs that each working path is to be switched to in the event of a
Chang et al. Expires January 2001 [Page 15]
IETF Draft A Path Protection Mechanism for MPLS Networks July 2000
failure on the working path. It need not maintain an inverse cross-
connect table (for those LSPs and working paths for which it is the
PSL).
PML: The PML, in general, has to remember all of its upstream
neighbors and associate them with the appropriate working paths and
RNTs. If the PML is also the root of the RNT, it has to associate
each of its upstream nodes with a working path and RNT, but it need
not maintain an inverse cross-connect table (for those LSPs and
working paths for which it is a PML).
Intermediate LSR: An intermediate LSR has to only remember all of
its upstream neighbors and associate them with the appropriate
working paths and RNTs, and has to maintain an "inverse" cross-
connect table.
5.3 Engineering a Protection Domain
For 1:1 protection, the bandwidth (if any) reserved for a
protection/recovery path should be the same as the bandwidth
reserved for its corresponding working path. This guarantees the
same bandwidth for the protected traffic after protection switching.
If the LSRs on the protection path support excess mode [3], the
bandwidth reserved on the protection path for protecting high
priority traffic can be used by other lower priority traffic
streams. That is, lower priority traffic that has a segment in
common with the recovery path, use the bandwidth of the recovery
path, as long as the recovery path is not called into use. When the
recovery path is pressed into service, the low priority traffic will
be discarded to allow for the actual working traffic to take its
place. Also, if delay, jitter or other QoS parameters are to be
satisfied, the protection path in 1:1 protection should be chosen
such that these requirements are satisfied.
Since the volume of signaling traffic (e.g., FIS/FRS messages, or
liveness messages) is small, in general bandwidth need not be
reserved for the signaling traffic provided that there exist other
mechanisms that can ensure that the delay requirements of signaling
messages are met (by using, for example, the highest priority for
signaling messages).
For bypass tunneling protection, multiple working LSPs may share the
same protection bandwidth by tunneling protection LSPs over a common
path. This requires that the working paths of these LSPs be
disjoint, except at the PSL and PML, so that they can be assumed to
not all fail at the same time. In this case, the bandwidth reserved
on the tunnel will be the maximum of all individual paths.
Otherwise, a bypass tunnel could be created to carry all the backup
paths, with the bandwidth reserved for the tunnel being the maximum
bandwidth required over all failure scenarios on the working LSPs.
3.4 Configuring Timers
Chang et al. Expires January 2001 [Page 16]
IETF Draft A Path Protection Mechanism for MPLS Networks July 2000
The purpose of timers t1/t1' is to control the tradeoff between
notification delay of the FIS/FRS and the resources consumed when
sending the FIS/FRS. If t1/t1' is large, it may take a relatively
long time for the node that initiated the FIS/FRS transmission to
send the second the FIS/FRS if the first FIS/FRS message is lost,
thereby increasing notification delay. On the other hand, if t1/t1'
is small, the repetitive sending of FIS/FRS messages may waste
bandwidth and processing power because the first message may already
have reached the PSL(s).
It is assumed that after t2/t2' it is not necessary to do protection
at MPLS layer, either because it is no longer useful or because by
that time an upper layer protection mechanism will have been
triggered.
The timers t4/t4' are used to control the frequency of liveness
messages sent between neighboring LSRs, so their purpose is the same
as those of timers t1/t1Æ. While frequent exchanges of liveness
messages can unnecessarily consume network resources, too few
exchanges may delay the discovery of faults. To accommodate delay
jitter, t4' may be set at a slightly different value from t4.
The timers t5/t6 are used to allow lower layer protection to take
effect before initiating MPLS layer recovery mechanisms (for
example, an automatic protection switching between fibers that
comprise a link between two LSRs). Following the detection of a
fault/fault repair S/FRS packet, respectively. This allows for the
lower layer protection to take effect and for the LSR to learn this
through one of several ways: via an indication from a lower layer,
or by the resumption of the reception of a liveness message, or by
the lack of LF, LD, PF or PD conditions (see definitions in [3]).
The threshold K helps to minimize false alarms due to the occasional
loss of a liveness message, which may occur, for example, either due
to a temporary impairment in a link or a peer LSR or due to a buffer
overflow.
6.0 Fault Detection
Each LSR must be able to detect certain types of faults, such as PF,
PD, LF, and LD [3] and propagate an FIS message towards the PSL.
Here we consider unidirectional link faults, bi-directional (or
complete) link faults, and node faults.
Essentially, the node upstream of the fault must be able to
detect/learn about the fault. This motivates the need for a
"liveness" message, which allows a node upstream of the fault to
detect the fault either directly or implicitly. Also, the fault
detection mechanism must provide the trigger for generating the FIS.
Broadly, the types of mechanisms that could be triggers for the FIS
are:
i) Lower layer mechanisms
Chang et al. Expires January 2001 [Page 17]
IETF Draft A Path Protection Mechanism for MPLS Networks July 2000
ii) MPLS-based detection mechanisms, which may be used to detect
link faults, via a liveness message for example.
iii) User-plane OAM mechanisms, such as a path continuity test,
which may be used to detect other faults, such as mis-
connections (arising from incorrect entries in the label
forwarding table, for example).
6.1 Unidirectional Link Fault
A uni-directional link fault implies that only one direction of a
bi-directional link has experienced a fault.
6.1.2 Downlink Fault
A fault on a link in the downstream direction will be detected by
the node downstream of the faulty link, either via the PF or PD
condition being detected at the MPLS layer, or via LF or LD signals
being propagated to the MPLS layer by the lower layer or via the
absence of liveness messages. The downstream node will then
periodically transmit FIS messages to its upstream neighbor (via the
uplink), which will propagate these further upstream (using its
inverse cross-connect table) until they eventually reach the
appropriate PSLs, which will perform the protection switch.
Therefore, in Fig. 1, if link L34 has a fault, LSR 4 will detect the
fault via one of the means described above, and start transmitting
an FIS packet once every t1 time units back to LSR 3 over link L43.
The traffic in the queues of LSR 4 will continue to be serviced. LSR
3 in turn will propagate the FIS over the RNT back to LSR 2 and LSR
9. The actual protection switch will be performed by LSRs 9 and 1,
t3 time units after the receipt of the first FIS. LSR 4 will stop
transmitting FIS messages t2 time units after the transmission of
the first FIS message.
6.1.3 Uplink Fault
A fault on a link in the upstream direction will be detected by a
node upstream of the faulty link, either via a LF or LD being
detected at the lower layer and propagated to the MPLS layer (if
there was traffic on this reverse link), or via the PD or PF
condition being detected at the MPLS layer, or via absence of
liveness messages. The upstream node will then periodically send out
FIS messages to the node upstream of it, which in turn will
propagate these further until eventually the PSL(s) learns of the
failure and performs the protection switch.
Therefore, in Fig. 1, if link L43 experiences a fault, LSR 3 will
detect the fault, and transmit an FIS to nodes 2 and 9. Node 2, in
turn, will transmit an FIS to node 1, and nodes 1 and 9 will perform
the actual protection switch
6.2 Bi-directional link fault or Node Fault
Chang et al. Expires January 2001 [Page 18]
IETF Draft A Path Protection Mechanism for MPLS Networks July 2000
When both directions of the link have a fault (as in the case of a
fiber cut), nodes at both ends of the link will detect the fault
either due to the LF or PF signal or due to the absence of liveness
messages. Both will transmit FIS messages to their upstream nodes.
However, it is only the node upstream of the failed link whose FIS
messages will propagate further upstream, eventually reaching the
appropriate PSLs, which will perform the protection switch to the
recovery path.
The case of a node fault is similar, with the node upstream of the
failed node detecting the failure (due to loss of liveness messages,
for example) and propagating that information via the FIS message.
For example in Fig. 1, when both directions of the link between
nodes 3 and 4 experience a fault (or when node 4has a fault), LSR 3
will detect this failure via the non-reception of the liveness
message, and transmit FIS messages to nodes 2 and 9 as before. When
nodes 1 and 9 receive the FIS message they will perform the
protection switch after waiting for an interval of t3 time units.
The roles of the various core protection components in failure
detection are the same. The PSL, PML, and intermediate LSR must all
be able to detect PF and PD conditions and/or be able to interpret
and respond to the LF and LD indications received from the lower
layers.
7.0 Fault Notification
The rapid notification of a fault is effected by the propagation of
the FIS message along the RNT. Due to the timers built into the
FIS/FRS propagation mechanism, the transportation of FIS/FRS
messages does not require a reliable mechanism like TCP. Any LSR
may generate an FIS, but a PSL is the only LSR that may terminate
it.
For instance, in Fig. 1 if link L23 fails, LSR 3 will detect it and
transmit a FIS to LSR 2 (after waiting for time T2), its upstream
neighbor along link L23. The FIS will contain the incoming labels
(at node 3) of those LSPs on link L23 that have protection enabled.
Upon receiving the FIS message, LSR 2 will consult its inverse-cross
connect table and generate an FIS message for LSR 1, which on
receiving the first FIS packet will wait for time t3 before
performing a protection switch. The node which initiates the FIS
will continue to send FIS messages at an interval of t1 until timer
t2 expires. After t2 expires it is assumed that either upper layer
protection will be triggered or enough number of FIS messages will
have been sent to reach the desired reliability in conveying fault
information to the PSL(s).
The roles of the various core protection switching components are:
PSL: The PSL does not generate a FIS message, but must be able to
detect FIS packets.
Chang et al. Expires January 2001 [Page 19]
IETF Draft A Path Protection Mechanism for MPLS Networks July 2000
PML: The PML must be able to generate the FIS packets in response to
detecting failure, and should transmit them over the RNT. The PML
begins FIS transmission after continuously detecting a fault for T2
time units, and does so every t1 time units for a maximum of t2 time
units.
Intermediate LSR: An intermediate LSR must be able to
generate/forward FIS packets, either as a result of continuously
detecting a fault for T2 time units or in response to a received FIS
packet. It must transmit these to all its affected upstream
neighbors as per its inverse cross-connect table. Again, it does so
every t1 time units for a maximum of t2 time units.
8.0 Switch Over
The switch over is the actual switching of the working traffic from
the working path to the recovery path. This is performed by a PSL,
t3 time units after the reception of the first FIS packet.
For example, in Fig. 1, consider protection domain (1-2-3-4-6-7, 1-
5-7). When link L34 fails, the PSL LSR 1 on learning of the failure
will perform a protection switch of the protected traffic from the
working path 1-2-3-4-6-7 to the backup path 1-5-7. Notice that LSR 7
acts as a protection merge LSR, merging traffic from the working and
backup paths. Since buffered packets from LSR 4 may continue to
arrive at LSR 7 even after the protection switch (the dampening
timer t43at the PSL tends to mitigate this), a short-term
misordering of packets may happen at LSR 7, until the buffers on the
working path drain out.
The role of the core protection components is as follows:
PSL: Performs the protection switch upon receipt of the FIS message,
but after waiting for time t3 following the first FIS message.
PML: The PML automatically merges protection traffic with working
traffic. For a short period of time this may cause misordering of
packets, since packets buffered at LSRs downstream of the fault may
continue to arrive at the PML along the working path.
Intermediate LSR: The intermediate LSR has no special action.
9.0 Switch Back
Switch back or restoration is the transfer of working traffic from
the recovery path to the working path, once the working path is
repaired. This may be because the recovery path may be a limited
recovery path [3], or because the working path is deemed to be
preferred [3] in some respect. Restoration may be automatic or it
may be performed by manual intervention (or not performed at all).
In the revertive mode, restoration is performed upon the receipt of
the FRS message. A path continuity test may be performed to ensure
the integrity of the entire path before switching. I n the non-
revertive mode it may be performed by operator intervention.
Chang et al. Expires January 2001 [Page 20]
IETF Draft A Path Protection Mechanism for MPLS Networks July 2000
The role of the core protection components is similar here to what
it is for protection switching. The PML does not need to do
anything, unless it was the node that detected the failure, in which
case it transmits a FRS upstream T8 time units after continuously
detecting recover signal from lower layer or after detecting
liveness messages from its peers. The intermediate LSR generates the
FRS message if it was the node that detected the recovery or
generates a FRS to relay the restoration status received from a
downstream node. The PSL performs the restoration switch t3Æ seconds
after receiving the first FIS message.
10.0 Protocol Specific Extensions
The signaling protocol specific extensions needed to implement the
mechanism outlined in this draft are discussed in separate documents
[8]
11.0 Security Considerations
The MPLS protection that is specified herein does not raise any
security issues that are not already present in the MPLS
architecture.
12.0 Intellectual Property Considerations
In accordance with the intellectual property rights procedures of
the IETF standards process, to the extent that Tellabs has patents,
pending applications and/or other intellectual property rights that
are essential to implementation of any subject matter submitted by
Tellabs that is included in a standard, Tellabs is prepared to
grant, on the basis of reciprocity (grantback), a license on such
subject matter under terms and conditions that are reasonable and
non-discriminatory.
13.0 Acknowledgements
We would like to thank our colleague Ben Mack-Crane, and members of
the MPLS WG list, in particular Dave Allan, Bora Aykol, Neil Harrisson,
and J. Noel Chiappa, for suggestions, feedback, and corrections to the
first version of this draft.
14.0 AuthorsÆ Addresses
Changcheng Huang Vishal Sharma
Tellabs Operations, Inc. Tellabs Research Center
4951 Indiana Avenue One Kendall Square
Lisle, IL 60532 Bldg. 100, Ste. 121
Phone: 630-512-7754 Cambridge, MA 02139-1562
Changcheng.Huang@tellabs.com Phone: 617-577-8760
Vishal.Sharma@tellabs.com
Chang et al. Expires January 2001 [Page 21]
IETF Draft A Path Protection Mechanism for MPLS Networks July 2000
Srinivas Makam Ken Owens
Tellabs Operations, Inc. Tellabs Operations, Inc.
4951 Indiana Avenue 1106 Fourth Street
Lisle, IL 60532 St. Louis, MO 63126
Phone: 630-512-7217 Phone: 314-918-1579
Srinivas.Makam@tellabs.com Ken.Owens@tellabs.com
15.0 References
[1] Rosen, E., Viswanathan, A., and Callon, R., "Multiprotocol Label
Switching Architecture", Work in Progress, Internet Draft <draft-
ietf-mpls-arch-06.txt>, August 1999.
[2] Callon, R., Doolan, P., Feldman, N., Fredette, A., Swallow, G.,
Viswanathan, A., "A Framework for Multiprotocol Label Switching",
Work in Progress, Internet Draft <draft-ietf-mpls-framework-
05.txt>, September 1999.
[3] Makam, V., Sharma, V., Huang, C., Owens, K., Mack-Crane, B., et
al, "A Framework for MPLS-based Recovery," Work in Progress,
Internet Draft <draft-makam-mpls-recovery-frmwrk-00.txt>,
February 2000.
[4] Andersson, L., Doolan, P., Feldman, N., Fredette, A., Thomas,
B., "LDP Specification", Work in Progress, Internet Draft <draft-
ietf-mpls-ldp-06.txt>, September 1999.
[5] Jamoussi, B. "Constraint-Based LSP Setup using LDP", Work in
Progress, Internet Draft <draft-ietf-mpls-cr-ldp-03.txt>,
September 1999.
[6] Braden, R., Zhang, L., Berson, S., Herzog, S., "Resource
ReSerVation Protocol (RSVP) -- Version 1 Functional
Specification", RFC 2205, September 1997.
[7] Awduche, D. et al "Extensions to RSVP for LSP Tunnels", Work in
Progress, Internet Draft <draft-ietf-mpls-rsvp-lsp-tunnel-04.txt,
September 1999.
[8] Huang, C., Sharma, V., Makam. V, and Owens, K., "Extensions to
RSVP-TE for MPLS Path Protection," Internet Draft, <draft-chang-
rsvpte-path-protection-ext-00.txt>, July, 2000.
Chang et al. Expires January 2001 [Page 22]