5.1. Transport vs. Control Plane Responsibilities
When applicable, recovery resources are provisioned, for both
protection and restoration, using GMPLS signaling capabilities.
Thus, these are control plane-driven actions (topological and
resource-constrained) that are always performed in this context.
The following tables give an overview of the responsibilities taken
by the control plane in case of LSP/span recovery:
1. LSP/span Protection
- Phase 1: Failure Detection Transport plane
- Phase 2: Failure Localization/Isolation Transport/Control plane
- Phase 3: Failure Notification Transport/Control plane
- Phase 4: Protection Switching Transport/Control plane
- Phase 5: Reversion (Normalization) Transport/Control plane
Note: in the context of LSP/span protection, control plane actions
can be performed either for operational purposes and/or
synchronization purposes (vertical synchronization between transport
and control plane) and/or notification purposes (horizontal
synchronization between end-nodes at control plane level). This
suggests the selection of the responsible plane (in particular for
protection switching) during the provisioning phase of the
protected/protection LSP.
2. LSP/span Restoration
- Phase 1: Failure Detection Transport plane
- Phase 2: Failure Localization/Isolation Transport/Control plane
- Phase 3: Failure Notification Control plane
- Phase 4: Recovery Switching Control plane
- Phase 5: Reversion (Normalization) Control plane
Therefore, this document primarily focuses on provisioning of LSP
recovery resources, failure notification mechanisms, recovery
switching, and reversion operations. Moreover, some additional
considerations can be dedicated to the mechanisms associated to the
failure localization/isolation phase.
5.2. Technology-Independent and Technology-Dependent Mechanisms
The present recovery mechanisms analysis applies to any circuit-
oriented data plane technology with discrete bandwidth increments
(like SONET/SDH, G.709 OTN, etc.) being controlled by a GMPLS-based
distributed control plane.
The following sub-sections are not intended to favor one technology
versus another. They list pro and cons for each technology in order
to determine the mechanisms that GMPLS-based recovery must deliver to
overcome their cons and make use of their pros in their respective
applicability context.
5.2.1. OTN Recovery
OTN recovery specifics are left for further consideration.
5.2.2. Pre-OTN Recovery
Pre-OTN recovery specifics (also referred to as "lambda switching")
present mainly the following advantages:
- They benefit from a simpler architecture, making it more suitable
for mesh-based recovery types and schemes (on a per-channel basis).
- Failure suppression at intermediate node transponders, e.g., use of
squelching, implies that failures (such as LoL) will propagate to
edge nodes. Thus, edge nodes will have the possibility to initiate
recovery actions driven by upper layers (vs. use of non-standard
masking of upstream failures).
The main disadvantage is the lack of interworking due to the large
amount of failure management (in particular failure notification
protocols) and recovery mechanisms currently available.
Note also, that for all-optical networks, combination of recovery
with optical physical impairments is left for a future release of
this document because corresponding detection technologies are under
specification.
5.2.3. SONET/SDH Recovery
Some of the advantages of SONET [T1.105]/SDH [G.707], and more
generically any Time Division Multiplexing (TDM) transport plane
recovery, are that they provide:
- Protection types operating at the data plane level that are
standardized (see [G.841]) and can operate across protected domains
and interwork (see [G.842]).
- Failure detection, notification, and path/section Automatic
Protection Switching (APS) mechanisms.
- Greater control over the granularity of the TDM LSPs/links that can
be recovered with respect to coarser optical channel (or whole
fiber content) recovery switching
Some of the limitations of the SONET/SDH recovery are:
- Limited topological scope: Inherently the use of ring topologies,
typically, dedicated Sub-Network Connection Protection (SNCP) or
shared protection rings, has reduced flexibility and resource
efficiency with respect to the (somewhat more complex) meshed
recovery.
- Inefficient use of spare capacity: SONET/SDH protection is largely
applied to ring topologies, where spare capacity often remains
idle, making the efficiency of bandwidth usage a real issue.
- Support of meshed recovery requires intensive network management
development, and the functionality is limited by both the network
elements and the capabilities of the element management systems
(thus justifying the development of GMPLS-based distributed
recovery mechanisms.)
5.3. Specific Aspects of Control Plane-Based Recovery Mechanisms
5.3.1. In-Band vs. Out-Of-Band Signaling
The nodes communicate through the use of IP terminating control
channels defining the control plane (transport) topology. In this
context, two classes of transport mechanisms can be considered here:
in-fiber or out-of-fiber (through a dedicated physically diverse
control network referred to as the Data Communication Network or
DCN). The potential impact of the usage of an in-fiber (signaling)
transport mechanism is briefly considered here.
In-fiber transport mechanisms can be further subdivided into in-band
and out-of-band. As such, the distinction between in-fiber in-band
and in-fiber out-of-band signaling reduces to the consideration of a
logically- versus physically-embedded control plane topology with
respect to the transport plane topology. In the scope of this
document, it is assumed that at least one IP control channel between
each pair of adjacent nodes is continuously available to enable the
exchange of recovery-related information and messages. Thus, in
either case (i.e., in-band or out-of-band) at least one logical or
physical control channel between each pair of nodes is always
expected to be available.
Therefore, the key issue when using in-fiber signaling is whether one
can assume independence between the fault-tolerance capabilities of
control plane and the failures affecting the transport plane
(including the nodes). Note also that existing specifications like
the OTN provide a limited form of independence for in-fiber signaling
by dedicating a separate optical supervisory channel (OSC, see
[G.709] and [G.874]) to transport the overhead and other control
traffic. For OTNs, failure of the OSC does not result in failing the
optical channels. Similarly, loss of the control channel must not
result in failing the data channels (transport plane).
5.3.2. Uni- vs. Bi-Directional Failures
The failure detection, correlation, and notification mechanisms
(described in Section 4) can be triggered when either a uni-
directional or a bi-directional LSP/Span failure occurs (or a
combination of both). As illustrated in Figures 1 and 2, two
alternatives can be considered here:
1. Uni-directional failure detection: the failure is detected on the
receiver side, i.e., it is detected by only the downstream node to
the failure (or by the upstream node depending on the failure
propagation direction, respectively).
2. Bi-directional failure detection: the failure is detected on the
receiver side of both downstream node AND upstream node to the
failure.
Notice that after the failure detection time, if only control-plane-
based failure management is provided, the peering node is unaware of
the failure detection status of its neighbor.
------- ------- ------- -------
| | | |Tx Rx| | | |
| NodeA |----...----| NodeB |xxxxxxxxx| NodeC |----...----| NodeD |
| |----...----| |---------| |----...----| |
------- ------- ------- -------
t0 >>>>>>> F
t1 x <---------------x
Notification
t2 <--------...--------x x--------...-------->
Up Notification Down Notification
Figure 1: Uni-directional failure detection
------- ------- ------- -------
| | | |Tx Rx| | | |
| NodeA |----...----| NodeB |xxxxxxxxx| NodeC |----...----| NodeD |
| |----...----| |xxxxxxxxx| |----...----| |
------- ------- ------- -------
t0 F <<<<<<< >>>>>>> F
t1 x <-------------> x
Notification
t2 <--------...--------x x--------...-------->
Up Notification Down Notification
Figure 2: Bi-directional failure detection
After failure detection, the following failure management operations
can be subsequently considered:
- Each detecting entity sends a notification message to the
corresponding transmitting entity. For instance, in Figure 1, node
C sends a notification message to node B. In Figure 2, node C
sends a notification message to node B while node B sends a
notification message to node C. To ensure reliable failure
notification, a dedicated acknowledgement message can be returned
back to the sender node.
- Next, within a certain (and pre-determined) time window, nodes
impacted by the failure occurrences may perform their correlation.
In case of uni-directional failure, node B only receives the
notification message from node C, and thus the time for this
operation is negligible. In case of bi-directional failure, node B
has to correlate the received notification message from node C with
the corresponding locally detected information (and node C has to
do the same with the message from node B).
- After some (pre-determined) period of time, referred to as the
hold-off time, if the local recovery actions (see Section 5.3.4)
were not successful, the following occurs. In case of uni-
directional failure and depending on the directionality of the LSP,
node B should send an upstream notification message (see [RFC3473])
to the ingress node A. Node C may send a downstream notification
message (see [RFC3473]) to the egress node D. However, in that
case, only node A would initiate an edge to edge recovery action.
Node A is referred to as the "master", and node D is referred to as
the "slave", per [RFC4427]. Note that the other LSP end-node (node
D in this case) may be optionally notified using a downstream
notification message (see [RFC3473]).
In case of bi-directional failure, node B should send an upstream
notification message (see [RFC3473]) to the ingress node A. Node C
may send a downstream notification message (see [RFC3473]) to the
egress node D. However, due to the dependence on the LSP
directionality, only ingress node A would initiate an edge-to-edge
recovery action. Note that the other LSP end-node (node D in this
case) should also be notified of this event using a downstream
notification message (see [RFC3473]). For instance, if an LSP
directed from D to A is under failure condition, only the
notification message sent from node C to D would initiate a
recovery action. In this case, per [RFC4427], the deciding and
recovering node D is referred to as the "master", while node A is
referred to as the "slave" (i.e., recovering only entity).
Note: The determination of the master and the slave may be based
either on configured information or dedicated protocol capability.
In the above scenarios, the path followed by the upstream and
downstream notification messages does not have to be the same as the
one followed by the failed LSP (see [RFC3473] for more details on the
notification message exchange). The important point concerning this
mechanism is that either the detecting/reporting entity (i.e., nodes
B and C) is also the deciding/recovery entity or the
detecting/reporting entity is simply an intermediate node in the
subsequent recovery process. One refers to local recovery in the
former case, and to edge-to-edge recovery in the latter one (see also
Section 5.3.4).
5.3.3. Partial vs. Full Span Recovery
When a given span carries more than one LSP or LSP segment, an
additional aspect must be considered. In case of span failure, the
LSPs it carries can be recovered individually, as a group (aka bulk
LSP recovery), or as independent sub-groups. When correlation time
windows are used and simultaneous recovery of several LSPs can be
performed using a single request, the selection of this mechanism
would be triggered independently of the failure notification
granularity. Moreover, criteria for forming such sub-groups are
outside of the scope of this document.
Additional complexity arises in the case of (sub-)group LSP recovery.
Between a given pair of nodes, the LSPs that a given (sub-)group
contains may have been created from different source nodes (i.e.,
initiator) and directed toward different destination nodes.
Consequently the failure notification messages following a bi-
directional span failure that affects several LSPs (or the whole
group of LSPs it carries) are not necessarily directed toward the
same initiator nodes. In particular, these messages may be directed
to both the upstream and downstream nodes to the failure. Therefore,
such span failure may trigger recovery actions to be performed from
both sides (i.e., from both the upstream and the downstream nodes to
the failure). In order to facilitate the definition of the
corresponding recovery mechanisms (and their sequence), one assumes
here as well that, per [RFC4427], the deciding (and recovering)
entity (referred to as the "master") is the only initiator of the
recovery of the whole LSP (sub-)group.
5.3.4. Difference between LSP, LSP Segment and Span Recovery
The recovery definitions given in [RFC4427] are quite generic and
apply for link (or local span) and LSP recovery. The major
difference between LSP, LSP Segment and span recovery is related to
the number of intermediate nodes that the signaling messages have to
travel. Since nodes are not necessarily adjacent in the case of LSP
(or LSP Segment) recovery, signaling message exchanges from the
reporting to the deciding/recovery entity may have to cross several
intermediate nodes. In particular, this applies to the notification
messages due to the number of hops separating the location of a
failure occurrence from its destination. This results in an
additional propagation and forwarding delay. Note that the former
delay may in certain circumstances be non-negligible; e.g., in a
copper out-of-band network, the delay is approximately 1 ms per
200km.
Moreover, the recovery mechanisms applicable to end-to-end LSPs and
to the segments that may compose an end-to-end LSP (i.e., edge-to-
edge recovery) can be exactly the same. However, one expects in the
latter case, that the destination of the failure notification message
will be the ingress/egress of each of these segments. Therefore,
using the mechanisms described in Section 5.3.2, failure notification
messages can be exchanged first between terminating points of the LSP
segment, and after expiration of the hold-off time, between
terminating points of the end-to-end LSP.
Note: Several studies provide quantitative analysis of the relative
performance of LSP/span recovery techniques. [WANG] for instance,
provides an analysis grid for these techniques showing that dynamic
LSP restoration (see Section 5.5.2) performs well under medium
network loads, but suffers performance degradations at higher loads
due to greater contention for recovery resources. LSP restoration
upon span failure, as defined in [WANG], degrades at higher loads
because paths around failed links tend to increase the hop count of
the affected LSPs and thus consume additional network resources.
Also, performance of LSP restoration can be enhanced by a failed
working LSP’s source node that initiates a new recovery attempt if an
initial attempt fails. A single retry attempt is sufficient to
produce large increases in the restoration success rate and ability
to initiate successful LSP restoration attempts, especially at high
loads, while not adding significantly to the long-term average
recovery time. Allowing additional attempts produces only small
additional gains in performance. This suggests using additional
(intermediate) crankback signaling when using dynamic LSP restoration
(described in Section 5.5.2 - case 2). Details on crankback
signaling are outside the scope of this document.
5.4. Difference between Recovery Type and Scheme
[RFC4427] defines the basic LSP/span recovery types. This section
describes the recovery schemes that can be built using these recovery
types. In brief, a recovery scheme is defined as the combination of
several ingress-egress node pairs supporting a given recovery type
(from the set of the recovery types they allow). Several examples
are provided here to illustrate the difference between recovery types
such as 1:1 or M:N, and recovery schemes such as (1:1)^n or (M:N)^n
(referred to as shared-mesh recovery).
1. (1:1)^n with recovery resource sharing
The exponent, n, indicates the number of times a 1:1 recovery type is
applied between at most n different ingress-egress node pairs. Here,
at most n pairs of disjoint working and recovery LSPs/spans share a
common resource at most n times. Since the working LSPs/spans are
mutually disjoint, simultaneous requests for use of the shared
(common) resource will only occur in case of simultaneous failures,
which are less likely to happen.
For instance, in the common (1:1)^2 case, if the 2 recovery LSPs in
the group overlap the same common resource, then it can handle only
single failures; any multiple working LSP failures will cause at
least one working LSP to be denied automatic recovery. Consider for
instance the following topology with the working LSPs A-B-C and F-G-H
and their respective recovery LSPs A-D-E-C and F-D-E-H that share a
common D-E link resource.
A---------B---------C
\ /
\ /
D-------------E
/ \
/ \
F---------G---------H
2. (M:N)^n with recovery resource sharing
The (M:N)^n scheme is documented here for the sake of completeness
only (i.e., it is not mandated that GMPLS capabilities support this
scheme). The exponent, n, indicates the number of times an M:N
recovery type is applied between at most n different ingress-egress
node pairs. So the interpretation follows from the previous case,
except that here disjointness applies to the N working LSPs/spans and
to the M recovery LSPs/spans while sharing at most n times M common
resources.
In both schemes, it results in a "group" of sum{n=1}^N N{n} working
LSPs and a pool of shared recovery resources, not all of which are
available to any given working LSP. In such conditions, defining a
metric that describes the amount of overlap among the recovery LSPs
would give some indication of the group’s ability to handle
simultaneous failures of multiple LSPs.
For instance, in the simple (1:1)^n case, if n recovery LSPs in a
(1:1)^n group overlap, then the group can handle only single
failures; any simultaneous failure of multiple working LSPs will
cause at least one working LSP to be denied automatic recovery. But
if one considers, for instance, a (2:2)^2 group in which there are
two pairs of overlapping recovery LSPs, then two LSPs (belonging to
the same pair) can be simultaneously recovered. The latter case can
be illustrated by the following topology with 2 pairs of working LSPs
A-B-C and F-G-H and their respective recovery LSPs A-D-E-C and
F-D-E-H that share two common D-E link resources.
A========B========C
\\ //
\\ //
D =========== E
// \\
// \\
F========G========H
Moreover, in all these schemes, (working) path disjointness can be
enforced by exchanging information related to working LSPs during the
recovery LSP signaling. Specific issues related to the combination
of shared (discrete) bandwidth and disjointness for recovery schemes
are described in Section 8.4.2.
5.5. LSP Recovery Mechanisms
5.5.1. Classification
The recovery time and ratio of LSPs/spans depend on proper recovery
LSP provisioning (meaning pre-provisioning when performed before
failure occurrence) and the level of overbooking of recovery
resources (i.e., over-provisioning). A proper balance of these two
operations will result in the desired LSP/span recovery time and
ratio when single or multiple failures occur. Note also that these
operations are mostly performed during the network planning phases.
The different options for LSP (pre-)provisioning and overbooking are
classified below to structure the analysis of the different recovery
mechanisms.
1. Pre-Provisioning
Proper recovery LSP pre-provisioning will help to alleviate the
failure of the working LSPs (due to the failure of the resources that
carry these LSPs). As an example, one may compute and establish the
recovery LSP either end-to-end or segment-per-segment, to protect a
working LSP from multiple failure events affecting link(s), node(s)
and/or SRLG(s). The recovery LSP pre-provisioning options are
classified as follows in the figure below:
(1) The recovery path can be either pre-computed or computed on-
demand.
(2) When the recovery path is pre-computed, it can be either pre-
signaled (implying recovery resource reservation) or signaled
on-demand.
(3) When the recovery resources are pre-signaled, they can be either