Upon receiving this message and object, the ingress node sends a
PathTear/Release message downstream to remove the LSP and normal
RSVP-TE/CR-LDP processing takes place.
In the second usage, the Admin Status object/TLV is carried in a
Notification/Label Mapping (with the modification action indicator
flag set to modify) message to request that the ingress node change
the administrative state of an LSP. This allows intermediate and
egress nodes triggering the setting of administrative status. In
particular, this allows intermediate or egress LSRs requesting a
release of an LSP initiated by the ingress node.
7.18. Control Channel Separation
In GMPLS, a control channel be separated from the data channel.
Indeed, the control channel can be implemented completely out-of-
band for various reason, e.g., when the data channel cannot carry
in-band control information. This issue was even originally
introduced to MPLS in the context of link bundling.
In traditional MPLS, there is an implicit one-to-one association of a
control channel to a data channel. When such an association is
present, no additional or special information is required to
associate a particular LSP setup transaction with a particular data
channel.
Otherwise, it is necessary to convey additional information in
signaling to identify the particular data channel being controlled.
GMPLS supports explicit data channel identification by providing
interface identification information. GMPLS allows the use of a
number of interface identification schemes including IPv4 or IPv6
addresses, interface indexes (for unnumbered interfaces) and
component interfaces (for bundled interfaces), unnumbered bundled
interfaces are also supported.
The choice of the data interface to use is always made by the sender
of the Path/Label Request message, and indicated by including the
data channel’s interface identifier in the message using a new
RSVP_HOP object sub-type/Interface TLV.
For bi-directional LSPs, the sender chooses the data interface in
each direction. In all cases but bundling, the upstream interface is
implied by the downstream interface. For bundling, the Path/Label
Request sender explicitly identifies the component interface used in
each direction. The new object/TLV is used in Resv/Label Mapping
message to indicate the downstream node’s usage of the indicated
interface(s).
The new object/TLV can contain a list of embedded TLVs, each embedded
TLV can be an IPv4 address, and IPv6 address, an interface index, a
downstream component interface ID or an upstream component interface
ID. In the last three cases, the embedded TLV contains itself an IP
address plus an Interface ID, the IP address being used to identify
the interface ID (it can be the router ID for instance).
There are cases where it is useful to indicate a specific interface
associated with an error. To support these cases the IF_ID
ERROR_SPEC RSVP Objects are defined.
8. Forwarding Adjacencies (FA)
To improve scalability of MPLS TE (and thus GMPLS) it may be useful
to aggregate multiple TE LSPs inside a bigger TE LSP. Intermediate
nodes see the external LSP only. They do not have to maintain
forwarding states for each internal LSP, less signaling messages need
to be exchanged and the external LSP can be somehow protected instead
(or in addition) to the internal LSPs. This can considerably
increase the scalability of the signaling.
The aggregation is accomplished by (a) an LSR creating a TE LSP, (b)
the LSR forming a forwarding adjacency out of that LSP (advertising
this LSP as a Traffic Engineering (TE) link into IS-IS/OSPF), (c)
allowing other LSRs to use forwarding adjacencies for their path
computation, and (d) nesting of LSPs originated by other LSRs into
that LSP (e.g., by using the label stack construct in the case of
IP).
ISIS/OSPF floods the information about "Forwarding Adjacencies" FAs
just as it floods the information about any other links. Consequently
to this flooding, an LSR has in its TE link state database the
information about not just conventional links, but FAs as well.
An LSR, when performing path computation, uses not just conventional
links, but FAs as well. Once a path is computed, the LSR uses RSVP-
TE/CR-LDP for establishing label binding along the path. FAs need
simple extensions to signaling and routing protocols.
8.1. Routing and Forwarding Adjacencies
Forwarding adjacencies may be represented as either unnumbered or
numbered links. A FA can also be a bundle of LSPs between two nodes.
FAs are advertised as GMPLS TE links such as defined in [HIERARCHY].
GMPLS TE links are advertised in OSPF and IS-IS such as defined in
[OSPF-TE-GMPLS] and [ISIS-TE-GMPLS]. These last two specifications
enhance [OSPF-TE] and [ISIS-TE] that defines a base TE link.
When a FA is created dynamically, its TE attributes are inherited
from the FA-LSP that induced its creation. [HIERARCHY] specifies how
each TE parameter of the FA is inherited from the FA-LSP. Note that
the bandwidth of the FA must be at least as big as the FA-LSP that
induced it, but may be bigger if only discrete bandwidths are
available for the FA-LSP. In general, for dynamically provisioned
forwarding adjacencies, a policy-based mechanism may be needed to
associate attributes to forwarding adjacencies.
A FA advertisement could contain the information about the path taken
by the FA-LSP associated with that FA. Other LSRs may use this
information for path computation. This information is carried in a
new OSPF and IS-IS TLV called the Path TLV.
It is possible that the underlying path information might change over
time, via configuration updates, or dynamic route modifications,
resulting in the change of that TLV.
If forwarding adjacencies are bundled (via link bundling), and if the
resulting bundled link carries a Path TLV, the underlying path
followed by each of the FA-LSPs that form the component links must be
the same.
It is expected that forwarding adjacencies will not be used for
establishing IS-IS/OSPF peering relation between the routers at the
ends of the adjacency.
LSP hierarchy could exist both with the peer and with the overlay
models. With the peer model, the LSP hierarchy is realized via FAs
and an LSP is both created and used as a TE link by exactly the same
instance of the control plane. Creating LSP hierarchies with
overlays does not involve the concept of FA. With the overlay model
an LSP created (and maintained) by one instance of the GMPLS control
plane is used as a TE link by another instance of the GMPLS control
plane. Moreover, the nodes using a TE link are expected to have a
routing and signaling adjacency.
8.2. Signaling Aspects
For the purpose of processing the explicit route in a Path/Request
message of an LSP that is to be tunneled over a forwarding adjacency,
an LSR at the head-end of the FA-LSP views the LSR at the tail of
that FA-LSP as adjacent (one IP hop away).
8.3. Cascading of Forwarding Adjacencies
With an integrated model, several layers are controlled using the
same routing and signaling protocols. A network may then have links
with different multiplexing/demultiplexing capabilities. For
example, a node may be able to multiplex/demultiplex individual
packets on a given link, and may be able to multiplex/demultiplex
channels within a SONET payload on other links.
A new OSPF and IS-IS sub-TLV has been defined to advertise the
multiplexing capability of each interface: PSC, L2SC, TDM, LSC or
FSC. This sub-TLV is called the Interface Switching Capability
Descriptor sub-TLV, which complements the sub-TLVs defined in
[OSPF-TE-GMPLS] and [ISIS-TE-GMPLS]. The information carried in this
sub-TLV is used to construct LSP regions, and determine region’s
boundaries.
Path computation may take into account region boundaries when
computing a path for an LSP. For example, path computation may
restrict the path taken by an LSP to only the links whose
multiplexing/demultiplexing capability is PSC. When an LSP need to
cross a region boundary, it can trigger the establishment of an FA at
the underlying layer (i.e., the L2SC layer). This can trigger a
cascading of FAs between layers with the following obvious order:
L2SC, then TDM, then LSC, and then finally FSC.
9. Routing and Signaling Adjacencies
By definition, two nodes have a routing (IS-IS/OSPF) adjacency if
they are neighbors in the IS-IS/OSPF sense.
By definition, two nodes have a signaling (RSVP-TE/CR-LDP) adjacency
if they are neighbors in the RSVP-TE/CR-LDP sense. Nodes A and B are
RSVP-TE neighbors if they directly exchange RSVP-TE messages
(Path/Resv) (e.g., as described in sections 7.1.1 and 7.1.2 of
[HIERARCHY]). The neighbor relationship includes exchanging RSVP-TE
Hellos.
By definition, a Forwarding Adjacency (FA) is a TE Link between two
GMPLS nodes whose path transits one or more other (G)MPLS nodes in
the same instance of the (G)MPLS control plane. If two nodes have
one or more non-FA TE Links between them, these two nodes are
expected (although not required) to have a routing adjacency. If two
nodes do not have any non-FA TE Links between them, it is expected
(although not required) that these two nodes would not have a routing
adjacency. To state the obvious, if the TE links between two nodes
are to be used for establishing LSPs, the two nodes must have a
signaling adjacency.
If one wants to establish routing and/or signaling adjacency between
two nodes, there must be an IP path between them. This IP path can
be, for example, a TE Link with an interface switching capability of
PSC, anything that looks likes an IP link (e.g., GRE tunnel, or a
(bi-directional) LSP that with an interface switching capability of
PSC).
A TE link may not be capable of being used directly for maintaining
routing and/or signaling adjacencies. This is because GMPLS routing
and signaling adjacencies requires exchanging data on a per frame/
packet basis, and a TE link (e.g., a link between OXCs) may not be
capable of exchanging data on a per packet basis. In this case, the
routing and signaling adjacencies are maintained via a set of one or
more control channels (see [LMP]).
Two nodes may have a TE link between them even if they do not have a
routing adjacency. Naturally, each node must run OSPF/IS-IS with
GMPLS extensions in order for that TE link to be advertised. More
precisely, the node needs to run GMPLS extensions for TE Links with
an interface switching capability (see [GMPLS-ROUTING]) other than
PSC. Moreover, this node needs to run either GMPLS or MPLS
extensions for TE links with an interface switching capability of
PSC.
The mechanisms for Control Channel Separation [RFC3471] should be
used (even if the IP path between two nodes is a TE link). I.e.,
RSVP-TE/CR-LDP signaling should use the Interface_ID (IF_ID) object
to specify a particular TE link when establishing an LSP.
The IP path could consist of multiple IP hops. In this case, the
mechanisms of sections 7.1.1 and 7.1.2 of [HIERARCHY] should be used
(in addition to Control Channel Separation).
10. Control Plane Fault Handling
Two major types of faults can impact a control plane. The first,
referred to as control channel fault, relates to the case where
control communication is lost between two neighboring nodes. If the
control channel is embedded with the data channel, data channel
recovery procedure should solve the problem. If the control channel
is independent of the data channel, additional procedures are
required to recover from that problem.
The second, referred to as nodal faults, relates to the case where
node loses its control state (e.g., after a restart) but does not
loose its data forwarding state.
In transport networks, such types of control plane faults should not
have service impact on the existing connections. Under such
circumstances, a mechanism must exist to detect a control
communication failure and a recovery procedure must guarantee
connection integrity at both ends of the control channel.
For a control channel fault, once communication is restored routing
protocols are naturally able to recover but the underlying signaling
protocols must indicate that the nodes have maintained their state
through the failure. The signaling protocol must also ensure that
any state changes that were instantiated during the failure are
synchronized between the nodes.
For a nodal fault, a node’s control plane restarts and loses most of
its state information. In this case, both upstream and downstream
nodes must synchronize their state information with the restarted
node. In order for any resynchronization to occur the node
undergoing the restart will need to preserve some information, such
as it’s mappings of incoming to outgoing labels.
These issues are addressed in protocol specific fashions, see
[RFC3473], [RFC3472], [OSPF-TE-GMPLS] and [ISIS-TE-GMPLS]. Note that
these cases only apply when there are mechanisms to detect data
channel failures independent of control channel failures.
The LDP Fault tolerance (see [RFC3479]) specifies the procedures to
recover from a control channel failure. [RFC3473] specifies how to
recover from both a control channel failure and a node failure.
11. LSP Protection and Restoration
This section discusses Protection and Restoration (P&R) issues for
GMPLS LSPs. It is driven by the requirements outlined in [RFC3386]
and some of the principles outlined in [RFC3469]. It will be
enhanced, as more GMPLS P&R mechanisms are defined. The scope of
this section is clarified hereafter:
- This section is only applicable when a fault impacting LSP(s)
happens in the data/transport plane. Section 10 deals with
control plane fault handling for nodal and control channel faults.
- This section focuses on P&R at the TDM, LSC and FSC layers. There
are specific P&R requirements at these layers not present at the
PSC layer.
- This section focuses on intra-area P&R as opposed to inter-area
P&R and even inter-domain P&R. Note that P&R can even be more
restricted, e.g., to a collection of like customer equipment, or a
collection of equipment of like capabilities, in one single
routing area.
- This section focuses on intra-layer P&R (horizontal hierarchy as
defined in [RFC3386]) as opposed to the inter-layer P&R (vertical
hierarchy).
- P&R mechanisms are in general designed to handle single failures,
which makes SRLG diversity a necessity. Recovery from multiple
failures requires further study.
- Both mesh and ring-like topologies are supported.
In the following, we assume that:
- TDM, LSC and FSC devices are more generally committing recovery
resources in a non-best effort way. Recovery resources are either
allocated (thus used) or at least logically reserved (whether used
or not by preemptable extra traffic but unavailable anyway for
regular working traffic).
- Shared P&R mechanisms are valuable to operators in order to
maximize their network utilization.
- Sending preemptable excess traffic on recovery resources is a
valuable feature for operators.
11.1. Protection Escalation across Domains and Layers
To describe the P&R architecture, one must consider two dimensions of
hierarchy [RFC3386]:
- A horizontal hierarchy consisting of multiple P&R domains, which
is important in an LSP based protection scheme. The scope of P&R
may extend over a link (or span), an administrative domain or
sub-network, an entire LSP.
An administrative domain may consist of a single P&R domain or as
a concatenation of several smaller P&R domains. The operator can
configure P&R domains, based on customers’ requirements, and on
network topology and traffic engineering constraints.
- A vertical hierarchy consisting of multiple layers of P&R with
varying granularities (packet flows, STS trails, lightpaths,
fibers, etc).
In the absence of adequate P&R coordination, a fault may propagate
from one level to the next within a P&R hierarchy. It can lead to
"collisions" and simultaneous recovery actions may lead to race
conditions, reduced resource utilization, or instabilities
[MANCHESTER]. Thus, a consistent escalation strategy is needed to
coordinate recovery across domains and layers. The fact that
GMPLS can be used at different layers could simplify this
coordination.
There are two types of escalation strategies: bottom-up and top-
down. The bottom-up approach assumes that "lower-level" recovery
schemes are more expedient. Therefore we can inhibit or hold off
higher-level P&R. The Top-down approach attempts service P&R at
the higher levels before invoking "lower level" P&R. Higher-layer
P&R is service selective, and permits "per-CoS" or "per-LSP" re-
routing.
Service Level Agreements (SLAs) between network operators and their
clients are needed to determine the necessary time scales for P&R at
each layer and at each domain.
11.2. Mapping of Services to P&R Resources
The choice of a P&R scheme is a tradeoff between network utilization
(cost) and service interruption time. In light of this tradeoff,
network service providers are expected to support a range of
different service offerings or service levels.
One can classify LSPs into one of a small set of service levels.
Among other things, these service levels define the reliability
characteristics of the LSP. The service level associated with a
given LSP is mapped to one or more P&R schemes during LSP
establishment. An advantage that mapping is that an LSP may use
different P&E schemes in different segments of a network (e.g., some
links may be span protected, whilst other segments of the LSP may
utilize ring protection). These details are likely to be service
provider specific.
An alternative to using service levels is for an application to
specify the set of specific P&R mechanisms to be used when
establishing the LSP. This allows greater flexibility in using
different mechanisms to meet the application requirements.
A differentiator between these service levels is service interruption
time in case of network failures, which is defined as the length of
time between when a failure occurs and when connectivity is re-
established. The choice of service level (or P&R scheme) should be
dictated by the service requirements of different applications.
11.3. Classification of P&R Mechanism Characteristics
The following figure provides a classification of the possible
provisioning types of recovery LSPs, and of the levels of overbooking
that is possible for them.
+-Computed on +-Established +-Resources pre-
| demand | on demand | allocated
| | |
Recovery LSP | | |
Provisioning -+-Pre computed +-Pre established +-Resources allocated
on demand
+--- Dedicated (1:1, 1+1)
|
|
+--- Shared (1:N, Ring, Shared mesh)
|
Level of |
Overbooking ---+--- Best effort
11.4. Different Stages in P&R
Recovery from a network fault or impairment takes place in several
stages as discussed in [RFC3469], including fault detection, fault
localization, notification, recovery (i.e., the P&R itself) and
reversion of traffic (i.e., returning the traffic to the original
working LSP or to a new one).
- Fault detection is technology and implementation dependent. In
general, failures are detected by lower layer mechanisms (e.g.,
SONET/SDH, Loss-of-Light (LOL)). When a node detects a failure,
an alarm may be passed up to a GMPLS entity, which will take
appropriate actions, or the alarm may be propagated at the lower
layer (e.g., SONET/SDH AIS).
- Fault localization can be done with the help of GMPLS, e.g., using
LMP for fault localization (see section 6.4).
- Fault notification can also be achieved through GMPLS, e.g., using
GMPLS RSVP-TE/CR-LDP notification (see section 7.12).
- This section focuses on the different mechanisms available for
recovery and reversion of traffic once fault detection,
localization and notification have taken place.
11.5. Recovery Strategies
Network P&R techniques can be divided into Protection and
Restoration. In protection, resources between the protection
endpoints are established before failure, and connectivity after
failure is achieved simply by switching performed at the protection
end-points. In contrast, restoration uses signaling after failure to
allocate resources along the recovery path.
- Protection aims at extremely fast reaction times and may rely on
the use of overhead control fields for achieving end-point
coordination. Protection for SONET/SDH networks is described in
[ITUT-G.841] and [ANSI-T1.105]. Protection mechanisms can be
further classified by the level of redundancy and sharing.
- Restoration mechanisms rely on signaling protocols to coordinate
switching actions during recovery, and may involve simple re-
provisioning, i.e., signaling only at the time of recovery; or
pre-signaling, i.e., signaling prior to recovery.
In addition, P&R can be applied on a local or end-to-end basis. In
the local approach, P&R is focused on the local proximity of the
fault in order to reduce delay in restoring service. In the end-to-
end approach, the LSP originating and terminating nodes control
recovery.
Using these strategies, the following recovery mechanisms can be
defined.
11.6. Recovery mechanisms: Protection schemes
Note that protection schemes are usually defined in technology
specific ways, but this does not preclude other solutions.
- 1+1 Link Protection: Two pre-provisioned resources are used in
parallel. For example, data is transmitted simultaneously on two
parallel links and a selector is used at the receiving node to
choose the best source (see also [GMPLS-FUNCT]).
- 1:N Link Protection: Working and protecting resources (N working,
1 backup) are pre-provisioned. If a working resource fails, the
data is switched to the protecting resource, using a coordination
mechanism (e.g., in overhead bytes). More generally, N working
and M protecting resources can be assigned for M:N link protection
(see also [GMPLS-FUNCT]).
- Enhanced Protection: Various mechanisms such as protection rings
can be used to enhance the level of protection beyond single link
failures to include the ability to switch around a node failure or
multiple link failures within a span, based on a pre-established
topology of protection resources (note: no reference available at
publication time).
- 1+1 LSP Protection: Simultaneous data transmission on working and
protecting LSPs and tail-end selection can be applied (see also
[GMPLS-FUNCT]).
11.7. Recovery mechanisms: Restoration schemes
Thanks to the use of a distributed control plane like GMPLS,
restoration is possible in multiple of tenths of milliseconds. It is
much harder to achieve when only an NMS is used and can only be done
in that case in a multiple of seconds.
- End-to-end LSP restoration with re-provisioning: an end-to-end
restoration path is established after failure. The restoration
path may be dynamically calculated after failure, or pre-