Abstract
Over the past few years, leading communications service providers and a number of NE (network element) suppliers have supported the development of an MPLS Transport profile (MPLS-TP) that will enable the technology to be operated in a similar manner to existing transport technologies and give it the capability to support packet transport services with a degree of predictability that is found in existing transport networks.
The fundamental idea is to extend
MPLS wherever necessary with
Operations, Administration and
Maintenance (OAM) tools that are
widely applied in existing transport
network technologies such as
SONET/SDH or OTN.
This paper provides a brief history
of the MPLS-TP standardization
activities and addresses the MPLS-TP
OAM functions. These functions are
targeted at making MPLS comparable
to SONET/SDH and OTN in terms of
reliability and monitoring capabilities,
i.e., MPLS-TP will become a true
carrier grade packet transport
technology.
Introduction
An MPLS-TP network can be
operated in an SDH-like fashion
and a network management
system (NMS) can be used to configure connections. Connection
management and restoration
functions, however, can alternatively
be provided utilising the Generalised
MPLS (GMPLS) control plane
protocols which are also applicable
to the MPLS-TP data plane. In
addition to the simplification of the
network operation leading to reduced
operational expenditures (OPEX),
the GMPLS control plane provides
network restoration capabilities, in
addition to the network protection
features that the MPLS-TP data plane
already provides.This results in a
further improved network resiliency.
The MPLS-TP technology is also
multi-service capable leveraging
the pseudo-wire technology
that has been developed at the
IETF and which is still evolving.
Some applications require
synchronization, e.g. mobile
services and interconnection of
telephony switches. Ethernet is an
asynchronous network protocol
and hence protocol extensions are
necessary. This paper discusses the
different emerging standards. One
of the key requirements is that the
new MPLS-TP network layer must
be capable to utilize the existing
physical infrastructure and the
paper lists the various adaptation
or encapsulation techniques that
allow MPLS-TP packets to be carried
over a variety of different physical
technologies ranging from SONET/
SDH and OTN to Gigabit Ethernet.
Overview of MPLS-TP
The goal of MPLS-TP is to provide
connection-oriented transport for
packet and TDM services over
optical networks leveraging the
widely deployed MPLS technology.
Key to this effort is the definition
and implementation of OAM and
resiliency features to ensure the
capabilities needed for carriergrade transport networks – scalable operations, high availability,
performance monitoring and multidomain support.
Key characteristics of MPLS-TP are:
- Is strictly connection oriented
- Is client-agnostic (can carry L3, L2,
L1 services)
- Is physical layer agnostic (can run
over IEEE Ethernet PHYs, SONET/
SDH [G.783] and OTN [G.709],
[G.872] using GFP [G.7041], WDM,
etc.)
- Provides strong operations,
administration and maintenance
(OAM) functions, similar to those
available in traditional optical
transport networks (e.g., SONET/
SDH, OTN); these OAM functions
are an integral part of the MPLSTP
data plane and are independent
from the control plane.
- Provides several protection
schemes at the data plane, similar
to those available in traditional
optical transport networks
- Allows network provisioning via a
centralized NMS and/or a
distributed control plane
- The GMPLS control plane is also
applicable to the MPLS-TP client
or server layers and allows to use a
common approach for management and control of multilayer transport networks.
Moreover, MPLS-TP provides dynamic
provisioning of MPLS-TP transport
paths via a control plane. The control
plane is mainly used to provide
restoration functions for improved
network survivability in the presence
of failures and it facilitates end-to-end
path provisioning across network
or operator domains. The operator
has the choice to enable the control
plane or to operate the network in a
traditional way, without control plane
by means of an NMS. It shall be noted
that the control plane does not make
the NMS obsolete – the NMS needs
to configure the control plane and
also needs to interact with the control
plane for connection management
purposes.
MPLS-TP Drivers
Carriers are experiencing an
unprecedented combination of
demand for service sophistication
and expansion (e.g. Triple Play, LTE
in mobile radio communications)
coupled with economic pressure
to minimise the cost for providing
these services. MPLS-TP is being
defined to meet these divergent
requirements by introducing SDHlike
OAM features to packet transport
networks.
Service providers have expressed
interest in MPLS-TP primarily
because they are looking to control
the costs if transporting packets that
are increasingly dominating their
traffic mix and – in many cases – are
currently being carried inefficiently
over legacy SDH networks that run at
constant bit rates even when there is
no traffic.
The ability to support multiple
services and applications over a
common MPLS enable infrastructure
provides flexibility to more rapidly
add new offerings and to costeffectively scale with demand
over time. While IP/MPLS is widely
deployed in carrier networks and
has rich multiservice and scalability
features, it generally offers more
than what is needed for transport
application. Major operators have encouraged development of a
transport-oriented version of MPLS
with a rich set of OAM features to
complement IP/MPLS deployed in
their networks.
MPLS-TP OAM
The MPLS-TP OAM tool set is
currently under definition at the
IETF and comprises the OAM
features listed in Figure below. The fundamental idea is that dedicated
OAM packets are interspersed into
the associated user traffic flows.
These OAM packets are created
and processed by maintenance end
point. Maintenance intermediate
points can also process these
OAM packets and may collect
data or raise alarms. The tools can
be categorised in proactive OAM
functions that are running all the
time and on-demand monitoring
functions.
One of the goals of MPLS-TP OAM
is to provide the tools needed to
monitor and manage the network
with the same attributes offered by
legacy transport technologies. For
example, the OAM is designed to
travel on the exact same path that
the data would take. In other words,
MPLS-TP OAM monitors PWs or
LSPs.
Two important components of the
OAM mechanisms are the G-ACh
and the Generic Alert Label (GAL).
As their names indicate, they allow
an operator to send any type of
control traffic into a PW or an LSP.
The G-ACh is used in both PWs and
MPLS-TP LSPs. The GAL is used
today in MPLS-TP LSPs to flag the
G-ACh.
MPLS-TP Control Plane
The IETF further defined
Generalized MPLS (GMPLS) as
a generalization of the MPLS
control plane to develop a dynamic
control plane that can be applied to packet switched and optical networks. The GMPLS architecture
is described in [RFC3945]. The
GMPLS control plane, or its ITU-T
counterpart, Automatically Switched
Optical Network (ASON) [G.8080],
supports connection management
functions as well as protection and
restoration techniques and thus
providing network survivability
across networks comprising routers,
MPLS-TP LSRs, optical ADMs, cross
connects, and WDM devices.
MPLS-TP may utilize the distributed
control plane to enable fast,
dynamic and reliable service
provisioning in multi-vendor and
multi-domain environments using
standardized protocols that ensure
interoperability.
The MPLS-TP control plane is
based on a combination of the
MPLS control plane for pseudowires
and the GMPLS control plane
for MPLS-TP LSPs, respectively.
This is illustrated in Figure above.
The distributed MPLS-TP control
plane provides the following basic
functions:
- Signalling
- Routing
- Traffic engineering and constraint-
based path computation
Moreover, the MPLS-TP control
plane is capable of performing fast
restoration in the event of network
failures.
The MPLS-TP control plane
provides features to ensure its
own survivability and to enable it
to recover gracefully from failures
and degradations. These include
graceful restart and hot redundant
configurations. The MPLS-TP control
plane is as much as possible
decoupled from the MPLS-TP data
plane such that failures in the control
plane do not impact the data plane
and vice versa.
Within the context of MPLS-TP, the
control plane is the mechanism used
to set up an LSP automatically across
a packet-switched network domain.
The use of a control plane protocol is
optional in MPLS-TP. Some operators
may prefer to configure the LSPs and
PWs using a Network Management
System in the same way that it
would be used to provision a SONET
network. In this case, no IP or routing
protocol is used.
On the other hand, it is possible to
use a dynamic control plane with
MPLS-TP so that LSPs and PWs
are set up by the network using
Generalized (G)-MPLS and Targeted
Label Distribution Protocol (T-LDP)
respectively. G-MPLS is based on
the TE extensions to MPLS (MPLSTE). It may also be used to set up the
OAM function and define recovery
mechanisms. T-LDP is part of the
PW architecture and is widely used
today to signal PWs and their status.
Physical Layers supporting MPLS-TP
It is mandatory for MPLS-TP that
it can be carried over the existing
and still evolving physical transport
technologies such as SONET/SDH,
OTN/WDM, and Gigabit Ethernet.
The encapsulation techniques
for these technologies are briefly
described below.
MPLS-TP over SONET/SDH, PDH and OTN
ITU-T Recommendation G.7041
[G.7041] defines a generic framing procedure (GFP) to encapsulate
variable length payload of various
client signals for subsequent
transport over SONET/SDH, PDH,
and OTN networks. The GFP header
contains a User Payload Identifier
(UPI) field for which values are
defined that indicate that the carried
protocol data unit is an MPLS
packet. MPLS-TP uses that same
UPI code point as MPLS. The OTN
[G.709] includes a WDM network
layer for the transport of a variety of
OTN client signals. In the SONET/
SDH case, virtual concatenation
can be applied to form transmission
pipes with larger capacities (n x 150
Mbit/s).
MPLS-TP over Gigabit Ethernet
Similar to GFP, MPLS-TP can be
carried across Ethernet links. A
two-octet Ether Type field has been
defined by the Ethernet II framing
networking standard to indicate
which protocol is encapsulated in the
payload area of the frame.
Conclusion
The increasing demand for highly
reliable and large-capacity packet
transport technology has facilitated
the development of MPLS-TP
technology. The key features of
MPLS-TP in comparison to IP/
MPLS include separation of the
data plane and control plane, OAM,
centralized operation by NMS, and
high-speed recovery of signals by
a protection mechanism. MPLS-TP
standardization has progressed
to the point of completion of some
common documents and two kinds
of solutions including G.8113.1 and
G.8113.2 for OAM and G.8121.1
and G.8121.2 for equipment
functional blocks. The differences
in these two solutions can be
characterised by their simplicity
and the behavioural inclination to
Ethernet/OTN/SDH or IP/MPLS and
a scenario for deployment of future
new technology such as SDN.
Thus, G.81xx.1 based MPLS-TP will
become a key technology for multilayer converged transport networks
driven by SDN as a promising
solution for future cost-effective
networks.
Shweta Chaturvedi
Presales & Solution Team
Telecom Services Business
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