Framework for Generalized Multi-Protocol Label Switching (GMPLS)-based Control of Synchronous Digital Hierarchy/Synchronous Optical Networking (SDH/SONET) Networks
draft-ietf-ccamp-sdhsonet-control-05
The information below is for an old version of the document that is already published as an RFC.
| Document | Type |
This is an older version of an Internet-Draft that was ultimately published as RFC 4257.
|
|
|---|---|---|---|
| Authors | Vishal Sharma , Greg M. Bernstein , Eric W. Gray , Eric Mannie | ||
| Last updated | 2013-03-02 (Latest revision 2005-02-23) | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
| Intended RFC status | Informational | ||
| Formats | |||
| Additional resources | Mailing list discussion | ||
| Stream | WG state | (None) | |
| Document shepherd | (None) | ||
| IESG | IESG state | Became RFC 4257 (Informational) | |
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(None)
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| Consensus boilerplate | Unknown | ||
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| Responsible AD | Alex D. Zinin | ||
| Send notices to | (None) |
draft-ietf-ccamp-sdhsonet-control-05
Network Working Group G. Bernstein (Grotto Networking)
Internet Draft E. Mannie (InterAir Link)
Category: Informational V. Sharma (Metanoia, Inc.)
E. Gray (Marconi Communications)
Expires August 2005 February 2005
Framework for GMPLS-based Control of SDH/SONET Networks
<draft-ietf-ccamp-sdhsonet-control-05.txt>
Status of this Memo
This document is an Internet-Draft and is subject to all provisions
of section 3 of RFC 3667 [1] and Section 6 of RFC 3668 [2].
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Abstract
Generalized MPLS (GMPLS) is a suite of protocol extensions to MPLS
(Multi-Protocol Label Switching) to make it generally applicable, to
include - for example - control of non packet-based switching, and
particularly, optical switching. One consideration is to use GMPLS
protocols to upgrade the control plane of optical transport networks.
This document illustrates this process by describing those extensions
to GMPLS protocols that are aimed at controlling Synchronous Digital
Hierarchy (SDH) or Synchronous Optical Networking (SONET) networks.
SDH/SONET networks make good examples of this process for a variety
of reasons. This document high-lights extensions to GMPLS-related
routing protocols to disseminate information needed in transport path
computation and network operations, together with (G)MPLS protocol
extensions required for the provisioning of transport circuits. New
capabilities that an GMPLS control plane would bring to SDH/SONET
networks, such as new restoration methods and multi-layer circuit
establishment, are also discussed.
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1. Introduction ................................................3
1.1. MPLS Overview .............................................3
1.2. SDH/SONET Overview ........................................4
1.3. The Current State of Circuit Establishment in SDH/SONET
Networks ..................................................7
1.3.1. Administrative Tasks ..................................7
1.3.2. Manual Operations .....................................7
1.3.3. Planning Tool Operation ...............................7
1.3.4. Circuit Provisioning ..................................8
1.4. Centralized Approach versus Distributed Approach ..........8
1.4.1. Topology Discovery and Resource Dissemination .........9
1.4.2. Path Computation (Route Determination).................9
1.4.3. Connection Establishment (Provisioning)...............10
1.5. Why SDH/SONET will not Disappear Tomorrow ................11
2. GMPLS Applied to SDH/SONET .................................12
2.1. Controlling the SDH/SONET Multiplex ......................12
2.2. SDH/SONET LSR and LSP Terminology ........................13
3. Decomposition of the GMPLS Circuit-Switching Problem Space .13
4. GMPLS Routing for SDH/SONET ................................14
4.1. Switching Capabilities ...................................15
4.1.1. Switching Granularity ................................15
4.1.2. Signal Concatenation Capabilities ....................16
4.1.3. SDH/SONET Transparency ...............................17
4.2. Protection ...............................................18
4.3. Available Capacity Advertisement .........................21
4.4. Path Computation .........................................22
5. LSP Provisioning/Signaling for SDH/SONET ...................22
5.1. What do we Label in SDH/SONET? Frames or Circuits?........23
5.2. Label Structure in SDH/SONET .............................24
5.3. Signaling Elements .......................................24
6. Summary and Conclusions ....................................26
7. Security Considerations ....................................27
8. Acknowledgments ............................................27
9. Author's Addresses .........................................27
10. References .................................................28
10.1. Normative References ...................................28
10.2. Informative References .................................28
11. Intellectual Property Statement ............................29
12. Disclaimer .................................................30
13. Copyright Statement ........................................30
14. IANA Considerations .........................................30
15. Acronyms ....................................................30
16. Acknowledgement .............................................31
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1. Introduction
The CCAMP Working Group of the IETF has the goal of extending MPLS
[3] protocols to support multiple network layers and new services.
This extended MPLS, which was initially known as Multi-Protocol
Lambda Switching, is now better referred to as Generalized MPLS (or
GMPLS).
The GMPLS effort is, in effect, extending IP/MPLS technology to
control and manage lower layers. Using the same framework and
similar signaling and routing protocols to control multiple layers
can not only reduce the overall complexity of designing, deploying
and maintaining networks, but can also make it possible to operate
two contiguous layers by using either an overlay model, a peer
model, or an integrated model. The benefits of using a peer or an
overlay model between the IP layer and its underlying layer(s) will
have to be clarified and evaluated in the future. In the mean time,
GMPLS could be used for controlling each layer independently.
The goal of this work is to highlight how GMPLS could be used to
dynamically establish, maintain, and tear down SDH/SONET circuits.
The objective of using these extended IP/MPLS protocols is to
provide at least the same kinds of SDH/SONET services as are
provided today, but using signaling instead of provisioning via
centralized management to establish those services. This will allow
operators to propose new services, and will allow clients to create
SDH/SONET paths on-demand, in real-time, through the provider
network. We first review the essential properties of SDH/SONET
networks and their operations, and we show how the label concept in
GMPLS can be extended to the SDH/SONET case. We then look at
important information to be disseminated by a link state routing
protocol and look at the important signal attributes that need to be
conveyed by a label distribution protocol. Finally, we look at some
outstanding issues and future possibilities.
1.1. MPLS Overview
A major advantage of the MPLS architecture [3] for use as a general
network control plane is its clear separation between the forwarding
(or data) plane, the signaling (or connection control) plane, and
the routing (or topology discovery/resource status) plane. This
allows the work on MPLS extensions to focus on the forwarding and
signaling planes, while allowing well-known IP routing protocols to
be reused in the routing plane. This clear separation also allows
for MPLS to be used to control networks that do not have a
packet-based forwarding plane.
An MPLS network consists of MPLS nodes called Label Switch Routers
(LSRs) connected via circuits called Label Switched Paths (LSPs). An
LSP is unidirectional and could be of several different types such
as point-to-point, point-to-multipoint, and multipoint-to-point.
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Border LSRs in an MPLS network act either as ingress or egress LSRs
depending on the direction of the traffic being forwarded.
Each LSP is associated with a Fowarding Equivalence Class (FEC),
which may be thought of as a set of packets that receive identical
forwarding treatment at an LSR. The simplest example of an FEC might
be the set of destination addresses lying in a given address range.
All packets that have a destination address lying within this
address range are forwarded identically at each LSR configured with
that FEC.
To establish an LSP, a signaling protocol (or label distribution
protocol) such as LDP or RSVP-TE is required. Between two adjacent
LSRs, an LSP is locally identified by a fixed length identifier
called a label, which is only significant between those two LSRs.
A signaling protocol is used for inter-node communication to assign
and maintain these labels.
When a packet enters an MPLS-based packet network, it is classified
according to its FEC and, possibly, additional rules, which together
determine the LSP along which the packet must be sent. For this
purpose, the ingress LSR attaches an appropriate label to the
packet, and forwards the packet to the next hop. The label may be
attached to a packet in different ways. For example, it may be in
the form of a header encapsulating the packet (the "shim" header) or
it may be written in the VPI/VCI field (or DLCI field) of the layer
2 encapsulation of the packet. In case of SDH/SONET networks, we
will see that a label is simply associated with a segment of a
circuit, and is mainly used in the signaling plane to identify this
segment (e.g. a time-slot) between two adjacent nodes.
When a packet reaches a packet LSR, this LSR uses the label as an
index into a forwarding table to determine the next hop and the
corresponding outgoing label (and, possibly, the QoS treatment to be
given to the packet), writes the new label into the packet, and
forwards the packet to the next hop. When the packet reaches the
egress LSR, the label is removed and the packet is forwarded using
appropriate forwarding, such as normal IP forwarding. We will see
that for a SDH/SONET network these operations do not occur in quite
the same way.
1.2. SDH/SONET Overview
There are currently two different multiplexing technologies in use
in optical networks: wavelength division multiplexing (WDM) and time
division multiplexing (TDM). This work focuses on TDM technology.
SDH and SONET are two TDM standards widely used by operators to
transport and multiplex different tributary signals over optical
links, thus creating a multiplexing structure, which we call the
SDH/SONET multiplex.
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ITU-T (G.707) [4] includes both the European ETSI SDH hierarchy and
the USA ANSI SONET hierarchy [5]. The ETSI SDH and SONET standards
regarding frame structures and higher-order multiplexing are the
same. There are some regional differences in terminology, on the use
of some overhead bytes, and lower-order multiplexing. Interworking
between the two lower-order hierarchies is possible using gateways.
The fundamental signal in SDH is the STM-1 that operates at a rate
of about 155 Mbps, while the fundamental signal in SONET is the STS-
1 that operates at a rate of about 51 Mbps. These two signals are
made of contiguous frames that consist of transport overhead
(header) and payload. To solve synchronization issues, the actual
data is not transported directly in the payload but rather in
another internal frame that is allowed to float over two successive
SDH/SONET payloads. This internal frame is named a Virtual Container
(VC) in SDH and a SONET Payload Envelope (SPE) in SONET.
The SDH/SONET architecture identifies three different layers, each
of which corresponds to one level of communication between SDH/SONET
equipment. These are, starting with the lowest, the regenerator
section/section layer, the multiplex section/line layer, and (at the
top) the path layer. Each of these layers in turn has its own
overhead (header). The transport overhead of a SDH/SONET frame is
mainly sub-divided in two parts that contain the regenerator
section/section overhead and the multiplex section/line overhead. In
addition, a pointer (in the form of the H1, H2 and H3 bytes)
indicates the beginning of the VC/SPE in the payload of the overall
STM/STS frame.
The VC/SPE itself is made up of a header (the path overhead) and a
payload. This payload can be further subdivided into sub-elements
(signals) in a fairly complex way. In the case of SDH, the STM-1
frame may contain either one VC-4 or three multiplexed VC-3s. The
SONET multiplex is a pure tree, while the SDH multiplex is not a
pure tree, since it contains a node that can be attached to two
parent nodes. The structure of the SDH/SONET multiplex is shown in
Figure 1. In addition, we show reference points in this figure that
are explained in later sections.
The leaves of these multiplex structures are time slots (positions)
of different sizes that can contain tributary signals. These
tributary signals (e.g. E1, E3, etc) are mapped into the leaves
using standardized mapping rules. In general, a tributary signal
does not fill a time slot completely, and the mapping rules define
precisely how to fill it.
What is important for the GMPLS-based control of SDH/SONET circuits
is to identify the elements that can be switched from an input
multiplex on one interface to an output multiplex on another
interface. The only elements that can be switched are those that can
be re-aligned via a pointer, i.e. a VC-x in the case of SDH and a
SPE in the case of SONET.
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xN x1
STM-N<----AUG<----AU-4<--VC4<------------------------------C-4 E4
^ ^
Ix3 Ix3
I I x1
I -----TUG-3<----TU-3<---VC-3<---I
I ^ C-3 DS3/E3
STM-0<------------AU-3<---VC-3<-- I ---------------------I
^ I
Ix7 Ix7
I I x1
-----TUG-2<---TU-2<---VC-2<---C-2 DS2/T2
^ ^
I I x3
I I----TU-12<---VC-12<--C-12 E1
I
I x4
I-------TU-11<---VC-11<--C-11 DS1/T1
xN
STS-N<-------------------SPE<------------------------------DS3/T3
^
Ix7
I x1
I---VT-Group<---VT-6<----SPE DS2/T2
^ ^ ^
I I I x2
I I I-----VT-3<----SPE DS1C
I I
I I x3
I I--------VT-2<----SPE E1
I
I x4
I-----------VT-1.5<--SPE DS1/T1
Figure 1. SDH and SONET multiplexing structure and typical PDH
payload signals.
An STM-N/STS-N signal is formed from N x STM-1/STS-1 signals via
byte interleaving. The VCs/SPEs in the N interleaved frames are
independent and float according to their own clocking. To transport
tributary signals in excess of the basic STM-1/STS-1 signal rates,
the VCs/SPEs can be concatenated, i.e., glued together. In this case
their relationship with respect to each other is fixed in time and
hence this relieves, when possible, an end system of any inverse
multiplexing bonding processes. Different types of concatenations
are defined in SDH/SONET.
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For example, standard SONET concatenation allows the concatenation
of M x STS-1 signals within an STS-N signal with M <= N, and M = 3,
12, 48, 192,...). The SPEs of these M x STS-1s can be concatenated
to form an STS-Mc. The STS-Mc notation is short hand for describing
an STS-M signal whose SPEs have been concatenated.
1.3. The Current State of Circuit Establishment in SDH/SONET Networks
In present day SDH and SONET networks, the networks are primarily
statically configured. When a client of an operator requests a
point-to-point circuit, the request sets in motion a process that
can last for several weeks or more. This process is composed of a
chain of shorter administrative and technical tasks, some of which
can be fully automated, resulting in significant improvements in
provisioning time and in operational savings. In the best case, the
entire process can be fully automated allowing, for example,
customer premise equipment (CPE) to contact a SDH/SONET switch to
request a circuit. Currently, the provisioning process involves the
following tasks.
1.3.1. Administrative Tasks
The administrative tasks represent a significant part of the
provisioning time. Most of them can be automated using IT
applications, e.g., a client still has to fill a form to request a
circuit. This form can be filled via a Web-based application and can
be automatically processed by the operator. A further enhancement is
to allow the client's equipment to coordinate with the operator's
network directly and request the desired circuit. This could be
achieved through a signaling protocol at the interface between the
client equipment and an operator switch, i.e., at the UNI, where
GMPLS signaling [6], [7] can be used.
1.3.2. Manual Operations
Another significant part of the time may be consumed by manual
operations that involve installing the right interface in the CPE
and installing the right cable or fiber between the CPE and the
operator switch. This time can be especially significant when a
client is in a different time zone than the operator's main office.
This first-time connection time is frequently accounted for in the
overall establishment time.
1.3.3. Planning Tool Operation
Another portion of the time is consumed by planning tools that run
simulations using heuristic algorithms to find an optimized
placement for the required circuits. These planning tools can
require a significant running time, sometimes on the order of days.
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These simulations are, in general, executed for a set of demands for
circuits, i.e., a batch mode, to improve the optimality of network
resource usage and other parameters. Today, we do not really have a
means to reduce this simulation time. On the contrary, to support
fast, on-line, circuit establishment, this phase may be invoked more
frequently, i.e., we will not "batch up" as many connection
requests before we plan out the corresponding circuits. This means
that the network may need to be re-optimized periodically, implying
that the signaling should support re-optimization with minimum
impact to existing services.
1.3.4. Circuit Provisioning
Once the first three steps discussed above have been completed, the
operator must provision the circuits using the outputs of the
planning process. The time required for provisioning varies greatly.
It can be fairly short, on the order of a few minutes, if the
operators already have tools that help them to do the provisioning
over heterogeneous equipment. Otherwise, the process can take days.
Developing these tools for each new piece of equipment and each
vendor is a significant burden on the service provider. A
standardized interface for provisioning, such as GMPLS signaling,
could significantly reduce or eliminate this development burden. In
general, provisioning is a batched activity, i.e., a few times per
week an operator provisions a set of circuits. GMPLS will reduce
this provisioning time from a few minutes to a few seconds and could
help to transform this periodic process into a real-time process.
When a circuit is provisioned, it is not delivered directly to a
client. Rather, the operator first tests its performance and
behavior and if successful, delivers the circuit to the client. This
testing phase lasts, in general, for up to 24 hours. The operator
installs test equipment at each end and uses pre-defined test
streams to verify performance. If successful, the circuit is
officially accepted by the client. To speed up the verification
(sometimes known as "proving") process, it would be necessary to
support some form of automated performance testing.
1.4. Centralized Approach versus Distributed Approach
Whether a centralized approach or a distributed approach will be
used to control SDH/SONET networks is an open question, since each
approach has its merits. The application of GMPLS to SDH/SONET
networks does not preclude either model, although GMPLS is itself a
distributed technology.
The basic tradeoff between the centralized and distributed
approaches is that of complexity of the network elements versus that
of the network management system (NMS). Since adding functionality
to existing SDH/SONET network elements may not be possible, a
centralized approach may be needed in some cases. The main issue
facing centralized control via an NMS is one of scalability. For
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instance, this approach may be limited in the number of network
elements that can be managed (e.g., one thousand). It is, therefore,
quite common for operators to deploy several NMS in parallel at
the Network Management Layer, each managing a different zone. In
that case, however, a Service Management Layer must be built on the
top of several individual NMS to take care of end-to-end on-demand
services. On the other hand, in a complex and/or dense network,
restoration could be faster with a distributed approach than with a
centralized approach.
Let's now look at how the major control plane functional components
are handled via the centralized and distributed approaches:
1.4.1. Topology Discovery and Resource Dissemination
Currently an NMS maintains a consistent view of all the networking
layers under its purview. This can include the physical topology,
such as information about fibers and ducts. Since most of this
information is entered manually, it remains error prone.
A link state GMPLS routing protocol, on the other hand, could
perform automatic topology discovery and disseminate the topology
as well as resource status. This information would be available to
all nodes in the network, and hence also the NMS. Hence one can
look at a continuum of functionality between manually provisioned
topology information (of which there will always be some) and fully
automated discovery and dissemination as in a link state protocol.
Note that, unlike the IP datagram case, a link state routing
protocol applied to the SDH/SONET network does not have any service
impacting implications. This is because in the SDH/SONET case, the
circuit is source-routed (so there can be no loops), and no traffic
is transmitted until a circuit has been established, and an
acknowledgement received at the source.
1.4.2. Path Computation (Route Determination)
In the SDH/SONET case, unlike the IP datagram case, there is no need
for network elements to all perform the same path calculation [9].
In addition, path determination is an area for vendors to provide a
potentially significant value addition in terms of network
efficiency, reliability, and service differentiation. In this sense,
a centralized approach to path computation may be easier to operate
and upgrade. For example, new features such as new types of path
diversity or new optimization algorithms can be introduced with a
simple NMS software upgrade. On the other hand, updating switches
with new path computation software is a more complicated task. In
addition, many of the algorithms can be fairly computationally
intensive and may be completely unsuitable for the embedded
processing environment available on most switches. In restoration
scenarios, the ability to perform a reasonably sophisticated level
of path computation on the network element can be particularly
useful for restoring traffic during major network faults.
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1.4.3. Connection Establishment (Provisioning)
The actual setting up of circuits, i.e., a coupled collection of
cross connects across a network, can be done either via the NMS
setting up individual cross connects or via a "soft permanent LSP"
(SPLSP) type approach. In the SPLSP approach, the NMS may just kick
off the connection at the "ingress" switch with GMPLS signaling
setting up the connection from that point onward. Connection
establishment is the trickiest part to distribute, however, since
errors in the connection setup/tear down process are service
impacting.
The table below compares the two approaches to connection
establishment.
Distributed approach Centralized approach
Packet-based control plane Management plane like TMN or
(like GMPLS or PNNI) useful? SNMP
Do we really need it? Being Always needed! Already there,
added/specified by several proven and understood.
standardization bodies
High survivability (e.g. in Potential single point(s) of
case of partition) failure
Distributed load Bottleneck: #requests and
actions to/from NMS
Individual local routing Centralized routing decision,
decision can be done per block of
requests
Routing scalable as for the Assumes a few big
Internet administrative domains
Complex to change routing Very easy local upgrade (non
protocol/algorithm intrusive)
Requires enhanced routing Better consistency
protocol (traffic
engineering)
Ideal for inter-domain Not inter-domain friendly
Suitable for very dynamic For less dynamic demands
demands (longer lived)
Probably faster to restore, Probably slower to restore,but
but more difficult to have could effect reliable
reliable restoration. restoration.
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High scalability Limited scalability: #nodes,
(hierarchical) links, circuits, messages
Planning (optimization) Planning is a background
harder to achieve centralized activity
Easier future integration
with other control plane
layers
Table 1. Qualitative comparison between centralized and distributed
approaches.
1.5. Why SDH/SONET will not Disappear Tomorrow
As IP traffic becomes the dominant traffic transported over the
transport infrastructure, it is useful to compare the statistical
multiplexing of IP with the time division multiplexing of SDH and
SONET.
Consider, for instance, a scenario where IP over WDM is used
everywhere and lambdas are optically switched. In such a case, a
carrier's carrier would sell dynamically controlled lambdas with
each customers building their own IP backbones over these lambdas.
This simple model implies that a carrier would sell lambdas instead
of bandwidth. The carrier's goal will be to maximize the number of
wavelengths/lambdas per fiber, with each customer having to fully
support the cost for each end-to-end lambda whether or not the
wavelength is fully utilized. Although, in the near future, we may
have technology to support up to several hundred lambdas per fiber,
a world where lambdas are so cheap and abundant that every
individual customer buys them, from one point to any other point,
appears an unlikely scenario today.
More realistically, there is still room for a multiplexing
technology that provides circuits with a lower granularity than a
wavelength. (Not everyone needs a minimum of 10 Gbps or 40 Gbps per
circuit, and IP does not yet support all telecom applications in
bulk efficiently.)
SDH and SONET possess a rich multiplexing hierarchy that permits
fairly fine granularity and that provides a very cheap and simple
physical separation of the transported traffic between circuits,
i.e., QoS. Moreover, even IP datagrams cannot be transported
directly over a wavelength. A framing or encapsulation is always
required to delimit IP datagrams. The Total Length field of an IP
header cannot be trusted to find the start of a new datagram, since
it could be corrupted and would result in a loss of synchronization.
The typical framing used today for IP over DWDM is defined in
RFC1619/RFC2615 and known as POS (Packet Over SDH/SONET), i.e., IP
over PPP (in HDLC-like format) over SDH/SONET. SDH and SONET are
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actually efficient encapsulations for IP. For instance, with an
average IP datagram length of 350 octets, an IP over GBE
encapsulation using an 8B/10B encoding results in 28% overhead, an
IP/ATM/SDH encapsulation results in 22% overhead and an IP/PPP/SDH
encapsulation results in only 6% overhead.
Any encapsulation of IP over WDM should, in the data plane, at least
provide error monitoring capabilities (to detect signal
degradation); error correction capabilities, such as FEC (Forward
Error Correction) that are particularly needed for ultra long haul
transmission; sufficient timing information, to allow robust
synchronization (that is, to detect the beginning of a packet). In
the case where associated signaling is used (that is the control and
data plane topologies are congruent) the encapsulation should also
provide the capacity to transport signaling, routing and management
messages, in order to control the optical switches. Rather SDH and
SONET cover all these aspects natively, except FEC, which tends to
be supported in a proprietary way. (We note, however, that
associated signaling is not a requirement for the GMPLS-based
control of SDH/SONET networks. Rather, it is just one option. Non
associated signaling, as would happen with an out-of-band control
plane network is another equally valid option.)
Since IP encapsulated in SDH/SONET is efficient and widely used, the
only real difference between an IP over WDM network and an IP over
SDH over WDM network is the layers at which the switching or
forwarding can take place. In the first case, it can take place at
the IP and optical layers. In the second case, it can take place at
the IP, SDH/SONET, and optical layers.
Almost all transmission networks today are based on SDH or SONET.
A client is connected either directly through an SDH or SONET
interface or through a PDH interface, the PDH signal being
transported between the ingress and the egress interfaces over SDH
or SONET. What we are arguing here is that it makes sense to do
switching or forwarding at all these layers.
2. GMPLS Applied to SDH/SONET
2.1. Controlling the SDH/SONET Multiplex
Controlling the SDH/SONET multiplex implies deciding which of the
different switchable components of the SDH/SONET multiplex do we
wish to control using GMPLS. Essentially, every SDH/SONET element
that is referenced by a pointer can be switched. These component
signals are the VC-4, VC-3, VC-2, VC-12 and VC-11 in the SDH case;
and the VT and STS SPEs in the SONET case. The SPEs in SONET do not
have individual names, although they can be referred to simply as
VT-N SPEs. We will refer to them by identifying the structure that
contains them, namely STS-1, VT-6, VT-3, VT-2 and VT-1.5.
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The STS-1 SPE corresponds to a VC-3, a VT-6 SPE corresponds to a VC-
2, a VT-2 SPE corresponds to a VC-12, and a VT-1.5 SPE corresponds
to a VC-11. The SONET VT-3 SPE has no correspondence in SDH, however
SDH's VC-4 corresponds to SONET's STS-3c SPE.
In addition, it is possible to concatenate some of the structures
that contain these elements to build larger elements. For instance,
SDH allows the concatenation of X contiguous AU-4s to build a VC-4-
Xc and of m contiguous TU-2s to build a VC-2-mc. In that case, a VC-
4-Xc or a VC-2-mc can be switched and controlled by GMPLS. SDH also
defines virtual (non-contiguous) concatenation of TU- 2s, however
in that case each constituent VC-2 is switched individually.
2.2. SDH/SONET LSR and LSP Terminology
Let a SDH or SONET Terminal Multiplexer (TM), Add-Drop Multiplexer
(ADM) or cross-connect (i.e. a switch) be called an SDH/SONET LSR. A
SDH/SONET path or circuit between two SDH/SONET LSRs now becomes a
GMPLS LSP. An SDH/SONET LSP is a logical connection between the
point at which a tributary signal (client layer) is adapted into its
virtual container, and the point at which it is extracted from its
virtual container.
To establish such an LSP, a signaling protocol is required to
configure the input interface, switch fabric, and output interface
of each SDH/SONET LSR along the path. An SDH/SONET LSP can be
point-to-point or point-to-multipoint, but not multipoint-to-point,
since no merging is possible with SDH/SONET signals.
To facilitate the signaling and setup of SDH/SONET circuits, an
SDH/SONET LSR must, therefore, identify each possible signal
individually per interface, since each signal corresponds to a
potential LSP that can be established through the SDH/SONET LSR. It
turns out, however, that not all SDH signals correspond to an LSP
and therefore not all of them need be identified. In fact, only
those signals that can be switched need identification.
3. Decomposition of the GMPLS Circuit-Switching Problem Space
Although those familiar with GMPLS may be familiar with its
application in a variety of application areas, e.g., ATM, Frame
Relay, and so on, here we quickly review its decomposition when
applied to the optical switching problem space.
(i) Information needed to compute paths must be made globally
available throughout the network. Since this is done via the link
state routing protocol, any information of this nature must either
be in the existing link state advertisements (LSAs) or the LSAs must
be supplemented to convey this information. For example, if it is
desirable to offer different levels of service in a network based on
whether a circuit is routed over SDH/SONET lines that are ring
protected versus being routed over those that are not ring protected
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(differentiation based on reliability), the type of protection on a
SDH/SONET line would be an important topological parameter that
would have to be distributed via the link state routing protocol.
(ii) Information that is only needed between two "adjacent" switches
for the purposes of connection establishment is appropriate for
distribution via one of the label distribution protocols. In fact,
this information can be thought of as the "virtual" label. For
example, in SONET networks, when distributing information to
switches concerning an end-to-end STS-1 path traversing a network,
it is critical that adjacent switches agree on the multiplex entry
used by this STS-1 (but this information is only of local
significance between those two switches). Hence, the multiplex entry
number in this case can be used as a virtual label. Note that the
label is virtual, in that it is not appended to the payload in any
way, but it is still a label in the sense that it uniquely
identifies the signal locally on the link between the two switches.
(iii) Information that all switches in the path need to know about a
circuit will also be distributed via the label distribution
protocol. Examples of such information include bandwidth, priority,
and preemption for instance.
(iv) Information intended only for end systems of the connection.
Some of the payload type information in may fall into this category.
4. GMPLS Routing for SDH/SONET
Modern SDH/SONET transport networks excel at interoperability in the
performance monitoring (PM) and fault management (FM) areas [10],
[11]. They do not, however, interoperate in the areas of topology
discovery or resource status. Although link state routing protocols,
such as IS-IS and OSPF, have been used for some time in the IP world
to compute destination-based next hops for routes (without routing
loops), they are particularly valuable for providing timely topology
and network status information in a distributed manner, i.e., at any
network node. If resource utilization information is disseminated
along with the link status (as done in ATM's PNNI routing protocol)
then a very complete picture of network status is available to a
network operator for use in planning, provisioning and operations.
The information needed to compute the path a connection will take
through a network is important to distribute via the routing
protocol. In the TDM case, this information includes, but is not
limited to: the available capacity of the network links, the
switching and termination capabilities of the nodes and interfaces,
and the protection properties of the link. This is what is being
proposed in the GMPLS extensions to IP routing protocols [12], [13],
[14].
When applying routing to circuit switched networks it is useful to
compare and contrast this situation with the datagram routing case
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[15]. In the case of routing datagrams, all routes on all nodes
must be calculated exactly the same to avoid loops and "black
holes". In circuit switching, this is not the case since routes are
established per circuit and are fixed for that circuit. Hence,
unlike the datagram case, routing is not service impacting in the
circuit switched case. This is helpful, because, to accommodate the
optical layer, routing protocols need to be supplemented with new
information, much more than the datagram case. This information is
also likely to be used in different ways for implementing different
user services. Due to the increase in information transferred in
the routing protocol, it may be useful to separate the relatively
static parameters concerning a link from those that may be subject
to frequent changes. The current GMPLS routing extensions
[12], [13], [14] do not make such a separation, however.
Indeed, from the carriers' perspective, the up-to-date dissemination
of all link properties is essential and desired, and the use of a
link-state routing protocol to distribute this information provides
timely and efficient delivery. If GMPLS-based networks got to the
point that bandwidth updates happen very frequently, it makes sense,
from an efficiency point of view, to separate them out for update.
This situation is not yet seen in actual networks; however, if GMPLS
signaling is put into widespread use then the need could arise.
4.1. Switching Capabilities
The main switching capabilities that characterize a SDH/SONET end
system and thus need to be advertised via the link state routing
protocol are: the switching granularity, supported forms of
concatenation, and the level of transparency.
4.1.1. Switching Granularity
From references [4], [5] and the overview section on SDH/SONET we
see that there are a number of different signals that compose the
SDH/SONET hierarchies. Those signals that are referenced via a
pointer, i.e., the VCs in SDH and the SPEs in SONET are those that
will actually be switched within a SDH/SONET network. These signals
are subdivided into lower order signals and higher order signals as
shown in Table 2.
Table 2. SDH/SONET switched signal groupings.
Signal Type SDH SONET
Lower Order VC-11, VC-12, VC-2 VT-1.5 SPE, VT-2 SPE,
VT-3 SPE, VT-6 SPE
Higher VC-3, VC-4 STS-1 SPE, STS-3c SPE
Order
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Manufacturers today differ in the types of switching capabilities
their systems support. Many manufacturers today switch signals
starting at VC-4 for SDH or STS-1 for SONET (i.e. the basic frame)
and above (see Section 5.1.2 on concatenation), but they do not
switch lower order signals. Some of them only allow the switching of
entire aggregates (concatenated or not) of signals such as 16 VC-4s,
i.e. a complete STM-16, and nothing finer. Some go down to the VC-3
level for SDH. Finally, some offer highly integrated switches that
switch at the VC-3/STS-1 level down to lower order signals such as
VC-12s. In order to cover the needs of all manufacturers and
operators, GMPLS signaling ([6], [7]) covers both higher order and
lower order signals.
4.1.2. Signal Concatenation Capabilities
As stated in the SDH/SONET overview, to transport tributary signals
with rates in excess of the basic STM-1/STS-1 signal, the VCs/SPEs
can be concatenated, i.e., glued together. Different types of
concatenations are defined: contiguous standard concatenation,
arbitrary concatenation, and virtual concatenation with different
rules concerning their size, placement, and binding.
Standard SONET concatenation allows the concatenation of M x STS-1
signals within an STS-N signal with M <= N, and M = 3, 12, 48, 192,
...). The SPEs of these M x STS-1s can be concatenated to form an
STS-Mc. The STS-Mc notation is short hand for describing an STS-M
signal whose SPEs have been concatenated. The multiplexing
procedures for SDH and SONET are given in references [4] and [5],
respectively. Constraints are imposed on the size of STS-Mc signals,
i.e., they must be a multiple of 3, and on their starting location
and interleaving.
This has the following advantages: (a) restriction to multiples of 3
helps with SDH compatibility (there is no STS-1 equivalent signal in
SDH); (b) the restriction to multiples of 3 reduces the number of
connection types; (c) the restriction on the placement and
interleaving could allow more compact representation of the "label";
The major disadvantages of these restrictions are:
(a) Limited flexibility in bandwidth assignment (somewhat inhibits
finer grained traffic engineering). (b) The lack of flexibility in
starting time slots for STS-Mc signals and in their interleaving
(where the rest of the signal gets put in terms of STS-1 slot
numbers) leads to the requirement for re-grooming (due to bandwidth
fragmentation).
Due to these disadvantages some SONET framer manufacturers now
support "flexible" or arbitrary concatenation, i.e., no restrictions
on the size of an STS-Mc (as long as M <= N) and no constraints on
the STS-1 timeslots used to convey it, i.e., the signals can use any
combination of available time slots.
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Standard and flexible concatenations are network services, while
virtual concatenation is a SDH/SONET end-system service approved by
the Committee T1 of ANSI [5] and the ITU-T [4]. The essence of this
service is to have SDH/SONET end systems "glue" together the VCs or
SPEs of separate signals rather than requiring that the signals be
carried through the network as a single unit. In one example of
virtual concatenation, two end systems supporting this feature could
essentially "inverse multiplex" two STS-1s into a STS-1-2v for the
efficient transport of 100 Mbps Ethernet traffic. Note that this
inverse multiplexing process (or virtual concatenation) can be
significantly easier to implement with SDH/SONET than packet switched
circuits, because ensuring that timing and in-order frame delivery is
preserved may be simpler to establish using SDH/SONET rather than
packet switched circuits, where more sophisticated techniques may be
needed.
Since virtual concatenation is provided by end systems, it is
compatible with existing SDH/SONET networks. Virtual concatenation
is defined for both higher order signals and low order signals.
Table 3 shows the nomenclature and capacity for several lower-order
virtually concatenated signals contained within different
higher-order signals.
Table 3 Capacity of Virtually Concatenated VTn-Xv (9/G.707)
Carried In X Capacity In steps
of
VT1.5/ STS-1/VC-3 1 to 28 1600kbit/s to 1600kbit/s
VC-11-Xv 44800kbit/s
VT2/ STS-1/VC-3 1 to 21 2176kbit/s to 2176kbit/s
VC-12-Xv 45696kbit/s
VT1.5/ STS-3c/VC-4 1 to 64 1600kbit/s to 1600kbit/s
VC-11-Xv 102400kbit/s
VT2/ STS-3c/VC-4 1 to 63 2176kbit/s to 2176kbit/s
VC-12-Xv 137088kbit/s
4.1.3. SDH/SONET Transparency
The purposed of SDH/SONET is to carry its payload signals in a
transparent manner. This can include some of the layers of SONET
itself. For example, situations where the path overhead can never be
touched, since it actually belongs to the client. This was another
reason for not coding an explicit label in the SDH/SONET path
overhead. It may be useful to transport, multiplex and/or switch
lower layers of the SONET signal transparently.
As mentioned in the introduction, SONET overhead is broken into
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three layers: Section, Line and Path. Each of these layers is
concerned with fault and performance monitoring. The Section
overhead is primarily concerned with framing, while the Line
overhead is primarily concerned with multiplexing and protection. To
perform pipe multiplexing (that is, multiplexing of 50 Mbps or 150
Mbps chunks), a SONET network element should be line terminating.
However, not all SONET multiplexers/switches perform SONET pointer
adjustments on all the STS-1s contained within a higher order SONET
signal passing through them. Alternatively, if they perform pointer
adjustments, they do not terminate the line overhead. For example, a
multiplexer may take four SONET STS-48 signals and multiplex them
onto an STS-192 without performing standard line pointer adjustments
on the individual STS-1s. This can be looked at as a service since
it may be desirable to pass SONET signals, like an STS-12 or STS-48,
with some level of transparency through a network and still take
advantage of TDM technology. Transparent multiplexing and switching
can also be viewed as a constraint, since some multiplexers and
switches may not switch with as fine a granularity as others. Table
4 summarizes the levels of SDH/SONET transparency.
Table 4. SDH/SONET transparency types and their properties.
Transparency Type Comments
Path Layer (or Line Standard higher order SONET path
Terminating) switching. Line overhead is terminated
or modified.
Line Level (or Section Preserves line overhead and switches
Terminating) the entire line multiplex as a whole.
Section overhead is terminated or
modified.
Section layer Preserves all section overhead,
Basically does not modify/terminate
any of the SDH/SONET overhead bits.
4.2. Protection
SONET and SDH networks offer a variety of protection options at both
the SONET line (SDH multiplex section) and SDH/SONET path level
[10], [11]. Standardized SONET line level protection techniques
include: Linear 1+1 and linear 1:N automatic protection switching
(APS) and both two-fiber and four-fiber bi-directional line switched
rings (BLSRs). At the path layer, SONET offers uni-directional path
switched ring protection. Likewise, standardized SDH multiplex
section protection techniques include linear 1+1 and 1:N automatic p
protection switching and both two-fiber and four-fiber bi-directional
MS-SPRings (Multiplex Section-Shared Protection Rings).
At the path layer, SDH offers SNCP (sub-network connection
protection) ring protection.
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Both ring and 1:N line protection also allow for "extra traffic" to
be carried over the protection line when that line is not being
used, i.e., when it is not carrying traffic for a failed working
line. These protection methods are summarized in Table 5. It should
be noted that these protection methods are completely separate from
any GMPLS layer protection or restoration mechanisms.
Table 5. Common SDH/SONET protection mechanisms.
Protection Type Extra Comments
Traffic
Optionally
Supported
1+1 No Requires no coordination
Unidirectional between the two ends of the
circuit. Dedicated
protection line.
1+1 Bi- No Coordination via K byte
directional protocol. Lines must be
consistently configured.
Dedicated protection line.
1:1 Yes Dedicated protection.
1:N Yes One Protection line shared
by N working lines
4F-BLSR (4 Yes Dedicated protection, with
fiber bi- alternative ring path.
directional
line switched
ring)
2F-BLSR (2 Yes Dedicated protection, with
fiber bi- alternative ring path
directional
line switched
ring)
UPSR (uni- No Dedicated protection via
directional alternative ring path.
path switched Typically used in access
ring) networks.
It may be desirable to route some connections over lines that
support protection of a given type, while others may be routed over
unprotected lines, or as "extra traffic" over protection lines.
Also, to assist in the configuration of these various protection
methods it can be extremely valuable to advertise the link
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protection attributes in the routing protocol, as is done in the
current GMPLS routing protocols. For example, suppose that a 1:N
protection group is being configured via two nodes. One must make
sure that the lines are "numbered the same" with respect to both
ends of the connection or else the APS (K1/K2 byte) protocol will
not correctly operate.
Table 6. Parameters defining protection mechanisms.
Protection Comments
Related Link
Information
Protection Type Indicates which of the protection types
delineated in Table 5.
Protection Indicates which of several protection
Group Id groups (linear or ring) that a node belongs
to. Must be unique for all groups that a
node participates in
Working line Important in 1:N case and to differentiate
number between working and protection lines
Protection line Used to indicate if the line is a
number protection line.
Extra Traffic Yes or No
Supported
Layer If this protection parameter is specific to
SONET then this parameter is unneeded,
otherwise it would indicate the signal
layer that the protection is applied.
An open issue concerning protection is the extent of information
regarding protection that must be disseminated. The contents of
Table 6 represent one extreme while a simple enumerated list of:
Extra-Traffic/Protection line, Unprotected, Shared (1:N)/Working
line, Dedicated (1:1, 1+1)/Working Line, Enhanced (Ring) /Working
Line, represents the other.
There is also a potential implication for link bundling [16], [18]
that is, for each link, the routing protocol could advertise whether
that link is a working or protection link and possibly some
parameters from Table 6. A possible drawback of this scheme is that
the routing protocol would be burdened with advertising properties
even for those protection links in the network that could not, in
fact, be used for routing working traffic, e.g., dedicated
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protection links. An alternative method would be to bundle the
working and protection links together, and advertise the bundle
instead. Now, for each bundled link, the protocol would have to
advertise the amount of bandwidth available on its working links, as
well as the amount of bandwidth available on those protection links
within the bundle that were capable of carrying "extra traffic."
This would reduce the amount of information to be advertised. An
issue here would be to decide which types of working and protection
links to bundle together. For instance, it might be preferable to
bundle working links (and their corresponding protection links) that
are "shared" protected separately from working links that are
"dedicated" protected.
4.3. Available Capacity Advertisement
Each SDH/SONET LSR must maintain an internal table per interface
that indicates each signal in the multiplex structure that is
allocated at that interface. This internal table is the most
complete and accurate view of the link usage and available capacity.
For use in path computation, this information needs to be advertised
in some way to all others SDH/SONET LSRs in the same domain. There
is a trade off to be reached concerning: the amount of detail in the
available capacity information to be reported via a link state
routing protocol, the frequency or conditions under which this
information is updated, the percentage of connection establishments
that are unsuccessful on their first attempt due to the granularity
of the advertised information, and the extent to which network
resources can be optimized. There are different levels of
summarization that are being considered today for the available
capacity information. At one extreme, all signals that are allocated
on an interface could be advertised, while at the other extreme, a
single aggregated value of the available bandwidth per link could be
advertised.
Consider first the relatively simple structure of SONET and its most
common current and planned usage. DS1s and DS3s are the signals most
often carried within a SONET STS-1. Either a single DS3 occupies
the STS-1 or up to 28 DS1s (4 each within the 7 VT groups) are
carried within the STS-1. With a reasonable VT1.5 placement
algorithm within each node it may be possible to just report on
aggregate bandwidth usage in terms of number of whole STS-1s
(dedicated to DS3s) used and the number of STS-1s dedicated to
carrying DS1s allocated for this purpose. This way a network
optimization program could try to determine the optimal placement of
DS3s and DS1s to minimize wasted bandwidth due to half-empty STS-1s
at various places within the transport network. Similarly consider
the set of super rate SONET signals (STS-Nc). If the links between
the two switches support flexible concatenation then the reporting
is particularly straightforward since any of the STS-1s within an
STS-M can be used to comprise the transported STS-Nc. However, if
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only standard concatenation is supported, then reporting gets
trickier since there are constraints on where the STS-1s can be
placed. SDH has still more options and constraints, hence it is not
yet clear which is the best way to advertise bandwidth resource
availability/usage in SDH/SONET. At present, the GMPLS routing
protocol extensions define minimum and maximum values for available
bandwidth, which allows a remote node to make some deductions about
the amount of capacity available at a remote link and the types of
signals it can accommodate. However, due to the multiplexed nature
of the signals, reporting of bandwidth particular to signal types
rather than as a single aggregate bit rate may be desirable. For
details on why this may be the case, we refer the reader to ITU-T
publications G.7715.1 [19] and to Chapter 12 of [20].
4.4. Path Computation
Although a link state routing protocol can be used to obtain network
topology and resource information, this does not imply the use of an
"open shortest path first" route [9]. The path must be open in the
sense that the links must be capable of supporting the desired
signal type and that capacity must be available to carry the signal.
Other constraints may include hop count, total delay (mostly
propagation), and underlying protection. In addition, it may be
desirable to route traffic in order to optimize overall network
capacity, or reliability, or some combination of the two. Dikstra's
algorithm computes the shortest path with respect to link weights
for a single connection at a time. This can be much different than
the paths that would be selected in response to a request to set up
a batch of connections between a set of endpoints in order to
optimize network link utilization. One can think of this along the
lines of global or local optimization of the network in time.
Due to the complexity of some of the connection routing algorithms
(high dimensionality, non-linear integer programming problems) and
various criteria by which one may optimize a network, it may not be
possible or desirable to run these algorithms on network nodes.
However, it may still be desirable to have some basic path
computation ability running on the network nodes, particularly for
use during restoration situations. Such an approach is in line with
the use of GMPLS for traffic engineering, but is much different than
typical OSPF or IS-IS usage where all nodes must run the same
routing algorithm.
5. LSP Provisioning/Signaling for SDH/SONET
Traditionally, end-to-end circuit connections in SDH/SONET networks
have been set up via network management systems (NMSs), which issue
commands (usually under the control of a human operator) to the
various network elements involved in the circuit, via an equipment
vendor's element management system (EMS). Very little multi-vendor
interoperability has been achieved via management systems. Hence,
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end-to-end circuits in a multi-vendor environment typically require
the use of multiple management systems and the infamous configuration
via "yellow sticky notes". As discussed in Section 3, a common
signaling protocol - such as RSVP with TE extensions or CR-LDP -
appropriately extended for circuit switching applications, could
therefore help to solve these interoperability problems. In this
section, we examine the various components involved in the automated
provisioning of SDH/SONET LSPs.
5.1. What do we Label in SDH/SONET? Frames or Circuits?
GMPLS was initially introduced to control asynchronous technologies
like IP, where a label was attached to each individual block of
data, such as an IP packet or a Frame Relay frame. SONET and SDH,
however, are synchronous technologies that define a multiplexing
structure (see Section 3), which we referred to as the SDH (or
SONET) multiplex. This multiplex involves a hierarchy of signals,
lower order signals embedded within successive higher order ones
(see Fig. 1). Thus, depending on its level in the hierarchy, each
signal consists of frames that repeat periodically, with a certain
number of byte time slots per frame.
The question then arises: is it these frames that we label in GMPLS?
It will be seen in what follows that each SONET or SDH "frame"
need not have its own label, nor is it necessary to switch frames
individually. Rather, the unit that is switched is a "flow"
comprised of a continuous sequence of time slots that appear at a
given position in a frame. That is, we switch an individual SONET or
SDH signal, and a label associated with each given signal.
For instance, the payload of an SDH STM-1 frame does not fully
contain a complete unit of user data. In fact, the user data is
contained in a virtual container (VC) that is allowed to float over
two contiguous frames for synchronization purposes. The H1-H2-H3
Au-n pointer bytes in the SDH overhead indicates the beginning of the
VC in the payload. Thus, frames are now inter-related, since each
consecutive pair may share a common virtual container. From the
point of view of GMPLS, therefore, it is not the successive frames
that are treated independently or labeled, but rather the entire
user signal. An identical argument applies to SONET.
Observe also that the GMPLS signaling used to control the SDH/SONET
multiplex must honor its hierarchy. In other words, the SDH/SONET
layer should not be viewed as homogeneous and flat, because this
would limit the scope of the services that SDH/SONET can provide.
Instead, GMPLS tunnels should be used to dynamically and
hierarchically control the SDH/SONET multiplex. For example, one
unstructured VC-4 LSP may be established between two nodes, and
later lower order LSPs (e.g. VC-12) may be created within that
higher order LSP. This VC-4 LSP can, in fact, be established
between two non-adjacent internal nodes in an SDH network, and later
advertised by a routing protocol as a new (virtual) link called a
Forwarding Adjacency (FA) [17].
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A SDH/SONET-LSR will have to identify each possible signal
individually per interface to fulfill the GMPLS operations. In order
to stay transparent the LSR obviously should not touch the SDH/SONET
overheads; this is why an explicit label is not encoded in the
SDH/SONET overheads. Rather, a label is associated with each
individual signal. This approach is similar to the one considered
for lambda switching, except that it is more complex, since SONET
and SDH define a richer multiplexing structure. Therefore a label
is associated with each signal, and is locally unique for each
signal at each interface. This signal could, and will most probably,
occupy different time-slots at different interfaces.
5.2. Label Structure in SDH/SONET
The signaling protocol used to establish an SDH/SONET LSP must have
specific information elements in it to map a label to the particular
signal type that it represents, and to the position of that signal
in the SDH/SONET multiplex. As we will see shortly, with a
carefully chosen label structure, the label itself can be made to
function as this information element.
In general, there are two ways to assign labels for signals between
neighboring SDH/SONET LSRs. One way is for the labels to be
allocated completely independently of any SDH/SONET semantics; e.g.
labels could just be unstructured 16 or 32 bit numbers. In that
case, in the absence of appropriate binding information, a label
gives no visible information about the flow that it represents. From
a management and debugging point of view, therefore, it becomes
difficult to match a label with the corresponding signal, since , as
we saw in Section 6.1, the label is not coded in the SDH/SONET
overhead of the signal.
Another way is to use the well-defined and finite structure of the
SDH/SONET multiplexing tree to devise a signal numbering scheme that
makes use of the multiplex as a naming tree, and assigns each
multiplex entry a unique associated value. This allows the unique
identification of each multiplex entry (signal) in terms of its type
and position in the multiplex tree. By using this multiplex entry
value itself as the label, we automatically add SDH/SONET semantics
to the label! Thus, simply by examining the label, one can now
directly deduce the signal that it represents, as well as its
position in the SDH/SONET multiplex. We refer to this as
multiplex-based labeling. This is the idea that was incorporated in
the GMPLS signaling specifications for SDH/SONET [18].
5.3. Signaling Elements
In the preceding sections, we defined the meaning of a SDH/SONET
label and specified its structure. A question that arises naturally
at this point is the following. In an LSP or connection setup
request, how do we specify the signal for which we want to establish
a path (and for which we desire a label)?
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Clearly, information that is required to completely specify the
desired signal and its characteristics must be transferred via the
label distribution protocol, so that the switches along the path can
be configured to correctly handle and switch the signal. This
information is specified in three parts [18], each of which refers
to a different network layer.
1. GENERALIZED_LABEL REQUEST (as in [6], [7]), which contains three
parts: LSP Encoding Type, Switching Type, and G-PID.
The first specifies the nature/type of the LSP or the desired
SDH/SONET channel, in terms of the particular signal (or collection
of signals) within the SDH/SONET multiplex that the LSP represents,
and is used by all the nodes along the path of the LSP.
The second specifies certain link selection constraints, which
control, at each hop, the selection of the underlying link that is
used to transport this LSP.
The third specifies the payload carried by the LSP or SDH/SONET
channel, in terms of the termination and adaptation functions
required at the end points, and is used by the source and
destination nodes of the LSP.
2. SONET/SDH TRAFFIC_PARAMETERS (as in [18], Section 2.1) used as a
SENDER_TSPEC/FLOWSPEC, which contains 7 parts: Signal Type,
(Requested Contiguous Concatenation (RCC), Number of Contiguous
Components (NCC), Number of Virtual Components (NVC)), Multiplier
(MT), Transparency, and Profile.
The Signal Type indicates the type of elementary signal comprising
the LSP, while the remaining fields indicate transforms that can be
applied to the basic signal to build the final signal that
corresponds to the LSP actually being requested. For instance (see
[18] for details):
- Contiguous concatenation (by using the RCC and NCC
fields) can be optionally applied on the Elementary Signal,
resulting in a contiguously concatenated signal.
- Then, virtual concatenation (by using the NVC field) can be
optionally applied on the Elementary Signal resulting in
a virtually concatenated signal.
- Third, some transparency (by using the Transparency field)
can be optionally specified when requesting a frame as
signal rather than an SPE or VC based signal.
- Fourth, a multiplication (by using the Multiplier field)
can be optionally applied either directly on the Elementary
Signal, or on the contiguously concatenated signal obtained
from the first phase, or on the virtually concatenated signal
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obtained from the second phase, or on these signals combined
with some transparency.
Transparency indicates precisely which fields in these overheads
must be delivered unmodified at the other end of the LSP. An ingress
LSR requesting transparency will pass these overhead fields that
must be delivered to the egress LSR without any change. From the
ingress and egress LSRs point of views, these fields must be seen as
unmodified.
Transparency is not applied at the interfaces with the initiating
and terminating LSRs, but is only applied between intermediate LSRs.
The transparency field is used to request an LSP that supports the
requested transparency type; it may also be used to setup the
transparency process to be applied at each intermediate LSR.
Finally, the profile field is intended particular capabilities that
must be supported for the LSP, for example monitoring capabilities.
No standard profile is currently defined, however.
3. UPSTREAM_LABEL for Bi-directional LSP's (as in [6], [7]).
4. Local Link Selection e.g. IF_ID_RSVP_HOP Object (as in [7]).
6. Summary and Conclusions
We provided a detailed account of the issues involved in applying
generalized GMPLS-based control (GMPLS) to TDM networks.
We began with a brief overview of GMPLS and SDH/SONET networks,
discussing current circuit establishment in TDM networks, and
arguing why SDH/SONET technologies will not be "outdated" in the
foreseeable future. Next, we looked at IP/MPLS applied to SDH/SONET
networks, where we considered why such an application makes sense,
and reviewed some GMPLS terminology as applied to TDM networks.
We considered the two main areas of application of IP/MPLS methods
to TDM networks, namely routing and signaling, and discussed how
Generalized MPLS routing and signaling are used in the context of
TDM networks. We reviewed in detail the switching capabilities of
TDM equipment, and the requirement to learn about the protection
capabilities of underlying links, and how these influence the
available capacity advertisement in TDM networks.
We focused briefly on path computation methods, pointing out that
these were not subject to standardization. We then examined optical
path provisioning or signaling, considering the issue of what
constitutes an appropriate label for TDM circuits and how this label
should be structured, and we focused on the importance of
hierarchical label allocation in a TDM network. Finally, we reviewed
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the signaling elements involved when setting up an TDM circuit,
focusing on the nature of the LSP, the type of payload it carries,
and the characteristics of the links that the LSP wishes to use at
each hop along its path for achieving a certain reliability.
7. Security Considerations
This document describes the framework for GMPLS extensions for use
in SDH/SONET control. As such, it introduces no new security issues
with respect to GMPLS specifications. GMPLS protocol specifications
should identify and address security issues specific to protocol.
Among the considerations that should be addressed by GMPLS protocol
specifications, are any security vulnerabilities that are introduced
by specific GMPLS extensions added in each specification.
8. Acknowledgments
We acknowledge all the participants of the MPLS and CCAMP WGs, whose
constant enquiry about GMPLS issues in TDM networks motivated the
writing of this document, and whose questions helped shape its
contents. Also, thanks to Kireeti Kompella for his careful reading
of the last version of this document, and for his helpful comments
and feedback, and to Dimitri Papadimitriou for his review on behalf
of the Routing Area Directorate, which provided many useful inputs
to help update the document to conform to the standards evolutions
since this document passed last call.
9. Author's Addresses
Greg Bernstein
Grotto Networking
Phone: +1 510 573-2237
E-mail: gregb@grotto-networking.com
Eric Mannie
InterAir Link
Phone: +32 2 790 34 25
E-mail: eric_mannie@hotmail.com
Vishal Sharma
Metanoia, Inc.
888 Villa Street, Suite 200B
Mountain View, CA 94041
Phone: +1 408 530 8313
Email: v.sharma@ieee.org
Eric Gray
Marconi Communications
E-mail: Eric.Gray@Marconi.com
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10. References
10.1. Normative References
[1] Bradner, S., "IETF Rights in Contributions" BCP 78, RFC 3667,
February, 2004.
[2] Bradner, S., "Intellectual Property Rights in IETF Technology",
BCP 79, RFC 3668, February, 2004.
10.2. Informative References
In the ITU references below, please see http://www.itu.int for
availability of ITU documents. For ANSI references, please see
the Library available through http://www.ansi.org.
[3] Rosen, E., Viswanathan, A., and Callon, R., "Multiprotocol
Label Switching Architecture", RFC 3031, January 2001.
[4] G.707, Network Node Interface for the Synchronous Digital
Hierarchy (SDH), International Telecommunication Union, March
1996.
[5] ANSI T1.105-1995, Synchronous Optical Network (SONET) Basic
Description including Multiplex Structure, Rates, and Formats,
American National Standards Institute.
[6] Berger, L. (Editor), "Generalized MPLS - Signaling Functional
Description," RFC 3471, January 2003.
[7] Berger, L. (Editor), "Generalized MPLS Signaling - RSVP-TE
Extensions," RFC 3473, January 2003.
[8] Omitted.
[9] Bernstein, G., Yates, J., Saha, D., "IP-Centric Control and
Management of Optical Transport Networks," IEEE Communications
Mag., Vol. 40, Issue 10, October 2000.
[10] ANSI T1.105.01-1995, Synchronous Optical Network (SONET)
Automatic Protection Switching, American National Standards
Institute.
[11] G.841, Types and Characteristics of SDH Network Protection
Architectures, ITU-T, July 1995.
[12] Kompella, K., et al, "Routing Extensions in Support of
Generalized MPLS," Internet Draft, Work-in-Progress,
draft-ietf-ccamp-gmpls-routing-09.txt, October 2003.
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Bernstein, et al GMPLS based Control of SDH/SONET February 2005
[13] Kompella, K., et al, "OSPF Extensions in Support of Generalized
MPLS," Internet Draft, Work-in-Progress,
draft-ietf-ccamp-ospf-extensions-12.txt, October 2003.
[14] Kompella, K., et al, "IS-IS Extensions in Support of
Generalized MPLS," Internet Draft, Work-in-Progress,
draft-ietf-isis-gmpls-extensions-16.txt, August 2002.
[15] Bernstein, G., Sharma, V., Ong, L., "Inter-domain Optical
Routing, " OSA J. of Optical Networking, vol. 1, no. 2, pp.
80-92.
[16] Kompella, K., Rekhter, Y., and Berger, L., "Link Bundling in
MPLS Traffic Engineering", Internet Draft, Work-in-Progress,
draft-ietf-mpls-bundle-04.txt, July 2002.
[17] Kompella, K., Rekhter, Y., "LSP Hierarchy with Generalized
MPLS-TE", Internet Draft, Work-in-Progress,
draft-ietf-mpls-lsp-hierarchy-08.txt, February 2002.
[18] Mannie, E. (Editor), "GMPLS Extensions for SONET and SDH
Control", Internet Draft, Work-in-Progress,
draft-ietf-ccamp-gmpls-sonet-sdh-08.txt, February 2003.
[19] G.7715.1, ASON Routing Architecture and Requirements for
Link-State Protocols, International Telecommunications Union,
February 2004.
[20] Bernstein, G., Rajagopalan, R., and Saha, D., "Optical Network
Control: Protocols, Architectures, and Standards,"
Addison-Wesley, July 2003.
11. Intellectual Property Statement
The IETF takes no position regarding the validity or scope of any
Intellectual Property Rights or other rights that might be claimed to
pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
might or might not be available; nor does it represent that it has
made any independent effort to identify any such rights. Information
on the procedures with respect to rights in RFC documents can be
found in BCP 78 and BCP 79.
Copies of IPR disclosures made to the IETF Secretariat and any
assurances of licenses to be made available, or the result of an
attempt made to obtain a general license or permission for the use
of such proprietary rights by implementers or users of this
specification can be obtained from the IETF on-line IPR repository
at http://www.ietf.org/ipr.
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The IETF invites any interested party to bring to its attention
any copyrights, patents or patent applications, or other
proprietary rights that may cover technology that may be required
to implement this standard. Please address the information to the
IETF at ietf-ipr@ietf.org.
12. Disclaimer
This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
13. Copyright Statement
Copyright (C) The Internet Society (2004). This document is
subject to the rights, licenses and restrictions contained in BCP
78, and except as set forth therein, the authors retain all their
rights.
14. IANA Considerations
There are no IANA considerations that apply to this document.
15. Acronyms
ANSI - American National Standards Institute
APS - Automatic Protection Switching
ATM - Asynchronous Transfer Mode
BLSR - Bi-directional Line Switch Ring
CPE - Customer Premise Equipment
DLCI - Data Link Connection Identifier
ETSI - European Telecommunication Standards Institute
FEC - Forwarding Equivalency Class
GMPLS - Generalized MPLS
IP - Internet Protocol
IS-IS - Intermediate System to Intermediate System (RP)
LDP - Label Distribution Protocol
LSP - Label Switched Path
LSR - Label Switching Router
MPLS - Multi-Protocol Label Switching
NMS - Network Management System
OSPF - Open Shortest Path First (RP)
PNNI - Private Network Node Interface
PPP - Point to Point Protocol
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QoS - Quality of Service
RP - Routing Protocol
RSVP - ReSerVation Protocol
SDH - Synchronous Digital Hierarchy
SNMP - Simple Network Management Protocol
SONET - Synchronous Optical NETworking
SPE - SONET Payload Envelope
STM - Synchronous Transport Module (or Terminal Multiplexer)
STS - Synchronous Transport Signal
TDM - Time Division Multiplexer
TE - Traffic Engineering
TMN - Telecommunication Management Network
UPSR - Uni-directional Path Switch Ring
VC - Virtual Container (SDH) or Virtual Circuit
VCI - Virtual Circuit Identifier (ATM)
VPI - Virtual Path Identifier (ATM)
VT - Virtual Tributary
WDM - Wave-length Division Multiplexing
16. Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
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