Network Working Group G. Bernstein
Internet Draft Grotto Networking
Intended status: Informational Y. Lee
Expires: December 2007 Huawei
June 25, 2007
Applicability of GMPLS and PCE to Wavelength Switched Optical
Networks
draft-bernstein-ccamp-wavelength-switched-00.txt
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Abstract
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This memo examines the applicability of Generalized Multi-Protocol
Label Switching (GMPLS) and the Path Computation Element (PCE)
architecture to the control of wavelength switched optical networks.
In particular we investigate how WDM based systems consisting of
tunable laser transmitters and reconfigurable optical add/drop
multiplexers (ROADM) or Wavelength Selective Switches (WSS) can be
controlled with the current GMPLS/PCE protocols. Minor protocol
extension requirements are identified where necessary.
The three cases of full wavelength conversion, no wavelength
conversion, and limited wavelength conversion and their impacts on
GMPLS signaling, GMPLS routing, and PCE communications protocol are
discussed.
Conventions used in this document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC-2119 [RFC2119].
Table of Contents
1. Introduction...................................................3
2. Terminology....................................................4
3. Routing and Wavelength Assignment..............................5
3.1. Implications for GMPLS signaling..........................6
3.1.1. No Wavelength Conversion.............................6
3.1.2. Limited Wavelength Conversion........................7
3.1.3. Full Wavelength Conversion...........................7
3.1.4. Future Issues for GMPLS Signaling....................7
3.2. Implications for GMPLS Routing............................8
3.2.1. Need for Wavelength-Specific Maximum Bandwidth
Information.................................................8
3.2.2. Need for Wavelength-Specific Availability Information8
3.2.3. Describing Wavelength Conversion Capabilities........9
3.2.4. Relationship to Link Bundling and Layering..........10
3.3. Optical Path Computation and Implications for PCE........10
3.3.1. No or Limited Wavelength Conversion.................10
3.3.2. Full Wavelength Conversion..........................11
3.3.3. PCE Discovery.......................................11
4. Security Considerations.......................................11
5. IANA Considerations...........................................12
6. Conclusions...................................................12
7. Acknowledgments...............................................12
8. References....................................................13
8.1. Normative References.....................................13
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8.2. Informative References...................................13
Author's Addresses...............................................15
Intellectual Property Statement..................................15
Disclaimer of Validity...........................................16
1. Introduction
Two key optical components have emerged that are making wavelength
switched optical networks more cost effective and dynamic. First
introduced to reduce inventory costs, tunable optical laser
transmitters are becoming widely deployed [Coldren04], [Buus06]. This
allows flexibility in the wavelength used for optical transmission.
Reconfigurable add/drop optical multiplexers (ROADM) have matured and
are available in different forms and technologies [Basch06]. This
allows wavelength based optical switching.
However, another optical component, the wavelength converter, has not
advanced as uniformly and different system designs may choose to
utilize this component to varying degrees or not at all. Wavelength
converters take an ingress optical signal at one wavelength and emit
an equivalent content optical signal at another wavelength on egress.
There are currently two approaches to building wavelength converters.
One approach is based on optical to electrical to optical (OEO)
conversion with tunable lasers on egress. This approach can be
dependent upon the signal rate and format, i.e., this is basically an
electrical regenerator combined with a tunable laser. The other
approach performs the wavelength conversion, optically via non-linear
optical effects, similar in spirit to the familiar frequency mixing
used in radio frequency systems, but significantly harder to
implement. Such processes/effects may place limits on the range of
achievable conversion. These may depend on the wavelength of the
input signal and the properties of the converter as opposed to the
only the properties of the converter in the OEO case.
The presence and amount of wavelength conversion available at a
wavelength switching interface has an impact on the information that
needs to be transferred by the control plane (Generalized
Multiprotocol Label Switching - GMPLS) and the Path Computation
Element (PCE) architecture. Figure 1, below, summarizes the current
capabilities of GMPLS signaling, GMPLS routing and the PCE
architecture to support the control of switched optical networks
consisting of (a) full wavelength conversion capabilities, (b) no
wavelength conversion capabilities, and (c) limited wavelength
conversion capabilities.
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Signaling Routing PCE
------------------------------------------------
Full | Yes | Yes | Yes |
Conversion | | | |
------------------------------------------------
No | Yes | No | Partial |
Conversion | | | |
------------------------------------------------
Limited | Yes | No | Partial |
Conversion | | | |
------------------------------------------------
Figure 1 Current support for wavelength switching in GMPLS & PCE.
The full wavelength conversion case occurs when all ROADMs or WSSs
have wavelength converters available on every interface. This, for
example, would occur in the case of OEO switches with WDM interfaces
featuring tunable lasers. Limited wavelength conversion capabilities
exist in a network when either wavelength conversion is either not
present on every port or not present at every switching node.
Finally, in the case of no conversion, none of the wavelength
switching nodes has wavelength conversion capability.
2. Terminology
ROADM: Reconfigurable optical add/drop multiplexer. A reduced port
count wavelength selective switching element featuring ingress and
egress line side ports as well as add/drop side ports.
Wavelength Conversion/Converters: The process of converting an
information bearing optical signal centered at a given wavelength to
one with "equivalent" content centered at a different wavelength.
Wavelength conversion can be implemented via an optical-electronic-
optical (OEO) process or via a strictly optical process.
Wavelength Switched Optical Networks: Wavelength Division Multiplex
(WDM) based optical networks in which switching is performed
selectively based on the center wavelength of an optical signal.
Wavelength Selective Switch (WSS): A general, multi-port, switch used
in wavelength switched optical networks. Switches data based on
ingress port and ingress lambda. May or may not have wavelength
conversion capabilities.
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3. Routing and Wavelength Assignment
In wavelength switched optical networks consisting of tunable lasers
and wavelength selective switches with wavelength converters on every
interface, path selection is similar to the MPLS and TDM circuit
switched cases in that the labels, in this case wavelengths
(lambdas), have only local significance. That is, a wavelength-
convertible network with full wavelength-conversion capability at
each node is equivalent to a circuit-switched TDM network with full
time slot interchange capability; thus, the routing problem needs to
be addressed only at the level of the TE link choice, and wavelength
assignment can be resolved by the switches on a hop-by-hop basis.
However, in the limiting case of an optical network with no
wavelength converters, a light path (optical channel - OCh -) needs a
route from source to destination and must pick a single wavelength
that can be used along that path without "colliding" with the
wavelength used by any other light path that may share an optical
span. This is sometimes referred to as a "wavelength continuity
constraint". To ease up on this constraint while keeping network
costs in check a limited number of wavelength converters maybe
introduce at key points in the network [Chu03].
In the general case of limited or no wavelength converters this
computation is known as the Routing and Wavelength Assignment (RWA)
problem [HZang00]. The "hardness" of this problem is well documented,
however, there exists a number of reasonable approximate methods for
its solution [HZang00].
The inputs to the basic RWA problem are the requested light paths
source and destination, the networks topology, the locations and
capabilities of any wavelength converters, and the wavelengths
available on each optical link. The output from an algorithm solving
the RWA problem is an explicit route through ROADMs or WSSs, a
wavelength for the optical transmitter, and a set of locations
(generally associated with switches) where wavelength conversion is
to occur and the new wavelength to be used on each component link
after that point in the route.
It is to be noted that the RWA algorithm is out of the scope for this
document. This document discusses GMPLS signaling and routing
requirements and PCE requirements that enable RWA aware light path
computation and the establishment of the LSPs in wavelength switched
optical networks.
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3.1. Implications for GMPLS signaling
In [RFC3471] a wavelength label is just a 32 bit integer that at a
minimum must have significance to the two neighbors, i.e., maps to a
specific wavelength or frequency. To set up a transparent network it
makes more sense to map labels to wavelengths at the network (domain)
level so we have an easy and consistent way to describe them in GMPLS
signaling. ITU-T recommendation [G.694.1] describes a WDM grid
defined in terms of frequency spacing of 12.5GHz, 25GHz, 50GHz,
100GHz, and other multiples of 100GHz. To see that the 32 bit GMPLS
label currently allocated is sufficient, consider a wideband fiber
such as that specified in [G.656] which is capable of operating over
a wavelength range of 1460-1625nm. This would correspond to a
frequency range of approximately 53.44THz, and with the currently
finest grid spacing of 12.5GHz would require approximately 4276 <
2^13 labels. This is far less than the possible 2^32 lambda labels
available via GMPLS hence a simplistic network wide map of
wavelengths to labels is feasible. An alternative to a global
network map of labels to wavelengths would be to use LMP to assign
the map for each link then convey that information to any path
computation entities, e.g., label switch routers or stand alone PCEs.
For use in GMPLS RSVP-TE path messages GMPLS already has the lambda
(value 9) LSP encoding type [RFC3471], or for G.709 compatible
optical channels, the LSP encoding type (value = 13) "G.709 Optical
Channel" from [RFC4328].
3.1.1. No Wavelength Conversion
Given a system-wide mapping between labels and lambdas and assuming
that the RWA problem has been solved to yield a path as a series of
links traversed by a single wavelength(explicit route). We can then
use the GMPLS signaling procedures [RFC3471] to set up the light path
with an appropriate interpretation of the parameters made at each
ROADM. In particular, the source of the light path would originate a
path message containing a label set consisting of a single label
(that corresponds to the assigned lambda). Upon reception at the
first ROADM or WSS this wavelength is confirmed to not be used on the
selected outgoing interface (fiber). Per [RFC3471] procedures for the
non-wavelength converter case the incoming label set (consisting of a
single label) forms the basis for the out-going label set and in this
way a path can be set up for the assigned wavelength and any
potential lambda collisions can be caught by GMPLS signaling
processing. Hence current GMPLS signaling can support the case with
no wavelength conversion.
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3.1.2. Limited Wavelength Conversion
When the optical network contains a limited number of wavelength
converters, the solution to the RWA problem will consist of a route
from the source to destination along with the wavelengths
(generalized labels) to be used along portions of the path. Current
GMPLS signaling supports an explicit route object (ERO) and within an
ERO an ERO Label subobject can be use to indicate the wavelength to
be used at a particular node. Hence current GMPLS signaling supports
the case of limited wavelength conversion.
3.1.3. Full Wavelength Conversion
When the optical network consists of full wavelength converters,
label assignment is strictly a link local matter and wavelength
assignment is not an issue beyond the local link, i.e., one doesn't
have to solve the wavelength assignment portion of the RWA problem.
Hence current GMPLS signaling (local) label assignment techniques can
be used and the current GMPLS signaling supports the case of full
wavelength conversion.
3.1.4. Future Issues for GMPLS Signaling
Although Non-Return to Zero (NRZ) is currently the dominant form of
optical modulation, new modulation formats are being researched
[Winzer06] and deployed. With a choice in modulation formats we no
longer have a one to one relationship between digital bandwidth in
bytes or bits per second and the amount of optical spectrum (optical
bandwidth) consumed. To simplify the specification of optical signals
the ITU-T, in recommendation G.959.1, combined a rate bound and
modulation format designator [G.959.1]. For example, two of the
signal classes defined in [G.959.1] are:
Optical tributary signal class NRZ 1.25G:
"Applies to continuous digital signals with non-return to zero line
coding, from nominally 622 Mbit/s to nominally 1.25 Gbit/s. Optical
tributary signal class NRZ 1.25G includes a signal with STM-4 bit
rate according to ITU-T Rec. G.707/Y.1322."
Optical tributary signal class RZ 40G:
"Applies to continuous digital signals with return to zero line
coding, from nominally 9.9 Gbit/s to nominally 43.02 Gbit/s.
Optical tributary signal class RZ 40G includes a signal with STM-
256 bit rate according to ITU-T Rec. G.707/Y.1322 and OTU3 bit rate
according to ITU-T Rec. G.709/Y.1331."
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Hence, as was done in reference [RFC4606] for SONET/SDH, in the
future it maybe worthwhile to define traffic parameters for lambda
LSPs that include a signal type field that includes modulation format
information.
3.2. Implications for GMPLS Routing
GMPLS routing [RFC4202] currently defines an interface capability
descriptor for "lambda switch capable" which we can use to describe
the interfaces on a ROADM or other type of wavelength selective
switch.
3.2.1. Need for Wavelength-Specific Maximum Bandwidth Information
Difficulties are encountered when trying to use the bandwidth
accounting methods of [RFC4202] and [RFC3630] to describe the
availability of wavelengths on a WDM link. The current RFCs give
three link resource measures: Maximum Bandwidth, Maximum Reservable
Bandwidth, and Unreserved Bandwidth. Although these can be used to
describe a WDM span they do not provide the fundamental information
needed for RWA. We are not given the maximum bandwidth per wavelength
for the span. If we did then we could use the aforementioned measures
to tell us the maximum wavelength count and the number of available
wavelengths.
For example, suppose we have a 32 channel WDM span, and that the
system in general supports ITU-T NRZ signals up to NRZ 10Gbps.
Further suppose that the first 20 channels are carrying 1Gbps
Ethernet, then the maximum bandwidth would be 320Gbps and the maximum
reservable bandwidth would be 120Gbps (12 wavelengths).
Alternatively, consider the case where the first 8 channels are
carrying 2.5Gbps SDH STM-16 channels, then the maximum bandwidth
would still be 320Gbps and the maximum reservable bandwidth would be
240Gbps (24 wavelengths).
3.2.2. Need for Wavelength-Specific Availability Information
Even if we know the number of available wavelengths on a link, we
actually need to know which specific wavelengths are available and
which are occupied so we can assign a wavelength that can be used
across the entire path from source to destination. This is currently
not possible with GMPLS routing extensions.
In the routing extensions for GMPLS [RFC4202], requirements for
layer-specific TE attributes are discussed. The RWA problem for
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optical networks without wavelength converters imposes an additional
requirement for the lambda (or optical channel) layer: that of
knowing which specific wavelengths are in use. Note that current
dense WDM (DWDM) systems range from 16 channels to 128 channels with
advanced laboratory systems with as many as 300 channels. Given these
channel limitations and if we take the approach of a global
wavelength to label mapping or furnishing the local mappings to the
PCEs then representing the use of wavelengths via a simple bit-map is
feasible.
In the GMPLS extensions for OSPF [RFC4203] the interface capability
descriptor sub-TLV contains a subfield that contains switching
capability specific information and is one possible place for a bit
map of available lambdas. However, current GMPLS routing extensions
do not provide enough information for the solution of the RWA
problem.
3.2.3. Describing Wavelength Conversion Capabilities
Topology, switching capabilities and resource status information are
typically disseminated via GMPLS extensions to routing. From the
point of view of an algorithm for RWA we are interested in the
following features associated with an interface to a wavelength
converter:
1. The number of wavelengths that can be converted, i.e., out of the
N channels supported by the WDM link how many can be converted to
a new lambda.
2. The range of conversion for a given lambda. In all optical
wavelength conversion this is typically a function of the input
lambda. In electro-optic wavelength conversion it is just a
property of the egress tunable laser.
A switching node may share a pool of wavelength converters amongst
many ports hence it would be appropriate to feed this overall node
constraint to a RWA algorithm particularly in the case of batch
processing of multiple light paths. See [TE-NODE] for examples of
currently shared TE node capabilities.
Currently the wavelength conversion capabilities/properties of a
lambda switch capable interface are not defined in GMPLS routing
extensions [RFC4202]. In reference [RFC4202] an interface can be
denoted as lambda switching capable (LSC), but the default assumption
seems to be that no constraints on wavelength conversion exist. A
simple way to indicate that a wavelength selective switch has no
wavelength conversion capabilities would be desirable. Note that OSPF
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extensions for GMPLS [RFC4203] does provide a placeholder for
"switching capability" specific information that could be used for
this purpose.
3.2.4. Relationship to Link Bundling and Layering
When dealing with static DWDM systems, particularly from a SONET/SDH
or G.709 digital wrapper layer, each lambda looks like a separate
link. Typically a bunch of unnumbered links, as supported in GMPLS
routing extensions [RFC4202], would be used to describe a static DWDM
system. In addition these links can be bundled into a TE link
([RFC4202], [RFC4201]) for more efficient dissemination of resource
information. However, in the case discussed here we want to control a
dynamic WDM layer and must deal with wavelengths as labels and not
just as links or component links from the perspective of an upper
(client) layer. In addition, a typical point to point optical cable
contains many optical fibers and hence it may be desirable to bundle
these separate fibers into a TE link. Note that in the no wavelength
conversion or limited wavelength conversion situations that we will
need information on wavelength usage on the individual component
links.
3.3. Optical Path Computation and Implications for PCE
As previously noted the RWA problem can be computationally intensive
[HZang00]. Such computationally intensive path computations and
optimizations were part of the impetus for the PCE (path computation
element) architecture.
3.3.1. No or Limited Wavelength Conversion
A network that consists of switches with no wavelength conversion is
referred to as a transparent optical network. From the perspective of
path computation, this type of network imposes an additional
constraint; that is, a wavelength continuity constraint. It is not
sufficient for a path that has available lambda channels on every
link to be considered as a candidate path. At least one channel of
the same wavelength must be available on every link of the path
within a transparency domain.
When the optical network contains a limited number of wavelength
converters, the complexity of path computation increases. That is,
the PCE needs to compute a route for a given source-destination pair
along with the wavelengths to be used over some segments of the
route.
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At a minimum to solve the RWA problem one needs the following
information: (a) tuning range of the source laser, (b) network
topology, (c) network resource availability (wavelengths in
use/available on particular spans), and (d) location and capabilities
of any wavelength converters. These attributes must be made available
to the path computation engine via configuration or advertising.
Tuning ranges of lasers can vary with product model and more
specifically are usually associated with specific optical bands such
as C band (1530-1562nm) and L band (1570-1605nm). Note that if we set
up a mapping between the system frequency grid and labels then we can
represent the tuning range of a laser by a range of labels.
With respect to the PCE architecture the tuning range of the source
laser could be configured via management or as a constraint furnished
to the PCE in a PCEP request message. After the PCE has performed the
RWA computation and wants to return the result to the PCC, it needs
an object/TLV in which to send back the assigned wavelength (label)in
the case of no conversion or a set of wavelengths corresponding to
the egress wavelengths at the wavelength converters. This can be done
with the ERO object in conjunction with the ERO label subobject given
that there is either a global mapping of labels to lambdas known to
the PCE or the PCE has a collection of local label to lambda mappings
for each interface.
3.3.2. Full Wavelength Conversion
When the optical network consists of full wavelength converters, only
the routing problem needs to be addressed, and wavelength assignment
can be handled locally. In this case the PCE would not necessarily
need to be involved with lambda/label assignments.
3.3.3. PCE Discovery
The algorithms and network information needed for solving the RWA are
somewhat specialized and computationally intensive hence not all PCEs
within a domain would necessarily need or want this capability.
Hence, it would be useful via the mechanisms being established for
PCE discovery [DISCO] to indicate that a PCE has the ability to deal
with the RWA problem. Reference [DISCO] indicates that a sub-TLV
could be allocated for this purpose.
4. Security Considerations
TBD
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5. IANA Considerations
TBD
6. Conclusions
TBD
7. Acknowledgments
The authors would like to thank Adrian Farrel for many helpful
comments that greatly improved the contents of this draft.
This document was prepared using 2-Word-v2.0.template.dot.
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8. References
8.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3471] Berger, L., "Generalized Multi-Protocol Label Switching
(GMPLS) Signaling Functional Description", RFC 3471,
January 2003.
[RFC3630] Katz, D., Kompella, K., and D. Yeung, "Traffic Engineering
(TE) Extensions to OSPF Version 2", RFC 3630, September
2003.
[RFC4201] Kompella, K., Rekhter, Y., and L. Berger, "Link Bundling in
MPLS Traffic Engineering (TE)", RFC 4201, October 2005.
[RFC4202] Kompella, K. and Y. Rekhter, "Routing Extensions in Support
of Generalized Multi-Protocol Label Switching (GMPLS)", RFC
4202, October 2005.
[RFC4203] Kompella, K. and Y. Rekhter, "OSPF Extensions in Support of
Generalized Multi-Protocol Label Switching (GMPLS)", RFC
4203, October 2005.
[RFC4328] Papadimitriou, D., "Generalized Multi-Protocol Label
Switching (GMPLS) Signaling Extensions for G.709 Optical
Transport Networks Control", RFC 4328, January 2006.
[G.694.1] ITU-T Recommendation G.694.1, "Spectral grids for WDM
applications: DWDM frequency grid", June, 2002.
[DISCO] J.L. Le Roux, J.P. Vasseur, Yuichi Ikejiri, and Raymond
Zhang, "OSPF protocol extensions for Path Computation
Element (PCE) Discovery", work in progress, draft-ietf-pce-
disco-proto-ospf-05.txt, May 2007.
8.2. Informative References
[TE-NODE] J.P. Vasseur and J.L. Le Roux (eds), "IGP Routing Protocol
Extensions for Discovery of Traffic Engineering Node
Capabilities", work in progress, draft-ietf-ccamp-te-node-
cap-05.txt, April 2007.
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[HZang00] H. Zang, J. Jue and B. Mukherjeee, "A review of routing and
wavelength assignment approaches for wavelength-routed
optical WDM networks", Optical Networks Magazine, January
2000.
[Coldren04] Larry A. Coldren, G. A. Fish, Y. Akulova, J. S.
Barton, L. Johansson and C. W. Coldren, "Tunable
Seiconductor Lasers: A Tutorial", Journal of Lightwave
Technology, vol. 22, no. 1, pp. 193-202, January 2004.
[Chu03] Xiaowen Chu, Bo Li and Chlamtac I, "Wavelength converter
placement under different RWA algorithms in wavelength-
routed all-optical networks", IEEE Transactions on
Communications, vol. 51, no. 4, pp. 607-617, April 2003.
[Buus06] Jens Buus EJM, "Tunable Lasers in Optical Networks",
Journal of Lightware Technology, vol. 24, no. 1, pp. 5-11,
January 2006.
[Basch06] E. Bert Bash, Roman Egorov, Steven Gringeri and Stuart
Elby, "Architectural Tradeoffs for Reconfigurable Dense
Wavelength-Division Multiplexing Systems", IEEE Journal of
Selected Topics in Quantum Electronics, vol. 12, no. 4, pp.
615-626, July/August 2006.
[Winzer06] Peter J. Winzer and Rene-Jean Essiambre, "Advanced
Optical Modulation Formats", Proceedings of the IEEE, vol.
94, no. 5, pp. 952-985, May 2006.
[G.959.1] ITU-T Recommendation G.959.1, Optical Transport Network
Physical Layer Interfaces, March 2006.
[RFC4606] Mannie, E. and D. Papadimitriou, "Generalized Multi-
Protocol Label Switching (GMPLS) Extensions for Synchronous
Optical Network (SONET) and Synchronous Digital Hierarchy
(SDH) Control", RFC 4606, August 2006.
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Author's Addresses
Greg Bernstein
Grotto Networking
Fremont, CA, USA
Phone: (510) 573-2237
Email: gregb@grotto-networking.com
Young Lee
Huawei Technologies
1700 Alma Drive, Suite 100
Plano, TX 75075
USA
Phone: (972) 509-5599 (x2240)
Email: ylee@huawei.com
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Copyright (C) The IETF Trust (2007).
This document is subject to the rights, licenses and restrictions
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Acknowledgment
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Bernstein & Lee Expires December 25, 2007 [Page 16]