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A Framework for QoS-based Routing in the Internet
RFC 2386

Document Type RFC - Informational (August 1998)
Authors Eric S. Crawley , Raj Nair , Dr. Bala Rajagopalan , Hal J. Sandick
Last updated 2013-03-02
RFC stream Internet Engineering Task Force (IETF)
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RFC 2386
RFC 2386           A Framework for QoS-based Routing         August 1998

   RSVP has been designed to operate independent of the underlying
   routing scheme. Under this model, RSVP PATH messages establish the
   reverse path for RESV messages.  In essence, this model is not
   compatible with QoS-based routing schemes that compute paths after
   receiver reservations are received. While this incompatibility can be
   resolved in a simple manner for unicast flows, multicast with
   heterogeneous receiver requirements is a more difficult case.  For
   this, reconciliation between RSVP and QoS-based routing models is
   necessary. Such a reconciliation, however, may require some changes
   to the RSVP model depending on the QoS-based routing model [ZES97,
   ZSSC97, GOA97]. On the other hand, QoS-based routing schemes may be
   designed with RSVP compatibility as a necessary goal. How this
   affects scalability and other performance measures must be
   considered.

8. SECURITY CONSIDERATIONS

   Security issues that arise with routing in general are about
   maintaining the integrity of the routing protocol in the presence of
   unintentional or malicious introduction of information that may lead
   to protocol failure [P88]. QoS-based routing requires additional
   security measures both to validate QoS requests for flows and to
   prevent resource-depletion type of threats that can arise when flows
   are allowed to make arbitratry resource requests along various paths
   in the network. Excessive resource consumption by an errant flow
   results in denial of resources to legitimate flows. While these
   situations may be prevented by setting up proper policy constraints,
   charging models and policing at various points in the network, the
   formalization of such protection requires work [BCCH94].

9. RELATED WORK

   "Adaptive" routing, based on network state, has a long history,
   especially in circuit-switched networks. Such routing has also been
   implemented in early datagram and virtual circuit packet networks.
   More recently, this type of routing has been the subject of study in
   the context of ATM networks, where the traffic characteristics and
   topology are substantially different from those of circuit-switched
   networks [MMR96]. It is instructive to review the adaptive routing
   methodologies, both to understand the problems encountered and
   possible solutions.

   Fundamentally, there are two aspects to adaptive, network state-
   dependent routing:

     1.  Measuring and gathering network state information, and
     2.  Computing routes based on the available information.

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   Depending on how these two steps are implemented, a variety of
   routing techniques are possible. These differ in the following
   respects:

   -  what state information is used
   -  whether local or global state is used
   -  what triggers the propagation of state information
   -  whether routes are computed in a distributed or centralized manner
   -  whether routes are computed on-demand, pre-computed, or in a
      hybrid manner
   -  what optimization criteria, if any, are used in computing routes
   -  whether source routing or hop by hop routing is used, and
   -  how alternate route choices are explored

   It should be noted that most of the adaptive routing work has focused
   on unicast routing. Multicast routing is one of the areas that would
   be prominent with Internet QoS-based routing. We treat this
   separately, and the following review considers only unicast routing.
   This review is not exhaustive, but gives a brief overview of some of
   the approaches.

9.1 Optimization Criteria

   The most common optimization criteria used in adaptive routing is
   throughput maximization or delay minimization. A general formulation
   of the optimization problem is the one in which the network revenue
   is maximized, given that there is a cost associated with routing a
   flow over a given path [MMR96, K88]. In general, global optimization
   solutions are difficult to implement, and they rely on a number of
   assumptions on the characteristics of the traffic being routed
   [MMR96]. Thus, the practical approach has been to treat the routing
   of each flow (VC, circuit or packet stream to a given destination)
   independently of the routing of other flows. Many such routing
   schemes have been implemented.

9.2  Circuit Switched Networks

   Many adaptive routing concepts have been proposed for circuit-
   switched networks. An example of a simple adaptive routing scheme is
   sequential alternate routing [T88]. This is a hop-by-hop
   destination-based routing scheme where only local state information
   is utilized.  Under this scheme, a routing table is computed for each
   node, which lists multiple output link choices for each destination.
   When a call set-up request is received by a node, it tries each
   output link choice in sequence, until it finds one that can
   accommodate the call. Resources are reserved on this link, and the
   call set-up is forwarded to the next node. The set-up either reaches
   the destination, or is blocked at some node. In the latter case, the

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   set-up can be cranked back to the previous node or a failure
   declared. Crankback allows the previous node to try an alternate
   path.  The routing table under this scheme can be computed in a
   centralized or distributed manner, based only on the topology of the
   network. For instance, a k-shortest-path algorithm can be used to
   determine k alternate paths from a node with distinct initial links
   [T88]. Some mechanism must be implemented during path computation or
   call set-up to prevent looping.

   Performance studies of this scheme illustrate some of the pitfalls of
   alternate routing in general, and crankback in particular [A84, M86,
   YS87]. Specifically, alternate routing improves the throughput when
   traffic load is relatively light, but adversely affects the
   performance when traffic load is heavy. Crankback could further
   degrade the performance under these conditions. In general,
   uncontrolled alternate routing (with or without crankback) can be
   harmful in a heavily utilized network, since circuits tend to be
   routed along longer paths thereby utilizing more capacity. This is an
   obvious, but important result that applies to QoS-based Internet
   routing also.

   The problem with alternate routing is that both direct routed (i.e.,
   over shortest paths) and alternate routed calls compete for the same
   resource.  At higher loads, allocating these resources to alternate
   routed calls result in the displacement of direct routed calls and
   hence the alternate routing of these calls. Therefore, many
   approaches have been proposed to limit the flow of alternate routed
   calls under high traffic loads. These schemes are designed for the
   fully-connected logical topology of long distance telephone networks
   (i.e., there is a logical link between every pair of nodes). In this
   topology, direct routed calls always traverse a 1-hop path to the
   destination and alternate routed calls traverse at most a 2-hop path.

   "Trunk reservation" is a scheme whereby on each link a certain
   bandwidth is reserved for direct routed calls [MS91]. Alternate
   routed calls are allowed on a trunk as long as the remaining trunk
   bandwidth is greater than the reserved capacity. Thus, alternate
   routed calls cannot totally displace direct routed calls on a trunk.
   This strategy has been shown to be very effective in preventing the
   adverse effects of alternate routing.

   "Dynamic alternate routing" (DAR) is a strategy whereby alternate
   routing is controlled by limiting the number of choices, in addition
   to trunk reservation [MS91]. Under DAR, the source first attempts to
   use the direct link to the destination. When blocked, the source
   attempts to alternate route the call via a pre-selected neighbor. If
   the call is still blocked, a different neighbor is selected for
   alternate routing to this destination in the future. The present call

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   is dropped. DAR thus requires only local state information. Also, it
   "learns" of good alternate paths by random sampling and sticks to
   them as long as possible.

   More recent circuit-switched routing schemes utilize global state to
   select routes for calls. An example is AT&T's Real-Time Network
   Routing (RTNR) scheme [ACFH92]. Unlike schemes like DAR, RTNR handles
   multiple classes of service, including voice and data at fixed rates.
   RTNR utilizes a sophisticated per-class trunk reservation mechanism
   with dynamic bandwidth sharing between classes. Also, when alternate
   routing a call, RTNR utilizes the loading on all trunks in the
   network to select a path. Because of the fully-connected topology,
   disseminating status information is simple under RTNR; each node
   simply exchanges status information directly with all others.

   From the point of view of designing QoS-based Internet routing
   schemes, there is much to be learned from circuit-switched routing.
   For example, alternate routing and its control, and dynamic resource
   sharing among different classes of traffic. It is, however, not
   simple to apply some of the results to a general topology network
   with heterogeneous multirate traffic. Work in the area of ATM network
   routing described next illustrates this.

9.3 ATM Networks

   The VC routing problem in ATM networks presents issues similar to
   that encountered in circuit-switched networks. Not surprisingly, some
   extensions of circuit-switched routing have been proposed. The goal
   of these routing schemes is to achieve higher throughput as compared
   to traditional shortest-path routing. The flows considered usually
   have a single QoS requirement, i.e., bandwidth.

   The first idea is to extend alternate routing with trunk reservation
   to general topologies [SD95].  Under this scheme, a distance vector
   routing protocol is used to build routing tables at each node with
   multiple choices of increasing hop count to each destination. A VC
   set-up is first routed along the primary ("direct") path. If
   sufficient resources are not available along this path, alternate
   paths are tried in the order of increasing hop count. A flag in the
   VC set-up message indicates primary or alternate routing, and
   bandwidth on links along an alternate path is allocated subject to
   trunk reservation. The trunk reservation values are determined based
   on some assumptions on traffic characteristics. Because the scheme
   works only for a single data rate, the practical utility of it is
   limited.

   The next idea is to import the notion of controlled alternate routing
   into traditional link state QoS-based routing [GKR96]. To do this,

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   first each VC is associated with a maximum permissible routing cost.
   This cost can be set based on expected revenues in carrying the VC or
   simply based on the length of the shortest path to the destination.
   Each link is associated with a metric that increases exponentially
   with its utilization. A switch computing a path for a VC simply
   determines a least-cost feasible path based on the link metric and
   the VC's QoS requirement.  The VC is admitted if the cost of the path
   is less than or equal to the maximum permissible routing cost. This
   routing scheme thus limits the extent of "detour" a VC experiences,
   thus preventing excessive resource consumption. This is a practical
   scheme and the basic idea can be extended to hierarchical routing.
   But the performance of this scheme has not been analyzed thoroughly.
   A similar notion of admission control based on the connection route
   was also incorporated in a routing scheme presented in [ACG92].

   Considering the ATM Forum PNNI protocol [PNNI96], a partial list of
   its stated characteristics are as follows:

            o   Scales to very large networks
            o   Supports hierarchical routing
            o   Supports QoS
            o   Uses source routed connection setup
            o   Supports multiple metrics and attributes
            o   Provides dynamic routing

   The PNNI specification is sub-divided into two protocols: a signaling
   and a routing protocol. The PNNI signaling protocol is used to
   establish point-to-point and point to multipoint connections and
   supports source routing, crankback and alternate routing. PNNI source
   routing allows loop free paths.  Also, it allows each implementation
   to use its own path computation algorithm. Furthermore, source
   routing is expected to support incremental deployment of future
   enhancements such as policy routing.

   The PNNI routing protocol is a dynamic, hierarchical link state
   protocol that propagates topology information by flooding it through
   the network.  The topology information is the set of resources (e.g.,
   nodes, links and addresses) which define the network. Resources are
   qualified by defined sets of metrics and attributes (delay, available
   bandwidth, jitter, etc.) which are grouped by supported traffic
   class.  Since some of the metrics used will change frequently, e.g.,
   available bandwidth, threshold algorithms are used to determine if
   the change in a metric or attribute is significant enough to require
   propagation of updated information.  Other features include, auto
   configuration of the routing hierarchy, connection admission control
   (as part of path calculation) and aggregation and summarization of
   topology and reachability information.

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   Despite its functionality, the PNNI routing protocol does not address
   the issues of multicast routing, policy routing and control of
   alternate routing. A problem in general with link state QoS-based
   routing is that of efficient broadcasting of state information. While
   flooding is a reasonable choice with static link metrics it may
   impact the performance adversely with dynamic metrics.

   Finally, Integrated PNNI [I-PNNI] has been designed from the start to
   take advantage of the QoS Routing capabilities that are available in
   PNNI and integrate them with routing for layer 3.  This would provide
   an integrated layer 2 and layer 3 routing protocol for networks that
   include PNNI in the ATM core.  The I-PNNI specification has been
   under development in the ATM Forum and, at this time, has not yet
   incorporated QoS routing mechanisms for layer 3.

9.4   Packet Networks

   Early attempts at adaptive routing in packet networks had the
   objective of delay minimization by dynamically adapting to network
   congestion.  Alternate routing based on k-shortest path tables, with
   route selection based on some local measure (e.g., shortest output
   queue) has been described [R76, YS81]. The original ARPAnet routing
   scheme was a distance vector protocol with delay-based cost metric
   [MW77]. Such a scheme was shown to be prone to route oscillations
   [B82]. For this and other reasons, a link state delay-based routing
   scheme was later developed for the ARPAnet [MRR80]. This scheme
   demonstrated a number of techniques such as triggered updates,
   flooding, etc., which are being used in OSPF and PNNI routing today.
   Although none of these schemes can be called QoS-based routing
   schemes, they had features that are relevant to QoS-based routing.

   IBM's System Network Architecture (SNA) introduced the concept of
   Class of Service (COS)-based routing [A79, GM79].  There were several
   classes of service:  interactive, batch, and network control.  In
   addition, users could define other classes. When starting a data
   session an application or device would request a COS.  Routing would
   then map the COS into a statically configured route which marked a
   path across the physical network.  Since SNA is connection oriented,
   a session was set up along this path and the application's or
   device's data would traverse this path for the life of the session.
   Initially, the service delivered to a session was based on the
   network engineering and current state of network congestion. Later,
   transmission priority was added to subarea SNA.  Transmission
   priority allowed more important traffic (e.g. interactive) to proceed
   before less time-critical traffic (e.g. batch) and improved link and
   network utilization. Transmission priority of a session was based on
   its COS.

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   SNA later evolved to support multiple or alternate paths between
   nodes.  But, although assisted by network design tools, the network
   administrator still had to statically configure routes. IBM later
   introduced SNA's Advanced Peer to Peer Networking (APPN) [B85]. APPN
   added new features to SNA including dynamic routing based on a link
   state database. An application would use COS to indicate it traffic
   requirements and APPN would calculate a path capable of meeting these
   requirements.  Each COS was mapped to a table of acceptable metrics
   and parameters that qualified the nodes and links contained in the
   APPN topology Database.  Metrics and parameters used as part of the
   APPN route calculation include, but are not limited to:  delay, cost
   per minute, node congestion and security.  The dynamic nature of APPN
   allowed it to route around failures and reduce network configuration.

   The service delivered by APPN was still based on the network
   engineering, transmission priority and network congestion.  IBM later
   introduced an extension to APPN, High Performance Routing
   (HPR)[IBM97]. HPR uses a congestion avoidance algorithm called
   adaptive rate based (ARB) congestion control.  Using predictive
   feedback methods, the ARB algorithm prevents congestion and improves
   network utilization.  Most recently, an extension to the COS table
   has been defined so that HPR routing could recognize and take
   advantage of ATM QoS capabilities.

   Considering IP routing, both IDRP [R92] and OSPF support  type of
   service (TOS)-based routing. While the IP header has a TOS field,
   there is no standardized way of utilizing it for TOS specification
   and routing. It seems possible to make use of the IP TOS feature,
   along with TOS-based routing and proper network engineering, to do
   QoS-based routing. The emerging differentiated services model is
   generating renewed interest in TOS support. Among the newer schemes,
   Source Demand Routing (SDR) [ELRV96] allows  on-demand path
   computation by routers and the implementation of strict and loose
   source routing. The Nimrod architecture [CCM96] has a number of
   concepts built in to handle scalability and specialized path
   computation. Recently, some work has been done on QoS-based routing
   schemes for the integrated services Internet. For example, in [M98],
   heuristic schemes for efficient routing of flows with bandwidth
   and/or delay constraints is described and evaluated.

9. SUMMARY AND CONCLUSIONS

   In this document, a framework for QoS-based Internet routing was
   defined.  This framework adopts the traditional separation between
   intra and interdomain routing. This approach is especially meaningful
   in the case of QoS-based routing, since there are many views on how
   QoS-based routing should be accomplished and many different needs.
   The objective of this document was to encourage the development of

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   different solution approaches for intradomain routing, subject to
   some broad requirements, while consensus on interdomain routing is
   achieved. To this end, the QoS-based routing issues were described,
   and some broad intradomain routing requirements and an interdomain
   routing model were defined. In addition, QoS-based multicast routing
   was discussed and a detailed review of related work was presented.

   The deployment of QoS-based routing across multiple administrative
   domains requires both the development of intradomain routing schemes
   and a standard way for them to interact via a well-defined
   interdomain routing mechanism. This document, while outlining the
   issues that must be addressed, did not engage in the specification of
   the actual features of the interdomain routing scheme. This would be
   the next step in the evolution of wide-area, multidomain QoS-based
   routing.

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            RFC 1636, June 1994.

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   [BCF94]  A. Ballardie, J. Crowcroft and P. Francis, "Core-Based
            Trees: A Scalable Multicast Routing Protocol", Proceedings
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   [BCS94]  Braden, R., Clark, D., and S. Shenker, "Integrated Services
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   [GKOP98] R. Guerin, S. Kamat, A. Orda, T. Przygienda, and D.
            Williams, "QoS Routing Mechanisms and OSPF extensions", work
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            Specification. af-96-0987r1, September 1996.

   [ISI81]  Postel, J., "Internet Protocol", STD 5, RFC 791, September
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            and Control, pp. 1345-1347, 1986.

   [M98]    Moy, J., "OSPF Version 2", STD 54, RFC 2328, April 1998.

   [M94]    Moy, J., "MOSPF: Analysis and Experience", RFC 1585,  March
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   [M98]    Q. Ma, "Quality-of-Service Routing in Integrated Services
            Networks", PhD thesis, Computer Science Department, Carnegie
            Mellon University, 1998.

   [MMR96]  D. Mitra, J. Morrison, and K. G. Ramakrishnan, "ATM Network
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            Framework", Proceedings of IEEE INFOCOM `96, 1996.

   [MRR80]  J. M. McQuillan, I. Richer and E. C. Rosen, "The New Routing
            Algorithm for the ARPANET", IEEE Trans.  Communications, pp.
            711-719, May, 1980.

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   [MS91]   D. Mitra and J. B. Seery, "Comparative Evaluations of
            Randomized and Dynamic Routing Strategies for Circuit
            Switched Networks", IEEE Trans. on Communications, pp. 102-
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            Decisions", Computer Networks, August, 1977.

   [NC94]   Nair, R. and Clemmensen, D. : "Routing in Integrated
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   [P88]    R. Perlman, "Network Layer Protocol with Byzantine
            Robustness", Ph.D. Thesis, Dept. of EE and CS, MIT, August,
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            Interface Spec.  v1.0 (PNNI 1.0)", afpnni-0055.00, March
            1996.

   [R76]    H. Rudin, "On Routing and "Delta Routing": A Taxonomy and
            Performance Comparison of Techniques for Packet-Switched
            Networks", IEEE Trans. Communications, pp. 43-59, January,
            1996.

   [R92]    Y. Rekhter, "IDRP Protocol Analysis: Storage Overhead", ACM
            Comp.  Comm.  Review, April, 1992.

   [R96]    B. Rajagopalan, "Efficient Link State Routing", Work in
            Progress, available from braja@ccrl.nj.nec.com.

   [RN98]   B. Rajagopalan and R. Nair, "Multicast Routing with Resource
            Reservation", to appear in J. of High Speed Networks, 1998.

   [SD95]   S. Sibal and A. Desimone, "Controlling Alternate Routing in
            General-Mesh Packet Flow Networks", Proceedings of ACM
            SIGCOMM, 1995.

   [SPG97]  Shenker, S., Partridge, C., and R. Guerin, "Specification of
            Guaranteed Quality of Service", RFC 2212, September 1997.

   [T88]    D. M. Topkis, "A k-Shortest-Path Algorithm for Adaptive
            Routing in Communications Networks", IEEE Trans.
            Communications, pp.  855-859, July, 1988.

   [W88]    B. M. Waxman, "Routing of Multipoint Connections", IEEE
            JSAC, pp. 1617-1622, December, 1988.

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   [W97]   Wroclawski, J., "Specification of the Controlled-Load Network
            Element Service", RFC 2211, September 1997.

   [WC96]   Z. Wang and J. Crowcroft, "QoS Routing for Supporting
            Resource Reservation", IEEE JSAC, September, 1996.

   [YS81]   T. P. Yum and M. Schwartz, "The Join-Based Queue Rule and
            its Application to Routing in Computer Communications
            Networks", IEEE Trans. Communications, pp. 505-511, 1981.

   [YS87]   T. G. Yum and M. Schwartz, "Comparison of Routing Procedures
            for Circuit-Switched Traffic in Nonhierarchical Networks",
            IEEE Trans. Communications, pp. 535-544, May, 1987.

   [ZES97]  Zappala, D., Estrin, D., and S. Shenker, "Alternate Path
            Routing and Pinning for Interdomain Multicast Routing", USC
            Computer Science Technical Report #97-655, USC, 1997.

   [ZSSC97] Zhang, Z., Sanchez, C., Salkewicz, B., and E. Crawley, "QoS
            Extensions to OSPF", Work in Progress.

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RFC 2386           A Framework for QoS-based Routing         August 1998

AUTHORS' ADDRESSES

   Bala Rajagopalan
   NEC USA, C&C Research Labs
   4 Independence Way
   Princeton, NJ 08540
   U.S.A

   Phone: +1-609-951-2969
   EMail: braja@ccrl.nj.nec.com

   Raj Nair
   Arrowpoint
   235 Littleton Rd.
   Westford, MA 01886
   U.S.A

   Phone: +1-508-692-5875, x29
   EMail: nair@arrowpoint.com

   Hal Sandick
   Bay Networks, Inc.
   1009 Slater Rd., Suite 220
   Durham, NC 27703
   U.S.A

   Phone: +1-919-941-1739
   EMail: Hsandick@baynetworks.com

   Eric S. Crawley
   Argon Networks, Inc.
   25 Porter Rd.
   Littelton, MA 01460
   U.S.A

   Phone: +1-508-486-0665
   EMail: esc@argon.com

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RFC 2386           A Framework for QoS-based Routing         August 1998

Full Copyright Statement

   Copyright (C) The Internet Society (1998).  All Rights Reserved.

   This document and translations of it may be copied and furnished to
   others, and derivative works that comment on or otherwise explain it
   or assist in its implementation may be prepared, copied, published
   and distributed, in whole or in part, without restriction of any
   kind, provided that the above copyright notice and this paragraph are
   included on all such copies and derivative works.  However, this
   document itself may not be modified in any way, such as by removing
   the copyright notice or references to the Internet Society or other
   Internet organizations, except as needed for the purpose of
   developing Internet standards in which case the procedures for
   copyrights defined in the Internet Standards process must be
   followed, or as required to translate it into languages other than
   English.

   The limited permissions granted above are perpetual and will not be
   revoked by the Internet Society or its successors or assigns.

   This document and the information contained herein is provided on an
   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
   TASK FORCE DISCLAIMS 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.

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