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Versions: 00                                                            
MPLS Working Group                                           B. Jamoussi
Internet Draft                                                Nortel Ltd
Expiration Date: February 1999
                                                              N. Feldman
                                                                IBM Corp

                                                            L. Andersson
                                                        Bay Networks Inc

                                                             August 1998

               MPLS Ships in the Night Operation with ATM

                    <draft-jamoussi-mpls-sin-00.txt>

Status of this Memo

   This document is an Internet-Draft. Internet-Drafts are working
   documents of the Internet Engineering Task Force (IETF), its areas,
   and its working groups.  Note that other groups may also distribute
   working documents as Internet-Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
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   To learn the current status of any Internet-Draft, please check the
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   ftp.isi.edu (US West Coast).

Abstract

   Multi-Protocol Label Switching (MPLS) can have several modes of
   operation over ATM. The MPLS Framework document [1] indicates that
   MPLS MUST allow 'ships in the night' (SIN) operation with existing L2
   switching protocols (e.g., ATM Forum Signaling).

   This document identifies the technical requirements that have to be
   resolved in order to allow for a successful SIN operation between
   MPLS and ATM Forum protocol stack. Solutions to the various
   challenges are proposed.

Table of Contents

   1          Introduction .........................................   2



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   1.1        Modes of Operation of MPLS over ATM ..................   2
   1.2        Benefits of Operating MPLS over ATM ..................   3
   1.3        Ships in the Night Mode of Operation .................   4
   2          VPI.VCI Space Partitioning   .........................   4
   3          Traffic management ...................................   5
   3.1        Bandwidth Management .................................   7
   3.2        Bandwidth Reservation ................................   8
   3.3        Queuing & Scheduling .................................   9
   3.4        Alignment with DS-Byte ...............................  10
   4          Processing Capacity ..................................  11
   5          Summary ..............................................  11
   6          Security Considerations ..............................  11
   7          Acknowledgment .......................................  11
   8          References ...........................................  11
   9          Author's Address .....................................  12
   Appendix A Example of Equivalent Rate Computation ...............  13
   Appendix B Example of ATM and MPLS COS mappings .................  14

1. Introduction

   In Section 1.1 we summarize the current models of using MPLS over
   ATM. Section 1.2 highlights the benefits of running MPLS on ATM. In
   Section 1.3, the requirements for running in Ships in the night (or
   hybrid) mode are outlined.

1.1 Modes of Operation of MPLS over ATM

   MPLS can have several modes of operation over ATM. This section
   outlines the various operation modes that have been described so far
   in various MPLS Internet Drafts. We classify MPLS over ATM modes of
   operation as follows:

      1. Use of ATM hardware as a Label-Controlled Interface:

        a. MPLS-Only:

           This mode has been described the most in the current MPLS
           documentation. A label switching control component is
           developed to control the ATM switching hardware as
           described in [2].

        b. Ships in the night:

           This mode of operation has not been described in any level of
           detail in any of the current MPLS drafts. The only mention
           was to divide the VPI.VCI space between the ATM control plane
           and the MPLS control plane [1, 2]. Other SIN operation
           considerations were out of scope in [2].



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      2. Use of ATM as a bearer service (B-ISDN):

        a. Use of a VP:

           A VP is established between two LSRs going across an ATM
           network. The VCI is used as label within the VP [2, 3].

        b. Use of a PVC or SVC and the VCID:

           A PVC or an SVC is established across an ATM network.
           The VCID is used to identify the two ends of the LSP [3, 4].

   The choice of the model of deployment of MPLS on ATM depends on
   various factors such as the pre-existence of an ATM network.

   In the remainder of this document we focus on the ships in the night
   mode of operation.

1.2 Benefits of Operating MPLS over ATM

   Operating MPLS over an ATM network has many benefits described in
   this section. Many of the ATM hardware features are readily available
   to MPLS implementations.

   - MPLS inherits the ATM hardware label (VPI/VCI) switching. This
   allows an ATM-LSR to forward packets at very high rates.

   - ATM's powerful traffic management features that are already
   embedded in ATM hardware are available for use by MPLS with the
   proper hooks in the MPLS control software (e.g., LDP). For example,
   the queuing and scheduling techniques embedded in ATM hardware allow
   it to provide many classes of service. This maps nicely to the COS
   field in the LDP protocol.  In addition, to queuing and scheduling,
   you can also make use of traffic shaping and traffic policing at the
   boundaries between MPLS domains.

   - The use of ATM-LSRs allows for multiple services (voice, video, and
   data) to be offered on a single platform. Operating MPLS over ATM
   reduces the number of infrastructures required to provide multiple
   services. It is hence possible to offer native ATM, a mixture of
   Frame and ATM services, as well as MPLS using the same
   infrastructure.

   - Protect the capital investment in ATM infrastructure by increasing
   the utilization of existing ATM network. Allow for an easy migration
   from IP over ATM to IP over MPLS over ATM. Protect the network
   management and operational knowledge.




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   - While the MPLS architectures evolves, ATM offers traffic-
   engineering solutions to solve today's bandwidth constraints.

1.3 Ships in the Night Mode

   Ships in the night (SIN) operation is described in [1].  Section 1.2
   of [1] includes the following requirement:

     "MPLS MUST allow "ships in the night" operation with existing layer
      2 switching protocols (e.g., ATM Forum Signaling) (i.e., MPLS must
      be capable of being used in the same network which is also
      simultaneously operating standard layer 2 protocols)."

   SIN operation means that both MPLS and ATM control and routing
   software are running on the same network. In addition, both MPLS and
   ATM  traffic share the same network infrastructure. However, both
   protocols are oblivious to each other.

   As the deployment of ATM networks continues to grow, it becomes
   necessary for the successful deployment of MPLS to address the
   question of how MPLS and ATM would co-exist.

   The key advantage of running in SIN mode is that an existing Multi-
   Service ATM network (running the ATM Forum software stack) can be
   easily shared with MPLS (running IP routing and LDP). However, the
   successful integration of MPLS and ATM on the same network requires
   the following issues to be resolved:

    1. VPI.VCI space partitioning
    2. Traffic Management
    3. Processing Capacity (Memory and CPU)

2. VPI.VCI Space Partitioning

   In a label-controlled ATM (LC-ATM) interface, labels are the VPI.VCI
   fields of the ATM cell. The entire  28-bit VPI.VCI field can be used
   as a label in a flat hierarchy environment. However,  a two level
   hierarchy can also be achieved by using the VPI and the VCI fields
   independently; each representing a level of the hierarchy.

   ATM Forum stack makes use of the same VPI.VCI field to switch ATM
   cells within a VP or a VC. Therefore, partitioning of the VPI/VCI
   space (label space) between ATM and MPLS is necessary.

   This issue is easily resolved through configuration.  Two VPI.VCI
   pools (Label-pool) are configured at start-up time.  A Label-pool (a
   sub-set of the VPI.VCI space) is allocated to ATM and another Label-
   pool to MPLS. Each system only allocates VPI.VCI resources from its



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   own Label-pool.

   In keeping with the idea that both ATM and MPLS control planes are
   oblivious to each other, it's necessary to avoid having one control
   plane infer parameters based on the configuration of the other
   control plane. Therefore, the division of label space between ATM and
   MPLS needs to be pre-configured per interface. The exchange of the
   valid VPI.VCI range between two adjacent LSRs/ATM switches is done as
   follows:

     - MPLS uses the LDP  to negotiate the range of
     valid VPI.VCI labels. This information is exchanged during the
     initialization phase of the LDP session between two peer LSRs as
     specified in LDP [5].

     - ATM uses the ILMI channel to negotiate the valid VPI.VCI range as
     specified in [6].

   Both LDP and ILMI are currently defined to take the intersection of
   the VPI.VCI range of two adjacent LSRs and ATM switches respectively.

   The choice of the boundary between ATM and MPLS is a Network
   Engineering exercise that takes into account  the number of ATM VCs
   that are expected, whether VC-merge is supported on the interface,
   the level of stream granularity, among possibly other parameters.

3. Traffic Management

   Traffic Management (TM) is one of the key components of MPLS.
   Currently, TM is proposed to rely on explicit routing (ER), Class of
   Service (COS) differentiation, and  bandwidth reservation.  [7]
   provides an overall framework for TM in MPLS.

   In [8], RSVP is used to setup ERs and reserve bandwidth. However, in
   [5], the LDP is used to setup the ER and to reserve bandwidth for the
   LSPs.

   This Section defines the necessary Traffic Management building block
   for providing CoS support in MPLS.  In addition, it extends the
   procedure defined in [5] by proposing a simple but effective  Label
   Switched Path Admission Control Mechanism that allows for high
   statistical multiplexing gain while providing a bandwidth guarantee.

   In order for MPLS to provide effective Class of Service (CoS)
   differentiation and guarantees, a set of traffic management function
   have to be implemented by the LSRs. These functions includes traffic
   shaping, traffic policing, queuing and scheduling, and LSP admission
   control.



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   Traffic shaping is the TM function invoked at the egress point of a
   network to limit the rate of traffic down to a specific value.
   Traffic shapers often have a buffer that absorbs traffic bursts
   beyond the shaping rate. Traffic shaping ensure that the outgoing
   traffic rate from LSR1 to LSR2 (shown in Figure 1) does not exceed
   the bandwidth agreement between ISP1 and ISP2. This function is
   usually needed at the boundary between two administrative domains
   (e.g., between ISP1 and ISP2).


               |                                 |
     LSR1      |      LSR2             LSR3      |         LSR4
    +------+   |     +------+         +------+   |      +------+
    |      |   |     |      |         |      |   |      |      |
    |      |   |     |      |         |      |   |      |      |
    |    S |---+-----| P  A |---------|    S |---+------| P    |
    |    A |   |     |    Q |         |    A |   |      |      |
    |    Q |   |     |      |         |    Q |   |      |      |
    |      |   |     |      |         |      |   |      |      |
    +------+   |     +------+         +------+   |      +------+
               |                                 |
        ISP 1  | ISP 2                           | ISP 3
               |                                 |
               |                                 |
            Boundary                          Boundary

        Legend

        S Shaping
        P Policing
        A Admission Control
        Q Queuing and Scheduling

                Figure 1. MPLS Traffic Management Strategy

   Traffic policing is the TM function invoked at the ingress point of a
   network to  ensures that only the agreed upon traffic rate is allowed
   to consume network resources.  Traffic policing is invoked by a
   network service provider to protect its network resources (e.g.,
   bandwidth) from traffic that exceeds the traffic contract. Traffic
   policing ensure that the incoming traffic rate from LSR1 to LSR2 does
   not exceed the bandwidth agreement between ISP1 and ISP2. This
   function is usually needed at the boundary between two administrative
   domains (e.g., between ISP1 and ISP2).

   A queuing and scheduling system is required in order to provide
   differentiated service levels. For instance three emission and four
   discard priorities provide twelve different service levels.  The



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   emission priorities affect the delay observed by the traffic. The
   discard priority affects the loss observed by the traffic. Delay
   sensitive traffic uses the higher emission priority queue than less
   delay sensitive traffic. Loss sensitive traffic uses the higher
   discard priority (last one to discard) than the less loss sensitive
   traffic. A sample queuing and scheduling system is described in
   Appendix B.

   Traffic shaping, policing, and queuing and scheduling are functions
   often developed in hardware so that they don't impact data traffic
   throughput. Most well-designed ATM hardware includes all of these
   function. Therefore, an MPLS implementation on Label-Controlled ATM
   interface (LC-ATM), inherits these functions.

   Admission control ensures that bandwidth guarantees are met. This
   function is often implemented in software and invoked at LSP setup
   time. This Internet Draft proposes a simple yet effective mechanism
   to reserve bandwidth for Explicitly Routed Label Switched Paths.  We
   focus in Sections 3.1-3.2  on LSP admission control which is the
   procedure used to decide if a request for an LSP can be accepted,
   based on the network capacity and the attributes of both the
   requested and the existing LSPs. LSRs must perform LSPAC during the
   LSP setup process as part of providing bandwidth guarantee.

3.1 Bandwidth Management

   An important aspect  of SIN operation is bandwidth management. ATM
   and MPLS traffic share the same network facilities. The support of
   SIN requires that the interaction between ATM traffic and MPLS
   traffic is handled carefully.

   A multi-service ATM network provides strict bandwidth guarantees to
   its connections. MPLS Label Switched Paths (LSPs) can also reserve
   bandwidth through the Label Distribution Protocol as defined in [5].

   The bandwidth reservation issue can be resolved in various ways
   through configuration at start-up time. The following configuration
   options are possible:

     - Hard partitioning between ATM and MPLS
       The aggregate interface bandwidth is partitioned between
       ATM and MPLS. Two bandwidth pools are created; one for ATM and
       another for MPLS. Each pool is allocated a percentage of the
       interface capacity. ATM makes its bandwidth reservations from the
       ATM pool and MPLS makes its bandwidth reservations from the MPLS
       pool.

       The choice of the bandwidth pool boundary between ATM and MPLS



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       is a Network Engineering exercise that takes into account the
       expected bandwidth requirement of ATM and MPLS. An over-booking
       factor can also be introduced where the sum of the percentages
       allocated to ATM and MPLS add to more than 100%.

     - Full sharing between ATM and MPLS
       A single bandwidth pool is kept for both MPLS and ATM per
       interface. As ATM connections or MPLS LSPs are admitted the
       available bandwidth in the pool gets decremented.

   The concept of bandwidth pools can be extended to provide a bandwidth
   pool per ATM service category and per MPLS class of service in both
   configuration modes.

3.2 Bandwidth Reservation in LDP

   In [5], the following Reservation (RES) object is defined in Section
   4.4.4.12:

     "
      +----------------+-------+--------------------------+----------+
      | OBJECT         | Type  |  Subtype(s)              | Length   |
      +----------------+-------+--------------------------+----------+
      | RES            | 0x0C  |  0x01  Raw Bandwidth     | Variable |
      +----------------+-------+--------------------------+----------+

      SubType = 0x01  Raw Bandwidth

      0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                      BW requirement                           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

     BW Requirement

     Unsigned 32 bit integer representing the bandwidth, in units
     of 64,000 bps, that must be reserved for the LSP at the LSR
     identified in the ERNH Object that contains this Reservation
     Object."

   There is no clear indication on what the value of the BW Requirement
   field in the RES object should be.

   Using a similar terminology to the one used in [9], we define a
   connection's traffic characteristics as follows:

    - R: peak rate,



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    - b: burst size,
    - r: average rate

   The BW requirement  can be set to the peak rate R,  average rate r,
   or to an equivalent rate (e) of the LSP. Setting the BW requirement
   to R of the LSP that has an On/Off traffic profile results in more
   bandwidth reserved than actually needed. However, setting the BW
   Requirement field to the r would result in high packet loss.

   Therefore, in order to determine the BW Requirement field, it is
   necessary to compute an equivalent rate  of the  Label Switched Path
   (LSP). The computation of this rate  needs to ensure two objectives
   simultaneously:

     1) Bandwidth guarantees; and
     2) Efficient network resource utilization.

   There are two ways of making the bandwidth reservation through LDP.
   In the first option, the equivalent bandwidth is computed only once
   at the LSR originating the ERLSP based on the traffic characteristics
   (R, r, and b) of the LSP. The value e is then carried by LDP in the
   RES object to inform the LSRs along the LSP of the bandwidth
   requirement. The second option is to extend LDP to carry all three
   traffic parameters and expect each LSR along the path to make its own
   computation of the required bandwidth for the given LSP.

   An extension to the current LDP specification [5] would be required
   to support option 2 and to allow for the signaling of more bandwidth
   parameters (R, r, and b)  to more accurately characterize the
   bandwidth requirements of an LSP.

   Appendix A describes an example of an algorithm for computing e.

3.3 Queuing and Scheduling

   ATM provides strict QoS guarantees. Hence, MPLS traffic should not
   interfere with ATM traffic in any way that affect its QoS guarantees.
   In addition, when MPLS provides bandwidth reservation and guarantees,
   it is necessary to ensure that native ATM traffic does not interfere
   with MPLS.

   Bandwidth management presented in Section 3 works at the reservation
   level. However, nodal-mechanisms of each LSR or ATM switch have to be
   setup such that the QoS guarantees of ATM and MPLS are both met.

   The nodal-mechanisms need to ensure that each service stays within
   its bandwidth allocation limits. Traffic shaping and policing are
   necessary to limit the throughput of each of the ATM and MPLS



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   services to the contracted values.

   In addition, loss and delay guarantees for both ATM and MPLS have to
   be met. This requires a careful mapping of ATM service categories and
   of MPLS Classes of Service on the shared queuing and scheduling
   system.

   Appendix B describes a Multi-Priority System (MPS) presented as an
   example of a queuing and scheduling mechanism and the mapping of MPLS
   COSs and ATM service categories to the MPS.

3.4 Alignment with the DS-byte

   The Diff-Serv model indicates that traffic conditioning is done at
   the edge of the network. A codepoint is assigned to define the
   behavior aggregate of the packet. Within the core, packets are
   forwarded based on the PHB associated with the DS codepoint. However,
   when going over an MPLS network where the underlying infrastructure
   is ATM, the DS codepoint is not accessible to the hardware making the
   forward decision. Therefore, a mapping between the DS codepoint and
   an MPLS LSP that would provide the same PHB is required as packets
   enter the MPLS domain. This means that multiple LSPs are established
   for a given FEC to accommodate the various DS codepoints.

   Currently, only two PHBs are proposed; the DE and the EF PHBs.  The
   [9] draft indicates that the mapping from the DS-byte codepoint to
   the behavior MUST be configurable. Since in an MPLS domain running on
   ATM, the mapping from the DS-codepoint to the LSP is only feasible on
   the edge of the MPLS domain, the mapping has to also be configurable.

   The DS-byte includes an in/out bit that indicates whether the packet
   is within its contracted rate or not. Setting this bit to 'out' means
   that under congestion, this packet should be discarded before packet
   that have the bit set to 'in'. The in/out bit in the DS-byte is
   similar to the CLP bit in ATM cells. Therefore, as part of the
   mapping function from the DS-byte codepoint to an LSP with a given
   priority, the In/Out bit should be mapped to the CLP bit of all the
   cells resulting from the segmentation of the frame.

   When going from an MPLS-ATM domain to a Diff-Serv domain, a mapping
   is also required from the various LSPs to a set of codepoints. In
   addition, if the CLP bit of any cells of a frame are set, then the
   "Out" bit needs to be set accordingly.

4. Processing Capacity

   When running in SIN mode, two routing and control stacks will reside
   on the same nodes.  ATM requires its signaling, and routing (IISP or



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   PNNI) software and topology database. MPLS requires its signaling
   (LDP) and IP routing (RIP, OSPF, or BGP) software and topology
   database.

   Therefore, SIN operation requires that nodal processing capacity
   (CPU, and memory) is adequate to simultaneously handle ATM and MPLS.

5. Summary

   Ships in the night operation support is a required element in MPLS
   [1]. This document highlights the requirements for SIN operation in
   MPLS. Solutions are proposed to carefully share various resources
   including VPI.VCI space, bandwidth, queuing and scheduling, and
   processing capacity.

   The traffic management building blocks that are necessary in order
   for MPLS to provide CoS differentiation and guarantees are
   identified. It proposed a simple yet effective mechanism to compute
   the Equivalent Rate (e) of an Explicitly Routed Label Switched Path
   to be signaled in the LDP RES object. The e computation allows for
   high network bandwidth utilization while minimizing packet loss. An
   admission control mechanism is described based on the e computation.

6. Security Considerations

   Security issues are not discussed in this draft.

7. Acknowledgement

   The authors would like to acknowledge the valuable comments of Pasi
   Vaananen, Osama Aboul-Magd, Ken Hayward, and Don Fedyk. In addition,
   recent MPLS Working Group discussions helped shape some sections of
   this draft.

8. References

   [1] R. Callon, et. al., "A Framework for Multiprotocol Label
   Switching", draft-ietf-mpls-framework-02.txt, November 21, 1997.

   [2] B. Davie, et. al., "Use of Label Switching With ATM", draft-
   davie-mpls-atm-01.txt, July 1998.

   [3] Ken-ichi Nagami, et. al., "VCID Notification over ATM link",
   <draft-ietf-mpls-vcid-atm-00.txt>, March 1998.

   [4] M. Suzuki, "The Assignment of the Information Field and Protocol
   Identifier in the Q.2941 Generic Identifier and Q.2957 User-to-user
   Signaling for the Internet Protocol",  <draft-ietf-mpls-git-uus-



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   00.txt>, June 1998.

   [5] Anderson, et. al., "Label Distribution Protocol", draft-mpls-
   ldp-00.txt, March 1998.

   [6] "Integrated Local Management Interface (ILMI) Specification
   Version 4", The ATM Forum technical committee, af-ilmi-0065.000, July
   1996.

   [7] P. Vaananen, et. al., "Framework for Traffic Management in MPLS
   Networks",  <draft-vaananen-mpls-tm-framework-00.txt>, March 1998.

   [8] B. Davie, et. al., "Use of Label Switching With RSVP", <draft-
   ietf-mpls-rsvp-00.txt>, March 1998.

   [9] Y. Bernet, et. al., "A Framework for Differentiated Services",
   <draft-ietf-diffserv-framework-00.txt>,  May 1998.

9. Authors' Addresses

   Bilel Jamoussi
   Nortel (Northern Telecom), Ltd.
   PO Box 3511 Station C
   Ottawa ON K1Y 4H7
   Canada

   EMail: jamoussi@nortel.ca

   Nancy Feldman
   IBM Corp.
   17 Skyline Drive
   Hawthorne NY 10532
   Phone:  914-784-3254

   EMail: nkf@us.ibm.com

   Loa Andersson
   Bay Networks Inc.
   3 Federal Street
   Billerica, MA  01821

   EMail: andersson@baynetworks.com









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Appendix A Example Equivalent Rate Computation

   The LSP admission problem is to find a nominal rate, called the
   equivalent rate (e), for each connection so that
   the system meets the specified performance objectives.
   This condition would be achieved as long as the sum of the e
   values of the accepted LSPs does not exceed the capacity of
   the designated link.

   If the number of admitted LSPs exceeds the number indicated by
   the bound, then the packet loss will likely exceed the required
   target. However, if the number of admitted calls is less than this bound,
   then link bandwidth is wasted.

   The estimate of e is computed by taking the average of the minimum and
   maximum number of LSPs that can be supported by the link.

   The minimum number of LSPs, min, that can be admitted without
   introducing any nodal loss or delay is obtained such that the
   sum of the peak rates (R) is less than the link rate
   (L) (min * R < L). Hence min=L/R.

   The maximum number of connections, max,
   is obtained when the sum of the average rates (r=A*R),
   where A is the source activity (A=r/R), exceeds the
   link rate (R * max < L), and there is sustained congestion.
   Hence, max = L / (A*R). When dividing the link rate by the average
   of min and max, we obtain the equivalent  rate (e), given by (EQ1).

                  e = R * [(2*A) / (1+A)]         (EQ 1)

   Note that the link rate L simplifies in the EQ 1 and is no longer
   needed in the computation of e.

   After computing (EQ 1), only once per LSP, the resulting e is signaled
   in the LDP message establishing the LSP. e is compared to the Available
   Rate (AvR) value for each hop (link) as follows:

     if (e < AvR)
        accept the LSP;
     else
        reject the LSP;









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Appendix B Example of ATM Service Categories and MPLS COS Mappings

   In this appendix, an example of how ATM and MPLS can be
   mapped in a multi-priority system is presented.

   Most well designed ATM hardware provides a way of differentiating the
   quality of service received by the various ATM
   service categories (CBR, VBR, UBR, etc.) through various queuing and
   scheduling techniques.

   In order to offer various delay and loss guarantees, ATM hardware often
   uses multiple emission and discard priorities. Let's say we have 3
   emission and 4 discard priorities. This MPS provides 12 different
   qualities of service as shown in Figure 2. Traffic is emitted in the
   order from E1 to E3. Therefore E1 has the lowest relative  delay and E3
   the highest. Under congestion, traffic is discard in the order from D4
   to D1. Therefore, D1 has the lowest relative packet loss and D4 the
   highest.

                                                   Emission
                                                   Priority

    +-------------------------------------------+  ^
    |          |          | CBR      | Control  |  E1
    |          |          |          |          |
    +-------------------------------------------+
    +-------------------------------------------+
    |          | CoS 2    | rt-VBR   | Control  |  E2
    |          |          |          |          |
    +-------------------------------------------+
    +-------------------------------------------+
    | UBR      | nrt-VBR  |          | Control  |  E3
    | CoS 0    | CoS 1    |          |          |
    +-------------------------------------------+
         D4         D3         D2        D1

                 Discard Priority

                   Figure 2 Multi-Priority System (MPS)

   Let's consider the open-loop ATM service categories (CBR, rt-VBR,
   nrt-VBR, and UBR). For MPLS, we define three different Classes of
   Service.

    CoS 0  Best Effort
    CoS 1  Low Loss Guarantee
    CoS 2  Low Delay Guarantee




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   Figure 2 also shows the mapping from the ATM service categories to the
   MPS as well as the mapping of MPLS CoSs to MPS. CoS 0 and UBR have a
   similar definition ("best effort"). Hence, they share the same emission
   and  discard priority (the lowest). CoS 1 and nrt-VBR  are both targeted
   for low loss guarantee but do not  care too much about delay. Therefore,
   they are mapped to the E3/D3 slot. CoS 2 and rt-VBR require Low delay
   guarantee. Hence, they are mapped to a higher  emission priority than
   CoS 0/ CoS 1. In order to provide rt-VBR with slightly better loss
   guarantee than CoS 2, rt-VBR is mapped to a higher discard priority
   (D2). Finally, CBR requires the best low delay and low loss
   guarantees. Hence it is mapped to the highest emission and discard
   priority.

   Network control traffic is in the case of ATM the signaling, ILMI, and
   RCC channels and in the case of MPLS the LDP channel etc.




































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