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RFC 5977

Experimental
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RMD-QOSM: The NSIS Quality-of-Service Model for Resource Management in Diffserv

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Internet Engineering Task Force (IETF)                          A. Bader
Request for Comments: 5977                                   L. Westberg
Category: Experimental                                          Ericsson
ISSN: 2070-1721                                           G. Karagiannis
                                                    University of Twente
                                                              C. Kappler
                                                  ck technology concepts
                                                               T. Phelan
                                                                   Sonus
                                                            October 2010


              RMD-QOSM: The NSIS Quality-of-Service Model
                  for Resource Management in Diffserv

Abstract

   This document describes a Next Steps in Signaling (NSIS) Quality-of-
   Service (QoS) Model for networks that use the Resource Management in
   Diffserv (RMD) concept.  RMD is a technique for adding admission
   control and preemption function to Differentiated Services (Diffserv)
   networks.  The RMD QoS Model allows devices external to the RMD
   network to signal reservation requests to Edge nodes in the RMD
   network.  The RMD Ingress Edge nodes classify the incoming flows into
   traffic classes and signals resource requests for the corresponding
   traffic class along the data path to the Egress Edge nodes for each
   flow.  Egress nodes reconstitute the original requests and continue
   forwarding them along the data path towards the final destination.
   In addition, RMD defines notification functions to indicate overload
   situations within the domain to the Edge nodes.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for examination, experimental implementation, and
   evaluation.

   This document defines an Experimental Protocol for the Internet
   community.  This document is a product of the Internet Engineering
   Task Force (IETF).  It represents the consensus of the IETF
   community.  It has received public review and has been approved for
   publication by the Internet Engineering Steering Group (IESG).  Not
   all documents approved by the IESG are a candidate for any level of
   Internet Standard; see Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc5977.

Page 2 
Copyright Notice

   Copyright (c) 2010 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1. Introduction ....................................................4
   2. Terminology .....................................................6
   3. Overview of RMD and RMD-QOSM ....................................7
      3.1. RMD ........................................................7
      3.2. Basic Features of RMD-QOSM ................................10
           3.2.1. Role of the QNEs ...................................10
           3.2.2. RMD-QOSM/QoS-NSLP Signaling ........................11
           3.2.3. RMD-QOSM Applicability and Considerations ..........13
   4. RMD-QOSM, Detailed Description .................................15
      4.1. RMD-QSPEC Definition ......................................16
           4.1.1. RMD-QOSM <QoS Desired> and <QoS Reserved> ..........16
           4.1.2. PHR Container ......................................17
           4.1.3. PDR Container ......................................20
      4.2. Message Format ............................................23
      4.3. RMD Node State Management .................................23
           4.3.1. Aggregated Operational and Reservation
                  States at the QNE Edges ............................23
           4.3.2. Measurement-Based Method ...........................25
           4.3.3. Reservation-Based Method ...........................27
      4.4. Transport of RMD-QOSM Messages ............................28
      4.5. Edge Discovery and Message Addressing .....................31
      4.6. Operation and Sequence of Events ..........................32
           4.6.1. Basic Unidirectional Operation .....................32
                  4.6.1.1. Successful Reservation ....................34
                  4.6.1.2. Unsuccessful Reservation ..................46
                  4.6.1.3. RMD Refresh Reservation ...................50
                  4.6.1.4. RMD Modification of Aggregated
                           Reservations ..............................54
                  4.6.1.5. RMD Release Procedure .....................55
                  4.6.1.6. Severe Congestion Handling ................64

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                  4.6.1.7. Admission Control Using Congestion
                           Notification Based on Probing .............70
           4.6.2. Bidirectional Operation ............................73
                  4.6.2.1. Successful and Unsuccessful Reservations ..77
                  4.6.2.2. Refresh Reservations ......................82
                  4.6.2.3. Modification of Aggregated Intra-Domain
                           QoS-NSLP Operational Reservation States ...82
                  4.6.2.4. Release Procedure .........................83
                  4.6.2.5. Severe Congestion Handling ................84
                  4.6.2.6. Admission Control Using Congestion
                           Notification Based on Probing .............87
      4.7. Handling of Additional Errors .............................89
   5. Security Considerations ........................................89
      5.1. Introduction ..............................................89
      5.2. Security Threats ..........................................91
           5.2.1. On-Path Adversary ..................................92
           5.2.2. Off-Path Adversary .................................94
      5.3. Security Requirements .....................................94
      5.4. Security Mechanisms .......................................94
   6. IANA Considerations ............................................97
      6.1. Assignment of QSPEC Parameter IDs .........................97
   7. Acknowledgments ................................................97
   8. References .....................................................97
      8.1. Normative References ......................................97
      8.2. Informative References ....................................98
   Appendix A. Examples .............................................101
      A.1. Example of a Re-Marking Operation during Severe
           Congestion in the Interior Nodes .........................101
      A.2. Example of a Detailed Severe Congestion Operation in the
           Egress Nodes .............................................107
      A.3. Example of a Detailed Re-Marking Admission Control
           (Congestion Notification) Operation in Interior Nodes ....111
      A.4. Example of a Detailed Admission Control (Congestion
           Notification) Operation in Egress Nodes ..................112
      A.5. Example of Selecting Bidirectional Flows for Termination
           during Severe Congestion .................................113
      A.6. Example of a Severe Congestion Solution for
           Bidirectional Flows Congested Simultaneously on Forward
           and Reverse Paths ........................................113
      A.7. Example of Preemption Handling during Admission Control ..117
      A.8. Example of a Retransmission Procedure within the RMD
           Domain ...................................................120
      A.9. Example on Matching the Initiator QSPEC to the Local
           RMD-QSPEC ................................................122

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1.  Introduction

   This document describes a Next Steps in Signaling (NSIS) QoS Model
   for networks that use the Resource Management in Diffserv (RMD)
   framework ([RMD1], [RMD2], [RMD3], and [RMD4]).  RMD adds admission
   control to Diffserv networks and allows nodes external to the
   networks to dynamically reserve resources within the Diffserv
   domains.

   The Quality-of-Service NSIS Signaling Layer Protocol (QoS-NSLP)
   [RFC5974] specifies a generic protocol for carrying QoS signaling
   information end-to-end in an IP network.  Each network along the end-
   to-end path is expected to implement a specific QoS Model (QOSM)
   specified by the QSPEC template [RFC5975] that interprets the
   requests and installs the necessary mechanisms, in a manner that is
   appropriate to the technology in use in the network, to ensure the
   delivery of the requested QoS.  This document specifies an NSIS QoS
   Model for RMD networks (RMD-QOSM), and an RMD-specific QSPEC (RMD-
   QSPEC) for expressing reservations in a suitable form for simple
   processing by internal nodes.

   They are used in combination with the QoS-NSLP to provide QoS
   signaling service in an RMD network.  Figure 1 shows an RMD network
   with the respective entities.

                          Stateless or reduced-state        Egress
   Ingress                RMD Nodes                         Node
   Node                   (Interior Nodes; I-Nodes)        (Stateful
   (Stateful              |          |            |         RMD QoS
   RMD QoS-NLSP           |          |            |         NSLP Node)
   Node)                  V          V            V
   +-------+   Data +------+      +------+       +------+     +------+
   |-------|--------|------|------|------|-------|------|---->|------|
   |       |   Flow |      |      |      |       |      |     |      |
   |Ingress|        |I-Node|      |I-Node|       |I-Node|     |Egress|
   |       |        |      |      |      |       |      |     |      |
   +-------+        +------+      +------+       +------+     +------+
            =================================================>
            <=================================================
                                  Signaling Flow

                   Figure 1: Actors in the RMD-QOSM

   Many network scenarios, such as the "Wired Part of Wireless Network"
   scenario, which is described in Section 8.4 of [RFC3726], require
   that the impact of the used QoS signaling protocol on the network
   performance should be minimized.  In such network scenarios, the
   performance of each network node that is used in a communication path

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   has an impact on the end-to-end performance.  As such, the end-to-end
   performance of the communication path can be improved by optimizing
   the performance of the Interior nodes.  One of the factors that can
   contribute to this optimization is the minimization of the QoS
   signaling protocol processing load and the minimization of the number
   of states on each Interior node.

   Another requirement that is imposed by such network scenarios is that
   whenever a severe congestion situation occurs in the network, the
   used QoS signaling protocol should be able to solve them.  In the
   case of a route change or link failure, a severe congestion situation
   may occur in the network.  Typically, routing algorithms are able to
   adapt and change their routing decisions to reflect changes in the
   topology and traffic volume.  In such situations, the rerouted
   traffic will have to follow a new path.  Interior nodes located on
   this new path may become overloaded, since they suddenly might need
   to support more traffic than for which they have capacity.  These
   severe congestion situations will severely affect the overall
   performance of the traffic passing through such nodes.

   RMD-QOSM is an edge-to-edge (intra-domain) QoS Model that, in
   combination with the QoS-NSLP and QSPEC specifications, is designed
   to support the requirements mentioned above:

      o Minimal impact on Interior node performance;

      o Increase of scalability;

      o Ability to deal with severe congestion

   Internally to the RMD network, RMD-QOSM together with QoS-NSLP
   [RFC5974] defines a scalable QoS signaling model in which per-flow
   QoS-NSLP and NSIS Transport Layer Protocol (NTLP) states are not
   stored in Interior nodes but per-flow signaling is performed (see
   [RFC5974]) at the Edges.

   In the RMD-QOSM, only routers at the Edges of a Diffserv domain
   (Ingress and Egress nodes) support the (QoS-NSLP) stateful operation;
   see Section 4.7 of [RFC5974].  Interior nodes support either the
   (QoS-NSLP) stateless operation or a reduced-state operation with
   coarser granularity than the Edge nodes.

   After the terminology in Section 2, we give an overview of RMD and
   the RMD-QOSM in Section 3.  This document specifies several RMD-QOSM/
   QoS-NSLP signaling schemes.  In particular, Section 3.2.3 identifies
   which combination of sections are used for the specification of each
   RMD-QOSM/QoS-NSLP signaling scheme.  In Section 4 we give a detailed
   description of the RMD-QOSM, including the role of QoS NSIS entities

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   (QNEs), the definition of the QSPEC, mapping of QSPEC generic
   parameters onto RMD-QOSM parameters, state management in QNEs, and
   operation and sequence of events.  Section 5 discusses security
   issues.

2.  Terminology

   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 [RFC2119].

   The terminology defined by GIST [RFC5971] and QoS-NSLP [RFC5974]
   applies to this document.

   In addition, the following terms are used:

   NSIS domain: an NSIS signaling-capable domain.

   RMD domain: an NSIS domain that is capable of supporting the RMD-QOSM
   signaling and operations.

   Edge node: a QoS-NSLP node on the boundary of some administrative
   domain that connects one NSIS domain to a node in either another NSIS
   domain or a non-NSIS domain.

   NSIS-aware node: a node that is aware of NSIS signaling and RMD-QOSM
   operations, such as severe congestion detection and Differentiated
   Service Code Point (DSCP) marking.

   NSIS-unaware node: a node that is unaware of NSIS signaling, but is
   aware of RMD-QOSM operations such as severe congestion detection and
   DSCP marking.

   Ingress node: an Edge node in its role in handling the traffic as it
   enters the NSIS domain.

   Egress node: an Edge node in its role in handling the traffic as it
   leaves the NSIS domain.

   Interior node: a node in an NSIS domain that is not an Edge node.

   Congestion: a temporal network state that occurs when the traffic (or
   when traffic associated with a particular Per-Hop Behavior (PHB))
   passing through a link is slightly higher than the capacity allocated
   for the link (or allocated for the particular PHB).  If no measures
   are taken, then the traffic passing through this link may temporarily
   slightly degrade in QoS.  This type of congestion is usually solved
   using admission control mechanisms.

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   Severe congestion: the congestion situation on a particular link
   within the RMD domain where a significant increase in its real packet
   queue situation occurs, such as when due to a link failure rerouted
   traffic has to be supported by this particular link.

3.  Overview of RMD and RMD-QOSM

3.1.  RMD

   The Differentiated Services (Diffserv) architecture ([RFC2475],
   [RFC2638]) was introduced as a result of efforts to avoid the
   scalability and complexity problems of IntServ [RFC1633].
   Scalability is achieved by offering services on an aggregate rather
   than per-flow basis and by forcing as much of the per-flow state as
   possible to the Edges of the network.  The service differentiation is
   achieved using the Differentiated Services (DS) field in the IP
   header and the Per-Hop Behavior (PHB) as the main building blocks.
   Packets are handled at each node according to the PHB indicated by
   the DS field in the message header.

   The Diffserv architecture does not specify any means for devices
   outside the domain to dynamically reserve resources or receive
   indications of network resource availability.  In practice, service
   providers rely on short active time Service Level Agreements (SLAs)
   that statically define the parameters of the traffic that will be
   accepted from a customer.

   RMD was introduced as a method for dynamic reservation of resources
   within a Diffserv domain.  It describes a method that is able to
   provide admission control for flows entering the domain and a
   congestion handling algorithm that is able to terminate flows in case
   of congestion due to a sudden failure (e.g., link, router) within the
   domain.

   In RMD, scalability is achieved by separating a fine-grained
   reservation mechanism used in the Edge nodes of a Diffserv domain
   from a much simpler reservation mechanism needed in the Interior
   nodes.  Typically, it is assumed that Edge nodes support per-flow QoS
   states in order to provide QoS guarantees for each flow.  Interior
   nodes use only one aggregated reservation state per traffic class or
   no states at all.  In this way, it is possible to handle large
   numbers of flows in the Interior nodes.  Furthermore, due to the
   limited functionality supported by the Interior nodes, this solution
   allows fast processing of signaling messages.

   The possible RMD-QOSM applicabilities are described in Section 3.2.3.
   Two main basic admission control modes are supported: reservation-
   based and measurement-based admission control that can be used in

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   combination with a severe congestion-handling solution.  The severe
   congestion-handling solution is used in the situation that a
   link/node becomes severely congested due to the fact that the traffic
   supported by a failed link/node is rerouted and has to be processed
   by this link/node.  Furthermore, RMD-QOSM supports both
   unidirectional and bidirectional reservations.

   Another important feature of RMD-QOSM is that the intra-domain
   sessions supported by the Edges can be either per-flow sessions or
   per-aggregate sessions.  In the case of the per-flow intra-domain
   sessions, the maintained per-flow intra-domain states have a one-to-
   one dependency to the per-flow end-to-end states supported by the
   same Edge.  In the case of the per-aggregate sessions the maintained
   per-aggregate states have a one-to-many relationship to the per-flow
   end-to-end states supported by the same Edge.

   In the reservation-based method, each Interior node maintains only
   one reservation state per traffic class.  The Ingress Edge nodes
   aggregate individual flow requests into PHB traffic classes, and
   signal changes in the class reservations as necessary.  The
   reservation is quantified in terms of resource units (or bandwidth).
   These resources are requested dynamically per PHB and reserved on
   demand in all nodes in the communication path from an Ingress node to
   an Egress node.

   The measurement-based algorithm continuously measures traffic levels
   and the actual available resources, and admits flows whose resource
   needs are within what is available at the time of the request.  The
   measurement-based algorithm is used to support a predictive service
   where the service commitment is somewhat less reliable than the
   service that can be supported by the reservation-based method.

   A main assumption that is made by such measurement-based admission
   control mechanisms is that the aggregated PHB traffic passing through
   an RMD Interior node is high and therefore, current measurement
   characteristics are considered to be an indicator of future load.
   Once an admission decision is made, no record of the decision need be
   kept at the Interior nodes.  The advantage of measurement-based
   resource management protocols is that they do not require pre-
   reservation state nor explicit release of the reservations at the
   Interior nodes.  Moreover, when the user traffic is variable,
   measurement-based admission control could provide higher network
   utilization than, e.g., peak-rate reservation.  However, this can
   introduce an uncertainty in the availability of the resources.  It is
   important to emphasize that the RMD measurement-based schemes
   described in this document do not use any refresh procedures, since
   these approaches are used in stateless nodes; see Section 4.6.1.3.

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   Two types of measurement-based admission control schemes are
   possible:

   * Congestion notification function based on probing:

   This method can be used to implement a simple measurement-based
   admission control within a Diffserv domain.  In this scenario, the
   Interior nodes are not NSIS-aware nodes.  In these Interior nodes,
   thresholds are set for the traffic belonging to different PHBs in the
   measurement-based admission control function.  In this scenario, an
   end-to-end NSIS message is used as a probe packet, meaning that the
   <DSCP> field in the header of the IP packet that carries the NSIS
   message is re-marked when the predefined congestion threshold is
   exceeded.  Note that when the predefined congestion threshold is
   exceeded, all packets are re-marked by a node, including NSIS
   messages.  In this way, the Edges can admit or reject flows that are
   requesting resources.  The frequency and duration that the congestion
   level is above the threshold resulting in re-marking is tracked and
   used to influence the admission control decisions.

   * NSIS measurement-based admission control:

   In this case, the measurement-based admission control functionality
   is implemented in NSIS-aware stateless routers.  The main difference
   between this type of admission control and the congestion
   notification based on probing is related to the fact that this type
   of admission control is applied mainly on NSIS-aware nodes.  With the
   measurement-based scheme, the requested peak bandwidth of a flow is
   carried by the admission control request.  The admission decision is
   considered as positive if the currently carried traffic, as
   characterized by the measured statistics, plus the requested
   resources for the new flow exceeds the system capacity with a
   probability smaller than a value alpha.  Otherwise, the admission
   decision is negative.  It is important to emphasize that due to the
   fact that the RMD Interior nodes are stateless, they do not store
   information of previous admission control requests.

   This could lead to a situation where the admission control accuracy
   is decreased when multiple simultaneous flows (sharing a common
   Interior node) are requesting admission control simultaneously.  By
   applying measuring techniques, e.g., see [JaSh97] and [GrTs03], which
   use current and past information on NSIS sessions that requested
   resources from an NSIS-aware Interior node, the decrease in admission
   control accuracy can be limited.  RMD describes the following
   procedures:

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   * classification of an individual resource reservation or a resource
     query into Per-Hop Behavior (PHB) groups at the Ingress node of the
     domain,

   * hop-by-hop admission control based on a PHB within the domain.
     There are two possible modes of operation for internal nodes to
     admit requests.  One mode is the stateless or measurement-based
     mode, where the resources within the domain are queried.  Another
     mode of operation is the reduced-state reservation or reservation-
     based mode, where the resources within the domain are reserved.

   * a method to forward the original requests across the domain up to
     the Egress node and beyond.

   * a congestion-control algorithm that notifies the Egress Edge nodes
     about congestion.  It is able to terminate the appropriate number
     of flows in the case a of congestion due to a sudden failure (e.g.,
     link or router failure) within the domain.

3.2.  Basic Features of RMD-QOSM

3.2.1.  Role of the QNEs

   The protocol model of the RMD-QOSM is shown in Figure 2.  The figure
   shows QoS NSIS initiator (QNI) and QoS NSIS Receiver (QNR) nodes, not
   part of the RMD network, that are the ultimate initiator and receiver
   of the QoS reservation requests.  It also shows QNE nodes that are
   the Ingress and Egress nodes in the RMD domain (QNE Ingress and QNE
   Egress), and QNE nodes that are Interior nodes (QNE Interior).

   All nodes of the RMD domain are usually QoS-NSLP-aware nodes.
   However, in the scenarios where the congestion notification function
   based on probing is used, then the Interior nodes are not NSIS aware.
   Edge nodes store and maintain QoS-NSLP and NTLP states and therefore
   are stateful nodes.  The NSIS-aware Interior nodes are NTLP
   stateless.  Furthermore, they are either QoS-NSLP stateless (for NSIS
   measurement-based operation) or reduced-state nodes storing per PHB
   aggregated QoS-NSLP states (for reservation-based operation).

   Note that the RMD domain MAY contain Interior nodes that are not
   NSIS-aware nodes (not shown in the figure).

   These nodes are assumed to have sufficient capacity for flows that
   might be admitted.  Furthermore, some of these NSIS-unaware nodes MAY
   be used for measuring the traffic congestion level on the data path.
   These measurements can be used by RMD-QOSM in the congestion control
   based on probing operation and/or severe congestion operation (see
   Section 4.6.1.6).

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   |------|   |-------|                           |------|   |------|
   | e2e  |<->| e2e   |<------------------------->| e2e  |<->| e2e  |
   | QoS  |   | QoS   |                           | QoS  |   | QoS  |
   |      |   |-------|                           |------|   |------|
   |      |   |-------|   |-------|   |-------|   |------|   |      |
   |      |   | local |<->| local |<->| local |<->| local|   |      |
   |      |   | QoS   |   |  QoS  |   |  QoS  |   |  QoS |   |      |
   |      |   |       |   |       |   |       |   |      |   |      |
   | NSLP |   | NSLP  |   | NSLP  |   | NSLP  |   | NSLP |   | NSLP |
   |st.ful|   |st.ful |   |st.less/   |st.less/   |st.ful|   |st.ful|
   |      |   |       |   |red.st.|   |red.st.|   |      |   |      |
   |      |   |-------|   |-------|   |-------|   |------|   |      |
   |------|   |-------|   |-------|   |-------|   |------|   |------|
   ------------------------------------------------------------------
   |------|   |-------|   |-------|   |-------|   |------|   |------|
   | NTLP |<->| NTLP  |<->| NTLP  |<->| NTLP  |<->| NTLP |<->|NTLP  |
   |st.ful|   |st.ful |   |st.less|   |st.less|   |st.ful|   |st.ful|
   |------|   |-------|   |-------|   |-------|   |------|   |------|
     QNI         QNE        QNE         QNE          QNE       QNR
   (End)     (Ingress)   (Interior)  (Interior)   (Egress)    (End)

       st.ful: stateful, st.less: stateless
       st.less red.st.: stateless or reduced-state

    Figure 2: Protocol model of stateless/reduced-state operation

3.2.2.  RMD-QOSM/QoS-NSLP Signaling

   The basic RMD-QOSM/QoS-NSLP signaling is shown in Figure 3.  The
   signaling scenarios are accomplished using the QoS-NSLP processing
   rules defined in [RFC5974], in combination with the Resource
   Management Function (RMF) triggers sent via the QoS-NSLP-RMF API
   described in [RFC5974].

   Due to the fact that within the RMD domain a QoS Model that is
   different than the end-to-end QoS Model applied at the Edges of the
   RMD domain can be supported, the RMD Interior node reduced-state
   reservations can be updated independently of the per-flow end-to-end
   reservations (see Section 4.7 of [RFC5974]).  Therefore, two
   different RESERVE messages are used within the RMD domain.  One
   RESERVE message that is associated with the per-flow end-to-end
   reservations and is used by the Edges of the RMD domain and one that
   is associated with the reduced-state reservations within the RMD
   domain.

   A RESERVE message is created by a QNI with an Initiator QSPEC
   describing the reservation and forwarded along the path towards the
   QNR.

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   When the original RESERVE message arrives at the Ingress node, an
   RMD-QSPEC is constructed based on the initial QSPEC in the message
   (usually the Initiator QSPEC).  The RMD-QSPEC is sent in a intra-
   domain, independent RESERVE message through the Interior nodes
   towards the QNR.  This intra-domain RESERVE message uses the GIST
   datagram signaling mechanism.  Note that the RMD-QOSM cannot directly
   specify that the GIST Datagram mode SHOULD be used.  This can however
   be notified by using the GIST API Transfer-Attributes, such as
   unreliable, low level of security and use of local policy.

   Meanwhile, the original RESERVE message is sent to the Egress node on
   the path to the QNR using the reliable transport mode of NTLP.  Each
   QoS-NSLP node on the data path processes the intra-domain RESERVE
   message and checks the availability of resources with either the
   reservation-based or the measurement-based method.

       QNE Ingress     QNE Interior     QNE Interior   QNE Egress
     NTLP stateful  NTLP stateless  NTLP stateless  NTLP stateful
            |               |               |              |
    RESERVE |               |               |              |
   -------->| RESERVE       |               |              |
            +--------------------------------------------->|
            | RESERVE'      |               |              |
            +-------------->|               |              |
            |               | RESERVE'      |              |
            |               +-------------->|              |
            |               |               | RESERVE'     |
            |               |               +------------->|
            |               |               |     RESPONSE'|
            |<---------------------------------------------+
            |               |               |              | RESERVE
            |               |               |              +------->
            |               |               |              |RESPONSE
            |               |               |              |<-------
            |               |               |     RESPONSE |
            |<---------------------------------------------+
    RESPONSE|               |               |              |
   <--------|               |               |              |

     Figure 3: Sender-initiated reservation with reduced-state
               Interior nodes

   When the message reaches the Egress node, and the reservation is
   successful in each Interior node, an intra-domain (local) RESPONSE'
   is sent towards the Ingress node and the original (end-to-end)
   RESERVE message is forwarded to the next domain.  When the Egress
   node receives a RESPONSE message from the downstream end, it is
   forwarded directly to the Ingress node.

Top      ToC       Page 13 
   If an intermediate node cannot accommodate the new request, it
   indicates this by marking a single bit in the message, and continues
   forwarding the message until the Egress node is reached.  From the
   Egress node, an intra-domain RESPONSE' and an original RESPONSE
   message are sent directly to the Ingress node.

   As a consequence, in the stateless/reduced-state domain only sender-
   initiated reservations can be performed and functions requiring per-
   flow NTLP or QoS-NSLP states, like summary and reduced refreshes,
   cannot be used.  If per-flow identification is needed, i.e.,
   associating the flow IDs for the reserved resources, Edge nodes act
   on behalf of Interior nodes.

3.2.3.  RMD-QOSM Applicability and Considerations

   The RMD-QOSM is a Diffserv-based bandwidth management methodology
   that is not able to provide a full Diffserv support.  The reason for
   this is that the RMD-QOSM concept can only support the (Expedited
   Forwarding) EF-like functionality behavior, but is not able to
   support the full set of (Assured Forwarding) AF-like functionality.
   The bandwidth information REQUIRED by the EF-like functionality
   behavior can be supported by RMD-QOSM carrying the bandwidth
   information in the <QoS Desired> parameter (see [RFC5975]).  The full
   set of (Assured Forwarding) AF-like functionality requires
   information that is specified in two token buckets.  The RMD-QOSM is
   not supporting the use of two token buckets and therefore, it is not
   able to support the full set of AF-functionality.  Note however, that
   RMD-QOSM could also support a single AF PHB, when the traffic or the
   upper limit of the traffic can be characterized by a single bandwidth
   parameter.  Moreover, it is considered that in case of tunneling, the
   RMD-QOSM supports only the uniform tunneling mode for Diffserv (see
   [RFC2983]).

   The RMD domain MUST be engineered in such a way that each QNE Ingress
   maintains information about the smallest MTU that is supported on the
   links within the RMD domain.

   A very important consideration on using RMD-QOSM is that within one
   RMD domain only one of the following RMD-QOSM schemes can be used at
   a time.  Thus, an RMD router can never process and use two different
   RMD-QOSM signaling schemes at the same time.

   However, all RMD QNEs supporting this specification MUST support the
   combination of the "per-flow RMD reservation-based" and the "severe
   congestion handling by proportional data packet marking" scheme.  If
   the RMD QNEs support more RMD-QOSM schemes, then the operator of that
   RMD domain MUST preconfigure all the QNE Edge nodes within one domain
   such that the <SCH> field included in the "PHR container" (Section

Top      ToC       Page 14 
   4.1.2) and the "PDR Container" (Section 4.1.3) will always use the
   same value, such that within one RMD domain only one of the below
   described RMD-QOSM schemes is used at a time.

   The congestion situations (see Section 2) are solved using an
   admission control mechanism, e.g., "per-flow congestion notification
   based on probing", while the severe congestion situations (see
   Section 2), are solved using the severe congestion handling
   mechanisms, e.g., "severe congestion handling by proportional data
   packet marking".

   The RMD domain MUST be engineered in such a way that RMD-QOSM
   messages could be transported using the GIST Query and DATA messages
   in Q-mode; see [RFC5971].  This means that the Path MTU MUST be
   engineered in such a way that the RMD-QOSM message are transported
   without fragmentation.  Furthermore, the RMD domain MUST be
   engineered in such a way to guarantee capacity for the GIST Query and
   Data messages in Q-mode, within the rate control limits imposed by
   GIST; see [RFC5971].

   The RMD domain has to be configured such that the GIST context-free
   flag (C-flag) MUST be set (C=1) for QUERY messages and DATA messages
   sent in Q-mode; see [RFC5971].

   Moreover, the same deployment issues and extensibility considerations
   described in [RFC5971] and [RFC5978] apply to this document.

   It is important to note that the concepts described in Sections
   4.6.1.6.2, 4.6.2.5.2, 4.6.1.6.2, and 4.6.2.5.2 contributed to the PCN
   WG standardization.

   The available RMD-QOSM/QoS-NSLP signaling schemes are:

   * "per-flow congestion notification based on probing" (see Sections
     4.3.2, 4.6.1.7, and 4.6.2.6).  Note that this scheme uses, for
     severe congestion handling, the "severe congestion handling by
     proportional data packet marking" (see Sections 4.6.1.6.2 and
     4.6.2.5.2).  Furthermore, the Interior nodes are considered to be
     Diffserv aware, but NSIS-unaware nodes (see Section 4.3.2).

   * "per-flow RMD NSIS measurement-based admission control" (see
     Sections 4.3.2, 4.6.1, and 4.6.2).  Note that this scheme uses, for
     severe congestion handling, the "severe congestion handling by
     proportional data packet marking" (see Sections 4.6.1.6.2 and
     4.6.2.5.2).  Furthermore, the Interior nodes are considered to be
     NSIS-aware nodes (see Section 4.3.2).

Top      ToC       Page 15 
   * "per-flow RMD reservation-based" in combination with the "severe
     congestion handling by the RMD-QOSM refresh" procedure (see
     Sections 4.3.3, 4.6.1, 4.6.1.6.1, and 4.6.2.5.1).  Note that this
     scheme uses, for severe congestion handling, the "severe congestion
     handling by the RMD-QOSM refresh" procedure (see Sections 4.6.1.6.1
     and 4.6.2.5.1).  Furthermore, the intra-domain sessions supported
     by the Edge nodes are per-flow sessions (see Section 4.3.3).

   * "per-flow RMD reservation-based" in combination with the "severe
     the congestion handling by proportional data packet marking"
     procedure (see Sections 4.3.3, 4.6.1, 4.6.1.6.2, and 4.6.2.5.2).
     Note that this scheme uses, for severe congestion handling, the
     "severe congestion handling by proportional data packet marking"
     procedure (see Sections 4.6.1.6.2 and 4.6.2.5.2).  Furthermore, the
     intra-domain sessions supported by the Edge nodes are per-flow
     sessions (see Section 4.3.3).

   * "per-aggregate RMD reservation-based" in combination with the
     "severe congestion handling by the RMD-QOSM refresh" procedure (see
     Sections 4.3.1, 4.6.1, 4.6.1.6.1, and 4.6.2.5.1).  Note that this
     scheme uses, for severe congestion handling, the "severe congestion
     handling by the RMD-QOSM refresh" procedure (see Sections 4.6.1.6.1
     and 4.6.2.5.1).  Furthermore, the intra-domain sessions supported
     by the Edge nodes are per-aggregate sessions (see Section 4.3.1).
     Moreover, this scheme can be considered to be a reservation-based
     scheme, since the RMD Interior nodes are reduced-state nodes, i.e.,
     they do not store NTLP/GIST states, but they do store per PHB-
     aggregated QoS-NSLP reservation states.

   * "per-aggregate RMD reservation-based" in combination with the
     "severe congestion handling by proportional data packet marking"
     procedure (see Sections 4.3.1, 4.6.1, 4.6.1.6.2, and 4.6.2.5.2).
     Note that this scheme uses, for severe congestion handling, the
     "severe congestion handling by proportional data packet marking"
     procedure (see Sections 4.6.1.6.2 and 4.6.2.5.2).  Furthermore, the
     intra-domain sessions supported by the Edge nodes are per-aggregate
     sessions (see Section 4.3.1).  Moreover, this scheme can be
     considered to be a reservation-based scheme, since the RMD Interior
     nodes are reduced-state nodes, i.e., they do not store NTLP/GIST
     states, but they do store per PHB-aggregated QoS-NSLP reservation
     states.

4.  RMD-QOSM, Detailed Description

   This section describes the RMD-QOSM in more detail.  In particular,
   it defines the role of stateless and reduced-state QNEs, the RMD-QOSM
   QSPEC Object, the format of the RMD-QOSM QoS-NSLP messages, and how
   QSPECs are processed and used in different protocol operations.

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4.1.  RMD-QSPEC Definition

   The RMD-QOSM uses the QSPEC format specified in [RFC5975].  The
   Initiator/Local QSPEC bit, i.e., <I> is set to "Local" (i.e., "1")
   and the <QSPEC Proc> is set as follows:

   * Message Sequence = 0: Sender initiated
   * Object combination = 0: <QoS Desired> for RESERVE and
     <QoS Reserved> for RESPONSE

   The <QSPEC Version> used by RMD-QOSM is the default version, i.e.,
   "0", see [RFC5975].  The <QSPEC Type> value used by the RMD-QOSM is
   specified in [RFC5975] and is equal to "2".  The <Traffic Handling
   Directives> contains the following fields:

   <Traffic Handling Directives> = <PHR container> <PDR container>

   The Per-Hop Reservation container (PHR container) and the Per-Domain
   Reservation container (PDR container) are specified in Sections 4.1.2
   and 4.1.3, respectively.  The <PHR container> contains the traffic
   handling directives for intra-domain communication and reservation.
   The <PDR container> contains additional traffic handling directives
   that are needed for edge-to-edge communication.  The parameter IDs
   used by the <PHR container> and <PDR container> are assigned by IANA;
   see Section 6.

   The RMD-QOSM <QoS Desired> and <QoS Reserved>, are specified in
   Section 4.1.1.  The RMD-QOSM <QoS Desired> and <QoS Reserved> and the
   <PHR container> are used and processed by the Edge and Interior
   nodes.  The <PDR container> field is only processed by Edge nodes.

4.1.1.  RMD-QOSM <QoS Desired> and <QoS Reserved>

   The RESERVE message contains only the <QoS Desired> object [RFC5975].
   The <QoS Reserved> object is carried by the RESPONSE message.

   In RMD-QOSM, the <QoS Desired> and <QoS Reserved> objects contain the
   following parameters:

   <QoS Desired> = <TMOD-1> <PHB Class> <Admission Priority>
   <QoS Reserved> = <TMOD-1> <PHB Class> <Admission Priority>

   The bit format of the <PHB Class> (see [RFC5975] and Figures 4 and 5)
   and <Admission Priority> complies with the bit format specified in
   [RFC5975].

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   Note that for the RMD-QOSM, a reservation established without an
   <Admission Priority> parameter is equivalent to a reservation
   established with an <Admission Priority> whose value is 1.

    0                   1
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | DSCP      |0 0 0 0 0 0 0 0 X 0|
   +---+---+---+---+---+---+---+---+

      Figure 4: DSCP parameter

    0                   1
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    PHB ID code        |0 0 X X|
   +---+---+---+---+---+---+---+---+

      Figure 5: PHB ID Code parameter

4.1.2.  PHR Container

   This section describes the parameters used by the PHR container,
   which are used by the RMD-QOSM functionality available at the
   Interior nodes.

   <PHR container> = <O> <K> <S> <M>, <Admitted Hops>, <B> <Hop_U> <Time
   Lag> <SCH> <Max Admitted Hops>

   The bit format of the PHR container can be seen in Figure 6.  Note
   that in Figure 6 <Hop_U> is represented as <U>.  Furthermore, in
   Figure 6, <Max Admitted Hops> is represented as <Max Adm Hops>.

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |M|E|N|r|       Parameter ID    |r|r|r|r|          2            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |S|M| Admitted  Hops|B|U| Time  Lag     |O|K| SCH |             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Max Adm  Hops |                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                        Figure 6: PHR container

   Parameter ID: 12-bit field, indicating the PHR type:
   PHR_Resource_Request, PHR_Release_Request, PHR_Refresh_Update.

Top      ToC       Page 18 
   "PHR_Resource_Request" (Parameter ID = 17): initiate or update the
   traffic class reservation state on all nodes located on the
   communication path between the QNE(Ingress) and QNE(Egress) nodes.

   "PHR_Release_Request" (Parameter ID = 18): explicitly release, by
   subtraction, the reserved resources for a particular flow from a
   traffic class reservation state.

   "PHR_Refresh_Update" (Parameter ID = 19): refresh the traffic class
   reservation soft state on all nodes located on the communication path
   between the QNE(Ingress) and QNE(Egress) nodes according to a
   resource reservation request that was successfully processed during a
   previous refresh period.

   <S> (Severe Congestion): 1 bit.  In the case of a route change,
   refreshing RESERVE messages follow the new data path, and hence
   resources are requested there.  If the resources are not sufficient
   to accommodate the new traffic, severe congestion occurs.  Severe
   congested Interior nodes SHOULD notify Edge QNEs about the congestion
   by setting the <S> bit.

   <O> (Overload): 1 bit.  This field is used during the severe
   congestion handling scheme that is using the RMD-QOSM refresh
   procedure.  This bit is set when an overload on a QNE Interior node
   is detected and when this field is carried by the
   "PHR_Refresh_Update" container.  <O> SHOULD be set to"1" if the <S>
   bit is set.  For more details, see Section 4.6.1.6.1.

   <M>: 1 bit.  In the case of unsuccessful resource reservation or
   resource query in an Interior QNE, this QNE sets the <M> bit in order
   to notify the Egress QNE.

   <Admitted Hops>: 8-bit field.  The <Admitted Hops> counts the number
   of hops in the RMD domain where the reservation was successful.  The
   <Admitted Hops> is set to "0" when a RESERVE message enters a domain
   and it MUST be incremented by each Interior QNE, provided that the
   <Hop_U> bit is not set.  However, when a QNE that does not have
   sufficient resources to admit the reservation is reached, the <M> bit
   is set, and the <Admitted Hops> value is frozen, by setting the
   <Hop_U> bit to "1".  Note that the <Admitted Hops> parameter in
   combination with the <Max Admitted Hops> and <K> parameters are used
   during the RMD partial release procedures (see Section 4.6.1.5.2).

   <Hop_U> (NSLP_Hops unset): 1 bit.  The QNE(Ingress) node MUST set the
   <Hop_U> parameter to 0.  This parameter SHOULD be set to "1" by a
   node when the node does not increase the <Admitted Hops> value.  This
   is the case when an RMD-QOSM reservation-based node is not admitting
   the reservation request.  When <Hop_U> is set to "1", the <Admitted

Top      ToC       Page 19 
   Hops> SHOULD NOT be changed.  Note that this flag, in combination
   with the <Admitted Hops> flag, are used to locate the last node that
   successfully processed a reservation request (see Section 4.6.1.2).

   <B>: 1 bit.  When set to "1", it indicates a bidirectional
   reservation.

   <Time Lag>: It represents the ratio between the "T_Lag" parameter,
   which is the time difference between the departure time of the last
   sent "PHR_Refresh_Update" control information container and the
   departure time of the "PHR_Release_Request" control information
   container, and the length of the refresh period, "T_period", see
   Section 4.6.1.5.

   <K>: 1 bit.  When set to "1", it indicates that the
   resources/bandwidth carried by a tearing RESERVE MUST NOT be
   released, and the resources/bandwidth carried by a non-tearing
   RESERVE MUST NOT be reserved/refreshed.  For more details, see
   Section 4.6.1.5.2.

   <Max Admitted Hops>: 8 bits.  The <Admitted Hops> value that has been
   carried by the <PHR container> field used to identify the RMD
   reservation-based node that admitted or processed a
   "PHR_Resource_Request".

   <SCH>: 3 bits.  The <SCH> value that is used to specify which of the
   6 RMD-QOSM scenarios (see Section 3.2.3) MUST be used within the RMD
   domain.  The operator of an RMD domain MUST preconfigure all the QNE
   Edge nodes within one domain such that the <SCH> field included in
   the "PHR container", will always use the same value, such that within
   one RMD domain only one of the below described RMD-QOSM schemes can
   be used at a time.  All the QNE Interior nodes MUST interpret this
   field before processing any other PHR container payload fields.  The
   currently defined <SCH> values are:

   o  0:     RMD-QOSM scheme MUST be "per-flow congestion notification
             based on probing";

   o  1:     RMD-QOSM scheme MUST be "per-flow RMD NSIS measurement-
             based admission control",

   o  2:     RMD-QOSM scheme MUST be "per-flow RMD reservation-based" in
             combination with the "severe congestion handling by the
             RMD-QOSM refresh" procedure;

   o  3 :    RMD-QOSM scheme MUST be "per-flow RMD reservation-based" in
             combination with the "severe congestion handling by
             proportional data packet marking" procedure;

Top      ToC       Page 20 
   o  4:     RMD-QOSM scheme MUST be "per-aggregate RMD reservation-
             based" in combination with the "severe congestion handling
             by the RMD-QOSM refresh" procedure;

   o  5:     RMD-QOSM scheme MUST be "per-aggregate RMD reservation-
             based" in combination with the "severe congestion handling
             by proportional data packet marking" procedure;

   o  6 - 7: reserved.

   The default value of the <SCH> field MUST be set to the value equal
   to 3.

4.1.3.  PDR Container

   This section describes the parameters of the PDR container, which are
   used by the RMD-QOSM functionality available at the Edge nodes.

   The bit format of the PDR container can be seen in Figure 7.

   <PDR container> = <O>  <S> <M>
   <Max Admitted Hops> <B> <SCH> [<PDR Bandwidth>]

   In Figure 7, note that <Max Admitted Hops> is represented as <Max Adm
   Hops>.

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |M|E|N|r|   Parameter ID        |r|r|r|r|          2            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |S|M| Max Adm  Hops |B|O| SCH |        EMPTY                    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |PDR Bandwidth(32-bit IEEE floating point.number)               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                        Figure 7: PDR container

   Parameter ID: 12-bit field identifying the type of <PDR container>
   field.

   "PDR_Reservation_Request" (Parameter ID = 20): generated by the
   QNE(Ingress) node in order to initiate or update the QoS-NSLP per-
   domain reservation state in the QNE(Egress) node.

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   "PDR_Refresh_Request" (Parameter ID = 21): generated by the
   QNE(Ingress) node and sent to the QNE(Egress) node to refresh, in
   case needed, the QoS-NSLP per-domain reservation states located in
   the QNE(Egress) node.

   "PDR_Release_Request" (Parameter ID = 22): generated and sent by the
   QNE(Ingress) node to the QNE(Egress) node to release the per-domain
   reservation states explicitly.

   "PDR_Reservation_Report" (Parameter ID = 23): generated and sent by
   the QNE(Egress) node to the QNE(Ingress) node to report that a
   "PHR_Resource_Request" and a "PDR_Reservation_Request" traffic
   handling directive field have been received and that the request has
   been admitted or rejected.

   "PDR_Refresh_Report" (Parameter ID = 24) generated and sent by the
   QNE(Egress) node in case needed, to the QNE(Ingress) node to report
   that a "PHR_Refresh_Update" traffic handling directive field has been
   received and has been processed.

   "PDR_Release_Report" (Parameter ID = 25) generated and sent by the
   QNE(Egress) node in case needed, to the QNE(Ingress) node to report
   that a "PHR_Release_Request" and a "PDR_Release_Request" traffic
   handling directive field have been received and have been processed.

   "PDR_Congestion_Report" (Parameter ID = 26): generated and sent by
   the QNE(Egress) node to the QNE(Ingress) node and used for congestion
   notification.

   <S> (PDR Severe Congestion): 1 bit.  Specifies if a severe congestion
   situation occurred.  It can also carry the <S> parameter of the
   <PHR_Resource_Request> or <PHR_Refresh_Update> fields.

   <O> (Overload): 1 bit.  This field is used during the severe
   congestion handling scheme that is using the RMD-QOSM refresh
   procedure.  This bit is set when an overload on a QNE Interior node
   is detected and when this field is carried by the
   "PDR_Congestion_Report" container.  <O> SHOULD be set to "1" if the
   <S> bit is set.  For more details, see Section 4.6.1.6.1.

   <M> (PDR Marked): 1 bit.  Carries the <M> value of the
   "PHR_Resource_Request" or "PHR_Refresh_Update" traffic handling
   directive field.

   <B>: 1 bit.  Indicates bidirectional reservation.

Top      ToC       Page 22 
   <Max Admitted Hops>: 8 bits.  The <Admitted Hops> value that has been
   carried by the <PHR container> field used to identify the RMD
   reservation-based node that admitted or processed a
   "PHR_Resource_Request".

   <PDR Bandwidth>: 32 bits.  This field specifies the bandwidth that
   either applies when the <B> flag is set to "1" and when this
   parameter is carried by a RESPONSE message or when a severe
   congestion occurs and the QNE Edges maintain an aggregated intra-
   domain QoS-NSLP operational state and it is carried by a NOTIFY
   message.  In the situation that the <B> flag is set to "1", this
   parameter specifies the requested bandwidth that has to be reserved
   by a node in the reverse direction and when the intra-domain
   signaling procedures require a bidirectional reservation procedure.
   In the severe congestion situation, this parameter specifies the
   bandwidth that has to be released.

   <SCH>: 3 bits.  The <SCH> value that is used to specify which of the
   6 RMD scenarios (see Section 3.2.3) MUST be used within the RMD
   domain.  The operator of an RMD domain MUST preconfigure all the QNE
   Edge nodes within one domain such that the <SCH> field included in
   the "PDR container", will always use the same value, such that within
   one RMD domain only one of the below described RMD-QOSM schemes can
   be used at a time.  All the QNE Interior nodes MUST interpret this
   field before processing any other <PDR container> payload fields.
   The currently defined <SCH> values are:

   o  0:     RMD-QOSM scheme MUST be "per-flow congestion notification
             based on probing";

   o  1:     RMD-QOSM scheme MUST be "per-flow RMD NSIS measurement-
             based admission control";

   o  2:     RMD-QOSM scheme MUST be "per-flow RMD reservation-based" in
             combination with the "severe congestion handling by the
             RMD-QOSM refresh" procedure;

   o  3 :    RMD-QOSM scheme MUST be "per-flow RMD reservation-based" in
             combination with the "severe congestion handling by
             proportional data packet marking" procedure;

   o  4:     RMD-QOSM scheme MUST be "per-aggregate RMD reservation-
             based" in combination with the "severe congestion handling
             by the RMD-QOSM refresh" procedure;

   o  5:     RMD-QOSM scheme MUST be "per-aggregate RMD reservation-
             based" in combination with the "severe congestion handling
             by proportional data packet marking" procedure;

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   o  6 - 7: reserved.

   The default value of the <SCH> field MUST be set to the value equal
   to 3.



(page 23 continued on part 2)

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