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RFC 6601 - Generic Connection Admission Control (GCAC) Algorithm


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Internet Engineering Task Force (IETF)                       G. Ash, Ed.
Request for Comments: 6601                                          AT&T
Category: Experimental                                        D. McDysan
ISSN: 2070-1721                                                  Verizon
                                                              April 2012

  Generic Connection Admission Control (GCAC) Algorithm Specification
                          for IP/MPLS Networks

Abstract

   This document presents a generic connection admission control (GCAC)
   reference model and algorithm for IP-/MPLS-based networks.  Service
   provider (SP) IP/MPLS networks need an MPLS GCAC mechanism, as one
   motivational example, to reject Voice over IP (VoIP) calls when
   additional calls would adversely affect calls already in progress.
   Without MPLS GCAC, connections on congested links will suffer
   degraded quality.  The MPLS GCAC algorithm can be optionally
   implemented in vendor equipment and deployed by service providers.
   MPLS GCAC interoperates between vendor equipment and across multiple
   service provider domains.  The MPLS GCAC algorithm uses available
   standard mechanisms for MPLS-based networks, such as RSVP, Diffserv-
   aware MPLS Traffic Engineering (DS-TE), Path Computation Element
   (PCE), Next Steps in Signaling (NSIS), Diffserv, and OSPF.  The MPLS
   GCAC algorithm does not include aspects of CAC that might be
   considered vendor proprietary implementations, such as detailed path
   selection mechanisms.  MPLS GCAC functions are implemented in a
   distributed manner to deliver the objective Quality of Service (QoS)
   for specified QoS constraints.  The objective is that the source is
   able to compute a source route with high likelihood that via-elements
   along the selected path will in fact admit the request.  In some
   cases (e.g., multiple Autonomous Systems (ASes)), this objective
   cannot always be met, but this document summarizes methods that
   partially meet this objective.  MPLS GCAC is applicable to any
   service or flow that must meet an objective QoS (delay, jitter,
   packet loss rate) for a specified quantity of traffic.

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/rfc6601.

Copyright Notice

   Copyright (c) 2012 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
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   publication of this document.  Please review these documents
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   Contributions published or made publicly available before November
   10, 2008.  The person(s) controlling the copyright in some of this
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   Without obtaining an adequate license from the person(s) controlling
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   it for publication as an RFC or to translate it into languages other
   than English.

Table of Contents

   1. Introduction ....................................................4
      1.1. Conventions Used in This Document ..........................5
   2. MPLS GCAC Reference Model and Algorithm Summary .................6
      2.1. Inputs to MPLS GCAC ........................................8
      2.2. MPLS GCAC Algorithm Summary ................................9
   3. MPLS GCAC Algorithm ............................................12
      3.1. Bandwidth Allocation Parameters ...........................12
      3.2. GCAC Algorithm ............................................15
   4. Security Considerations ........................................18
   5. Acknowledgements ...............................................20
   6. Normative References ...........................................20
   7. Informative References .........................................21
   Appendix A: Example MPLS GCAC Implementation Including Path
               Selection, Bandwidth Management, QoS Signaling, and
               Queuing ...............................................24
      A.1 Example of Path Selection and Bandwidth Management
          Implementation .............................................26
      A.2 Example of QoS Signaling Implementation ....................32
      A.3 Example of Queuing Implementation ..........................34

1.  Introduction

   This document presents a generic connection admission control (GCAC)
   reference model and algorithm for IP-/MPLS-based networks.  Service
   provider (SP) IP/MPLS networks need an MPLS GCAC mechanism, as one
   motivational example, to reject Voice over IP (VoIP) calls when
   additional calls would adversely affect calls already in progress.
   Without MPLS GCAC, connections on congested links will suffer
   degraded quality.  Given the capital constraints in some SP networks,
   over-provisioning is not acceptable.  MPLS GCAC supports all access
   technologies, protocols, and services while meeting performance
   objectives with a cost-effective solution and operates across routing
   areas, autonomous systems, and service provider boundaries.

   This document defines an MPLS GCAC reference model, algorithm, and
   functions implemented in one or more types of network elements in
   different domains that operate together in a distributed manner to
   deliver the objective QoS for specified QoS constraints, such as
   bandwidth.  With MPLS GCAC, the source router/server is able to
   compute a source route with high likelihood that via-elements along
   the selected path will in fact admit the request.  MPLS GCAC includes
   nested CAC actions, such as RSVP aggregation, nested RSVP - Traffic
   Engineering (RSVP-TE) for scaling between provider edge (PE) routers,
   and pseudowire (PW) CAC within traffic-engineered tunnels.  MPLS GCAC
   focuses on MPLS and PW-level CAC functions, rather than application-
   level CAC functions.

   MPLS GCAC is applicable to any service or flow that must meet an
   objective QoS (latency, delay variation, loss) for a specified
   quantity of traffic.  This would include, for example, most real-
   time/RTP services (voice, video, etc.) as well as some non-real-time
   services.  Real-time/RTP services are typically interactive,
   relatively persistent traffic flows.  Other services subject to MPLS
   GCAC could include, for example, manually provisioned label switched
   paths (LSPs) or PWs and automatic bandwidth assignment for
   applications that automatically build LSP meshes among PE routers.
   MPLS GCAC is applicable to both access and backbone networks, for
   example, to slow-speed access networks and to broadband DSL, cable,
   and fiber access networks.

   This document is Experimental.  It is intended that service providers
   and vendors experiment with the GCAC concept and the algorithm
   described in this document in a controlled manner to determine the
   benefits of such a mechanism.  That is, they should first experiment
   with the GCAC algorithm in their laboratories and test networks.
   When testing in live networks, they should install the GCAC algorithm
   on selected routers in only part of their network, and they should

   carefully monitor the effects.  The installation should be managed
   such that the routers can quickly be switched back to normal
   operation if any problem is seen.

   Since application of GCAC is most likely in Enterprise VPNs and/or
   internal TE infrastructure, it is RECOMMENDED that the experiment be
   conducted in such applications, and it is NOT RECOMMENDED that the
   experiment be conducted in the Internet.  If possible, the
   experimental configuration will address interoperability issues, such
   as, for example, the use of different constraint models across
   different traffic domains.

   The experiment can monitor various measures of quality of service
   before and after deployment of GCAC, particularly when the
   experimental network is under stress during an overload or failure
   condition.  These quality-of-service measures might include, for
   example, dropped packet rate and end-to-end packet delay.  The
   results of such experiments may be fed back to the IETF community to
   refine this document and to move it to the Standards Track (probably
   within the MPLS working group) if the experimental results are
   positive.

   It should be noted that the algorithm might have negative effects on
   live deployments if the experiment is a failure.  Effects might
   include blockage of traffic that would normally be handled or
   congestion caused by allowing excessive traffic on a link.  For these
   reasons, experimentation in production networks needs to be treated
   with caution as described above and should only be carried out after
   successful simulation and experimentation in test environments.  In
   Section 2, we describe the MPLS GCAC reference model, and in Section
   3, we specify the MPLS GCAC algorithm based on the principles in the
   reference model and requirements.  Appendix A gives an example of
   MPLS GCAC implementation including path selection, bandwidth
   management, QoS signaling, and queuing implementation.

1.1.  Conventions Used in This Document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119].

2.  MPLS GCAC Reference Model and Algorithm Summary

   Figure 1 illustrates the reference model for the MPLS GCAC algorithm:

                                   ,-.        ,-.
                               ,--+   `--+--'-   --'\
          +----+_____+------+  {   +----+   +----+   `. +------+
          |GEF1|     |      |______| P  |___| P  |______|      |
          |    |-----| PE1  |  {   +----+   +----+    /+| PE2  |
          |    |     |      |==========================>| ASBR |
          +-:--+     |      |<==========================|      |
           _|..__    +------+  {  DS-TE/MAR Tunnels  ;  +------+
         _,'    \-|          ./                    -'._    !|
         | Access  \         /        +----+           \,  !|
         | Network   |       \_       | P  |             | !|
         |          /          `|     +----+            /  !|
         `--.  ,.__,|           |    IP/MPLS Network   /   !|
            '`'  ''             ' .._,,'`.__   _/ '---'    !|
             |                             '`'''           !|
             C1                                            !|
                                    ,-.        ,-.         !|
                               ,--+   `--+--'-   --'\      !|
          +----+_____+------+  {   +----+   +----+   `. +------+
          |GEF2|     |      |______| P  |___| P  |______|      |
          |    |-----| PE4  |  {   +----+   +----+    /+| PE3  |
          |    |     |      |==========================>| ASBR |
          +-:--+     |      |<==========================|      |
           _|..__    +------+  {  DS-TE/MAM Tunnels  ;  +------+
         _,'    \-|          ./                    -'._
         | Access  \         /        +----+           \,
         | Network   |       \_       | P  |             |
         |          /          `|     +----+            /
         `--.  ,.__,|           |    IP/MPLS Network   /
            '`'  ''             ' .._,,'`.__   _/ '---'
             |                             '`'''
             C2

      CUSTOMER I/F PARAMETERS: BW, QoS, CoS, priority

      NOTE: PE, P, ASBR, GEF elements all support GCFs

      LEGEND:
      ---------
      ASBR:  Autonomous System Border Router
      BW:    bandwidth
      CoS:   class of service
      DS-TE: Diffserv-aware MPLS Traffic Engineering
      GCAC:  generic connection admission control
      GCF:   GCAC core function
      GEF:   GCAC edge function
      I/F:   interface
      MAM:   Maximum Allocation Model
      MAR:   Maximum Allocation with Reservation
      P:     provider router
      PE:    provider edge router
      ---    connection signaling
      ___    bearer/media flows

                    Figure 1:  MPLS GCAC Reference Model

   MPLS GCAC is applicable to any service or flow for which MPLS GCAC is
   required to meet a given QoS.  As such, the reference model applies
   to most real-time/RTP services (voice, video, etc.) as well as some
   non-real-time services.  Real-time/RTP services are typically
   interactive, relatively persistent traffic flows.  Non-real-time
   applications subject to MPLS GCAC could include, for example,
   manually provisioned LSPs or PWs and automatic bandwidth assignment
   for applications that automatically build LSP meshes among PE
   routers.  The reference model also applies to MPLS GCAC when MPLS is
   used in access networks, which include, for example, slow-speed
   access networks and broadband DSL, cable, and fiber access networks.
   Endpoints will be IP/PBXs (Private Branch Exchanges) and individual-
   usage SIP/RTP end devices (hard and soft SIP phones, Integrated
   Access Devices (IADs)).  This traffic will enter and leave the core
   via possibly bandwidth-constrained access networks, which may also be
   MPLS aware but may use some other admission control technology.

   The basic elements considered in the reference model are the MPLS
   GCAC edge function (GEF), GCAC core functions (GCFs), the PE routers,
   Autonomous System Border Routers (ASBRs), and provider (P) routers.
   As illustrated in Figure 1, the GEF interfaces to the application at
   the source and destination PE, and the GCF exists at the PE, P, and
   ASBR routers.  GEF has an end-to-end focus and deals with whether
   individual connection requests fit within an MPLS tunnel, and GCF has
   a hop-by-hop focus and deals with whether an MPLS tunnel can be
   established across specific core network elements on a path.  The GEF
   functionality may be implemented in the PE, ASBR, or a stand-alone
   network element.  The source/destination routers (or external devices

   through a router interface) support both GEF and GCF, while internal
   routers (or external devices through a router interface) support GCF.
   In Figure 1, the GEF handles both signaling and bearer control.

2.1.  Inputs to MPLS GCAC

   Inputs to the GEF and GCF include the following, where most are
   inputs to both GEF and GCF, except as noted.  Most of the parameters
   apply to the specific flow/LSP being calculated, while some
   parameters, such as request type, apply to the calculation method.
   Required inputs are marked with (*); all other inputs are optional:

   Traffic Description
      * Bandwidth per DS-TE class type [RFC4124] (GEF, GCF)
      * Bandwidth for LSP from [RFC3270] (GEF, GCF)
      * Aggregated RSVP bandwidth requirements from [RFC4804] (GEF)
      Variance Factor (GEF, GCF)

   Class of Service (CoS) and Quality of Service (QoS)
      * Class Type (CT) from [RFC4124] (GEF, GCF)
      Signaled or configured Traffic Class (TC) [RFC5462] to Per Hop
         Behavior (PHB) mapping from [RFC3270] (GEF, GCF)
      Signaled or configured PHB from [RFC3270] (GEF, GCF)
      QoS requirements from NSIS/Y.1541 [RFC5971][RFC5974][RFC5975]
         [RFC5976] (GEF)

   Priority
      Admission priority (high, normal, best effort) from NSIS/Y.1541
         [RFC5971][RFC5974][RFC5975][RFC5976] (GEF, GCF)
      Preemption priority from [RFC4124] (GEF, GCF)

   Request type
      Primary tunnel (GEF, GCF)
      Backup tunnel and fraction of capacity reserved for backup (GEF,
         GCF)

   Oversubscription method (see [RFC3270])
      Over/undersubscribe requested capacity (GEF, GCF)
      Over/undersubscribe available bandwidth (GEF, GCF)

   These inputs can be received by the GEF and GCF from a signaling
   interface (such as SIP or H.323), RSVP, or an NMS.  They can also be
   derived from measured traffic levels or from elsewhere.

2.2.  MPLS GCAC Algorithm Summary

   Figure 1 is a reference model for MPLS GCAC and illustrates the GEF
   to GEF MPLS GCAC algorithm to determine whether there is sufficient
   bandwidth to complete a connection.  The originating GEF receives a
   connection request including the above input parameters over the
   input interface, for example, via an RSVP bandwidth request as
   specified in [RFC4804].  The GEF a) determines whether there is
   enough bandwidth on the route between the originating and terminating
   GEFs via routing and signaling communication with the GCFs at the P,
   PE, and ASBR network elements along the path to accommodate the
   connection, b) communicates the accept/reject decision on the input
   interface for the connection request, and c) keeps account of network
   resource allocations by tracking bandwidth use and allocations per
   CoS.  Optionally, the GEF may dynamically adjust the tunnel size by
   signaling communication with the GCFs at nodes along the candidate
   paths.  For example, the GEF could a) maintain per-CoS tunnel
   capacity based on aggregated connection requests and respond on a
   connection-by-connection basis based on the available capacity, b)
   periodically adjust the tunnel capacity upward, when needed, and
   downward when spare capacity exists in the tunnel, and c) use a 'make
   before break' mechanism to adjust tunnel capacity in order to
   minimize disruption to the bearer traffic.

   In the reference model, DS-TE [RFC4124] tunnels are configured
   between the GEFs based on the traffic forecast and current network
   utilization.  These guaranteed bandwidth DS-TE tunnels are created
   using RSVP-TE [RFC3209].  DS-TE bandwidth constraints models are
   applied uniformly within each domain, such as the Maximum Allocation
   with Reservation (MAR) Bandwidth Constraints Model [RFC4126], the
   Maximum Allocation Model (MAM) [RFC4125], and the Russian Dolls Model
   (RDM) [RFC4127].  An IGP such as OSPF or IS-IS is used to advertise
   bandwidth availability by CT for use by the GCF to determine MPLS
   tunnel bandwidth allocation and admission on core (backbone) links.
   These DS-TE tunnels are configured based on the forecasted traffic
   load, and when needed, LSPs for different CTs can take different
   paths.

   As described in Section 3, the unreserved link bandwidth on CTc on
   link k (ULBCck) is the only bandwidth allocation parameter that must
   be available to the MPLS GCAC algorithm.  In the case that a
   connection is set up across multiple service provider networks, i.e.,
   across multiple routing domains/autonomous systems (ASes), there are
   several options to enable MPLS GCAC to be implemented:

   1.  Use [OIF-E-NNI] to advertise ULBCck parameters to the originating
       GEF, for the full topology of adjacent domains/areas/ASes, as
       described in Section 3.3.2.1.2 of [OIF-E-NNI].  Note that the

       option of abstract node summarization described in [OIF-E-NNI]
       will not suffice since the process of summarization results in
       loss of topology and capacity usage information.  In this manner,
       the originating GEF can implement the MPLS GCAC algorithm
       described in Section 3 across multiple domains/areas/ASes.

   2.  Use [BGP-TE] to advertise ULBCck parameters via BGP to the
       originating GEF for the full topology of adjacent
       domains/areas/ASes.  In this manner, the originating GEF can
       implement the MPLS GCAC algorithm described in Section 3 across
       multiple domains/areas/ASes.  However, network providers may be
       reluctant to divulge full topology and capacity usage information
       to other providers.  Furthermore, [BGP-TE] was never intended to
       provide full TE topology distribution across ASBRs.  Such a
       mechanism would be neither stable nor scalable.

   3.  Use individual AS control and MPLS crankback [RFC4920] to retain
       originating GEF control.  For example, in Figure 1, if a
       connection crosses the two ASes shown (call them AS1 and AS2),
       the source GEF1 applies the GCAC algorithm described in Section 3
       to the links in AS1, that is, between PE1 and PE2/ASBR in Figure
       1.  Then, in AS2, the GCF in PE3/ASBR applies the MPLS GCAC
       algorithm to the links in AS2, that is, between PE3 and PE4 in
       Figure 1.  If the flow is rejected in AS2, crankback signaling is
       used to inform GEF1.  In routing a connection across multiple
       ASes, e.g., across AS1-->AS2-->AS3, if the flow is rejected, say
       in AS2, the originating GEF1 can seek an alternate route, perhaps
       AS1-->AS4-->AS3.  This option does not achieve full originating
       GEF control with the desired full topology visibility across ASes
       but avoids possible issues with obtaining full topology
       visibility across ASes.

   4.  Use Path Computation Elements (PCEs) [RFC4655] across multiple
       ASes.  PCEs could potentially execute the GCAC algorithm within
       each AS and communicate/interwork across domains to determine
       which high-level path can supply the requested bandwidth.

   In the reference model, the GEFs implement RSVP aggregation [RFC4804]
   for scalability.  The GEF RSVP aggregator keeps a running total of
   bandwidth usage on the DS-TE tunnel, adding the bandwidth
   requirements during connection setup and subtracting during
   connection teardown.  The aggregator determines whether or not there
   is sufficient bandwidth for the connection from that originating GEF
   to the destination GEF.  The destination GEF also checks whether
   there is enough bandwidth on the DS-TE tunnel from the destination
   GEF to the originating GEF.  The aggregate bandwidth usage on the DS-
   TE tunnel is also available to the DS-TE bandwidth constraints model.
   If the available bandwidth is insufficient, then the GEF sends a PATH

   message through the tunnel to the other end, requesting bandwidth
   using GCFs, and if successful, the source would then complete a new
   explicit route with a PATH message along the path with increased
   bandwidth, again invoking GCFs on the path.  If the size of the DS-TE
   tunnel cannot be increased on the primary and alternate LSPs, then
   when the DS-TE tunnel bandwidth is exhausted, the GEF aggregator
   sends a message to the endpoint denying the reservation.  If the DS-
   TE tunnels are underutilized, the tunnel bandwidth may be reduced
   periodically to an appropriate level.  In the case of a basic single
   class TE scenario, there is a single TE tunnel rather than multiple-
   CT DS-TE tunnels; otherwise, the above GCAC functions remain the
   same.

   Optionally, the reference model implements separate queues with
   Diffserv based on Traffic Class (TC) bits [RFC5462].  For example,
   these queues may include two Expedited Forwarding (EF) priority
   queues, with the highest priority assigned to Emergency
   Telecommunications Service (ETS) traffic and the second priority
   assigned to normal-priority real-time traffic (alternatively, there
   could be a single EF queue with dual policers [RFC5865]).  Several
   Assured Forwarding (AF) queues may be used for various data traffic,
   for example, premium private data traffic and premium public data
   traffic.  A separate best-effort queue may be used for the best-
   effort traffic.  Several DS-TE tunnels may share the same physical
   link and therefore share the same queue.

   The MPLS GCAC algorithm increases the likelihood that the route
   selected by the GEF will succeed, even when the LSP traverses
   multiple service provider networks.

   Path computation is not part of the GCAC algorithm; rather, it is
   considered as a vendor proprietary function, although standard
   IP/MPLS functions may be included in path computation, such as the
   following:

   a)  Path Computation Element (PCE) [RFC4655][RFC4657][RFC5440] to
       implement inter-area/inter-AS/inter-SP path selection algorithms,
       including alternate path selection, path reoptimization, backup
       path computation to protect DS-TE tunnels, and inter-area/inter-
       AS/inter-SP traffic engineering.

   b)  Backward-Recursive PCE-Based Computation (BRPC) [RFC5441].

   c)  Per-Domain Path Computation [RFC5152].

   d)  MPLS fast reroute [RFC4090] to protect DS-TE LSPs against
       failure.

   e)  MPLS crankback [RFC4920] to trigger alternate path selection and
       enable explicit source routing.

3.  MPLS GCAC Algorithm

   MPLS GCAC is performed at the GEF during the connection setup attempt
   phase to determine if a connection request can be accepted without
   violating existing connections' QoS and throughput requirements.  To
   enable routing to produce paths that will likely be accepted, it is
   necessary for nodes to advertise some information about their
   internal CAC states.  Such advertisements should not require nodes to
   expose detailed and up-to-date CAC information, which may result in
   an unacceptably high rate of routing updates.  MPLS GCAC advertises
   CAC information that is generic (e.g., independent of the actual path
   selection algorithms used) and rich enough to support any CAC.

   MPLS GCAC defines a set of parameters to be advertised and a common
   admission interpretation of these parameters.  This common
   interpretation is in the form of an MPLS GCAC algorithm to be
   performed during MPLS LSP path selection to determine if a link or
   node can be included for consideration.  The algorithm uses the
   advertised MPLS GCAC parameters (available from the topology
   database) and the characteristics of the connection being requested
   (available from QoS signaling) to determine if a link/node will
   likely accept or reject the connection.  A link/node is included if
   the MPLS GCAC algorithm determines that it will likely accept the
   connection and excluded otherwise.

3.1.  Bandwidth Allocation Parameters

   MPLS GCAC bandwidth allocation parameters for each DS-TE CT are as
   defined within DS-TE [RFC4126], OSPF-TE extensions [RFC4203], and IS-
   IS-TE extensions [RFC5307].  The following parameters are available
   from DS-TE/TE extensions, advertised by the IGP, and available to the
   GEF and GCF [RFC4124].  Note that the approach presented in this
   section is adapted from [PNNI], Appendix B.

   MRBk    Maximum reservable bandwidth on link k specifies the maximum
           bandwidth that may be reserved; this may be greater than the
           maximum link bandwidth, in which case the link may be
           oversubscribed.

   BWCck   Bandwidth constraint for CTc on link k = allocated (minimum
           guaranteed) bandwidth for CTc on link k.

   ULBCck  Unreserved link bandwidth on CTc on link k specifies the
           amount of bandwidth not yet reserved for CTc.

   Note that BWCck and ULBCck are the only DS-TE parameters flooded by
   the IGP [RFC4124][RFC4203][RFC5307].  For example, when bandwidth
   reservation is used [RFC4126], ULBCck is calculated and flooded by
   the IGP as follows:

   RBTk    Reservation bandwidth threshold for link k.

   ULBCck  Unreserved link bandwidth on CTc on link k specifies the
           amount of bandwidth not yet reserved for CTc, taking RBTk
           into account,

           ULBCck = ULBk - delta0/1(CTck) * RBTk
           where
           delta0/1(CTck) = 0 if RBWck < BWCck
           delta0/1(CTck) = 1 if RBWck >= BWCck

   Also derivable at the GEF and GCF is MRBCck, the maximum reservable
   link bandwidth for CTc.  For example, when bandwidth reservation is
   used [RFC4126], MRBCck is calculated as follows:

   MRBCck  Maximum reservable link bandwidth for CTc on link k specifies
           the amount of bandwidth not yet reserved for CTc.

           MRBCck = MRBk - delta0/1(CTck) * RBTk
           where
           delta0/1(CTck) = 0 if RBWck < BWCck
           delta0/1(CTck) = 1 if RBWck >= BWCck

   Note that these bandwidth parameters must be configured in a
   consistent way within domains and across domains.  GEF routing of
   LSPs is based on ULBCck, where ULBk is available and RBTk can be
   accounted for by configuration, e.g., RBTk typically = .05 * MRBk.

   Also available are administrative weight (denoted as "link cost" in
   [RFC2328]), TE metric [RFC3630], and administrative group (also
   called color) 4-octet mask [RFC3630].

   The following quantities can be derived from information advertised
   by the IGP and otherwise available to the GEF and GCF:

   RBWck   Reserved bandwidth on CTc on link k (0 <= c <= MaxCT-1).

           RBWck = total amount of bandwidth reserved by all established
           LSPs that belong to CTc
           RBWck = BWCck - ULBCck.

   ULBk    Unreserved link bandwidth on link k specifies the amount of
           bandwidth not yet reserved for any CT.

           ULBk = MRBk - sum [RBWck (0 <= c <= MaxCT-1)].

   The GCAC algorithm assumes that a DS-TE bandwidth constraints model
   is used uniformly within each domain (e.g., MAR [RFC4126], MAM
   [RFC4125], or RDM [RFC4127]).  European Advanced Networking Test
   Center (EANTC) testing [EANTC] has shown that interoperability is
   problematic when different DS-TE bandwidth constraints models are
   used by different network elements within a domain.  Specific testing
   of MAM and RDM across different vendor equipment showed the
   incompatibility.  However, while the characteristics of the 3 DS-TE
   bandwidth constraints models are quite different, it is necessary to
   specify interworking between them even though it could be complex.

   The following parameters are also defined and available to GCF and
   are assumed to be locally configured to be a consistent value across
   all nodes and domain(s):

   SBWck   Sustained bandwidth for CTc on link k (aggregate of existing
           connections).

           SBWck = factor * RBWck where factor is configured based on
           standard 'demand overbooking' factors.

   VFck    Variance factor for CTc on link k; VFck is BWMck normalized
           by variance of SBWck.  VFck is configured based on typical
           traffic variability statistics.

   In many implementations of the Private Network-Network Interface
   (PNNI) GCAC algorithm, the variance factor is not included, or
   equivalently, VFck is assumed to be zero.  A simplified MPLS GCAC
   algorithm is also derived assuming VFck = 0.

   Note that different demand overbooking factors can be specified for
   each CT, e.g., no overbooking might be used for constant bitrate
   services, while a large overbooking factor might be used for bursty
   variable bitrate services.  We specify demand overbooking rather than
   link overbooking for the GCAC algorithm; one advantage is the demand
   overbooking is compatible with source routing used by the GCAC
   algorithm.

   Also defined is

   BWMck   bandwidth margin for CTc on link k; BWMck = RBWck - SBWck

   GEF uses BWCck, RBWck, ULBCck, SBWck, BWMck, and VFck for LSP/IGP
   routing.  GEF also needs to track per-CT LSP bandwidth allocation and
   reserved bandwidth parameters, which are defined as follows:

   RBWLcl  reserved bandwidth for CTc on LSP l

   UBWLcl  unreserved bandwidth for CTc on LSP l

3.2.  GCAC Algorithm

   The assumption behind the MPLS GCAC is that the ratio between the
   bandwidth margin that the node is putting above the sustained
   bandwidth and the standard deviation of the sustained bandwidth does
   not change significantly as one new aggregate flow is added on the
   link.  Any ingress node doing path selection can then compute the new
   standard deviation of the aggregate rate (from the old value and the
   aggregate flow's traffic descriptors) and an estimate of the new
   BWMck.  From this, the increase in bandwidth required to carry the
   new aggregate flow can be computed and compared to BWCck.

   To expand on the discussion above, let RBWck denote the reserved
   bandwidth capacity, i.e., the amount of bandwidth that has been
   allocated to existing aggregate flows for CTc on link k by the actual
   CAC used in the node.  BWMck is the difference between RBWck and the
   aggregate sustained bandwidth (SBWck) of the existing aggregate
   flows.  SBWck can be either the sum of existing aggregate flows'
   declared sustainable bandwidth (SBWi for aggregate flow i) or a
   smaller (possibly measured or estimated) value.  Let MRBCck denote
   the maximum reservable bandwidth that is usable by aggregate flows
   for CTc on link k.  The following diagram illustrates the
   relationship among MRBCck, RBWck, BWMck, SBWck, and ULBCck:

                     |<-- BWMck-->|<----- ULBCck ----->|
     |---------------|------------|--------------------|
     0              SBWck        RBWck               MRBCck

   The assumption is that BWMck is proportional to some measure of the
   burstiness of the traffic generated by the existing aggregate flows,
   this measure being the standard deviation of the aggregate traffic
   rate defined as the square root of the sum of SBWi(PBWi - SBWi) over
   all existing aggregate flows, where SBWi and PBWi are declared
   sustainable and peak bandwidth for aggregate flow i, respectively.
   This assumption is based on the simple argument that RBWck needs to
   be some multiple of the standard deviation above the mean aggregate

   traffic rate to guarantee some level of packet loss ratio and packet
   queuing time.  Depending on the actual CAC used, the BWMck-to-
   standard-deviation ratio may vary as aggregate flows are established
   and taken down.  It is reasonable to assume, however, that with a
   sufficiently large value of RBWck, this ratio will not vary
   significantly.  What this means is a link can advertise its current
   BWMck-to-standard-deviation ratio (actually in the form of VF, which
   is the square of this number), and the MPLS GCAC algorithm can use
   this number to estimate how much bandwidth is required to carry an
   additional aggregate flow.

   Following the derivation given in [PNNI], Appendix B, the MPLS GCAC
   algorithm is derived as follows.  Consider an aggregate flow
   bandwidth change request DBWi with peak bandwidth PBWi and
   sustainable bandwidth SBWi and a link with the following MPLS GCAC
   parameters:  ULBCck, BWMck, and VFck for CTc and link k.  Denote the
   variance (i.e., square of standard deviation) of the aggregate
   traffic rate by VARk (not advertised).  Denote other unadvertised
   MPLS GCAC quantities by RBWck and SBWck.  Then,

   VARk = SUM  SBWi*(PBWi-SBWi)                                      (1)
          over existing
          aggregate flows i

   and

           BWMck**2
   VFck = ----------                                                 (2)
            VARk

   Using the above equation, VARk can be computed from the advertised
   VFck and BWMck as:

   VARk = (BWMck**2)/VFck.

   Let DBWi be the additional bandwidth capacity needed to carry the
   flow within aggregate sustainable bandwidth SBWi.  The MPLS GCAC
   algorithm basically computes DBWi from the advertised MPLS GCAC
   parameters and the new aggregate flow's traffic descriptors, and
   compares it with ULBCck.  If ULBCck >= DBWi, then the link is
   included for path selection consideration; otherwise, it is excluded,
   i.e.,

   If (ULBCck >= DBWi), then include link k; else exclude link k     (3)

   Let BWMcknew denote the bandwidth margin if the new aggregate flow
   were accepted.  Denote other 'new' quantities by RBWcknew, SBWcknew,
   and VARnew.  Then,

   DBWi = BWMcknew - BWMck + SBWi                                    (4)

   since BWMcknew = RBWcknew - SBWcknew, BWMck = RBWck - SBWck, and
   SBWcknew - SBWck = SBWi.  Substituting (4) into (3), rearranging
   terms, and squaring both sides yield:

   If ((ULBCck+BWMck-SBWck)**2 >= BWMcknew**2), then include link k;
                                                else exclude link k  (5)

   Using the MPLS GCAC assumption made earlier, BWMcknew**2 can be
   computed as:

   BWMcknew**2 = VFck * VARnew,                                      (6)

   Where

   VARnew = VARk + SBWck * (PBWi-SBWi).                              (7)

   Substituting (2), (6) and (7) into (5) yields:

   If ((ULBCck+BWMck-SBWi)**2 >= BWMck**2 + VFck*SBWi(PBWi-SBWi)),
                                              then include link k;
                                              else exclude link k    (8)

   and moving BWMck**2 to the left-hand side and rearranging terms yield

   If ((ULBCck-SBWi) * (ULBCck-SBWi+2*BWMck) >= VFck*SBWi(PBWi-SBWi),
                                              then include link k;
                                              else exclude link k    (9)

   Equation (9) represents the Constrained Shortest Path First (CSPF)
   method implemented by most vendors and deployed by most service
   providers in MPLS networks.  In general, DBWi is between SBWi and
   PBWi.  So, the above test is not necessary for the cases ULBCck >=
   PBWi and ULBCck < SBWi.  In the former case, the link is included; in
   the latter case, the link is excluded.

          Exclude                         Include
     |<--- link ---->|<-- Test (9)-->|<--- link ----->|
     |---------------|---------------|----------------| ULBCck
                    SBWi            PBWi

   Note that VF and BWM are frequently not implemented; equivalently,
   these quantities are assumed to be zero, in which case Equation (9)
   becomes

   If (ULBCck >= SBWi), then include link k; else exclude link k    (10)

             Exclude         Include
        |<--- link ---->|<--- link ----->|
        |---------------|----------------| ULBCck
                       SBWi            PBWi

   PNNI GCAC implementations often do not incorporate the variance
   factor VF, in which case Equation (10) is used.

   MPLS GCAC must not reject a best-effort (BE, unassigned bandwidth)
   aggregate flow request based on bandwidth availability, but it may
   reject based on other reasons such as the number of BE flows
   exceeding a chosen threshold.  MPLS GCAC defines only one parameter
   for the BE service category -- maximum bandwidth (MBW) -- to
   advertise how much capacity is usable for BE flows.  The purpose of
   advertising this parameter is twofold:  MBW can be used for path
   optimization, and MBW = 0 is used to indicate that a link is not
   accepting any (additional) BE flows.

   Demand overbooking of LSP bandwidth is employed and must be compliant
   with [RFC4124] and [RFC3270] to over-/undersubscribe requested
   capacity.  It is simplest to use only one oversubscription method,
   i.e., the GCAC algorithm assumes oversubscription of demands per CT,
   both within domains and for interworking between domains.  The
   motivation is that interworking may be infeasible between domains if
   different overbooking models are used.  Note that the same assumption
   was made for DS-TE bandwidth constraints models, in that the GCAC
   algorithm assumes a consistent DS-TE bandwidth constraints model is
   used within each domain and interoperability of bandwidth constraints
   models across domains.

4.  Security Considerations

   It needs to be clearly understood that all routers contain local and
   implementation-specific rules (or algorithms) to help them determine
   what to do with traffic that exceeds capacity and how to admit new
   flows.  If these rules are poorly designed or implemented with
   defects, then problems may be observed in the network.  Furthermore,
   the implementation of such algorithms provides a mechanism for
   attacking the delivery of traffic within the network.  In view of
   this, routers and their software are usually extensively tested
   before deployment, router vendors are extended a degree of trust, and
   a "compromised router" (i.e., one on which an attacker has installed

   their own code) is considered a weak spot in the system.  Note that
   if a router is compromised, it can be made to do substantially more
   problematic things than simply modifying the admission control
   algorithm.  Implementers are RECOMMENDED to ensure that software
   modifications to routers are fully secured, and operators are
   RECOMMENDED to apply security measures (that are outside the scope of
   this document) to prevent unauthorized updates to router software.
   Nothing in this document suggests any change to normal software
   security practices.

   The use of a GCAC priority parameter raises possibilities for theft-
   of-service attacks because users could claim an emergency priority
   for their flows without real need, thereby effectively preventing
   serious emergency calls to get through.  Several options exist for
   countering such user attacks at the interface to the user, for
   example:

   -  Only some user groups (e.g., police) are authorized to set the
      emergency priority bit using a policy applied to RSVP-TE
      signaling.

   -  Any user is authorized to employ the emergency priority bit for
      particular destination addresses (e.g., police) using a policy
      applied to RSVP-TE signaling.

   -  If an attack occurs, the user/group and actions taken should be
      logged to trace the attack.

   -  [RFC5069] identifies a number of security threats against
      emergency call marking and mapping.  Section 6 of [RFC5069] lists
      security requirements to counter these threats, and those
      requirements should be followed by implementations of this
      document.

   -  The security requirements listed in Section 11 of [RFC4412] should
      be followed.  These requirements apply to use of the
      Communications Resource Priority Header for the Session Initiation
      Protocol (SIP) and concern aspects of authentication and
      authorization, confidentiality and privacy requirements,
      protection against denial-of-service attacks, and anonymity.

   Within the network, the policy and integrity mechanisms already
   present in RSVP-TE [RFC3209] can be used to ensure that the MPLS LSP
   has the right policy and security credentials to assume the signaled
   priority and bandwidth.  Further discussion of this topic for the
   signaling of priority levels using RSVP can be found in [RFC6401].
   Some similarities may also be drawn to the security issues

   surrounding the placement of emergency calls in Internet multimedia
   systems [RFC5069] although the concepts are only comparable at the
   highest levels.

   Like any algorithm, the algorithm specified in this document operates
   on data that is supplied as input parameters.  That data is assumed
   to be collected and stored locally (i.e., on the router performing
   the algorithm).  It is a fundamental assumption of the secure
   operation of any router that the data stored on that router cannot be
   externally modified.  In this particular case, it is important that
   the input parameters to the algorithm cannot be influenced by an
   outside party.  Thus, as with all configuration parameters on a
   router, the implementer MUST supply and the operator is RECOMMENDED
   to use security mechanisms to protect writing of the configuration
   parameters for this algorithm.

5.  Acknowledgements

   The authors greatly appreciate Adrian Farrel's support in serving as
   the sponsoring Area Director for this document and for his valuable
   comments and suggestions on the document.  The authors also greatly
   appreciate Young Lee serving as the document shepherd and his
   valuable comments and suggestions.  Finally, Robert Sparks' thorough
   review and helpful suggestions are sincerely appreciated.

6.  Normative References

   [RFC2119]    Bradner, S., "Key words for use in RFCs to Indicate
                Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2328]    Moy, J., "OSPF Version 2", STD 54, RFC 2328, April 1998.

   [RFC3031]    Rosen, E., Viswanathan, A., and R. Callon,
                "Multiprotocol Label Switching Architecture", RFC 3031,
                January 2001.

   [RFC3209]    Awduche, D., Berger, L., Gan, D., Li, T., Srinivasan,
                V., and G. Swallow, "RSVP-TE: Extensions to RSVP for LSP
                Tunnels", RFC 3209, December 2001.

   [RFC3270]    Le Faucheur, F., Wu, L., Davie, B., Davari, S.,
                Vaananen, P., Krishnan, R., Cheval, P., and J. Heinanen,
                "Multi-Protocol Label Switching (MPLS) Support of
                Differentiated Services", RFC 3270, May 2002.

   [RFC3630]    Katz, D., Kompella, K., and D. Yeung, "Traffic
                Engineering (TE) Extensions to OSPF Version 2", RFC
                3630, September 2003.

   [RFC4124]    Le Faucheur, F., Ed., "Protocol Extensions for Support
                of Diffserv-aware MPLS Traffic Engineering", RFC 4124,
                June 2005.

   [RFC4203]    Kompella, K., Ed., and Y. Rekhter, Ed., "OSPF Extensions
                in Support of Generalized Multi-Protocol Label Switching
                (GMPLS)", RFC 4203, October 2005.

   [RFC4804]    Le Faucheur, F., Ed., "Aggregation of Resource
                ReSerVation Protocol (RSVP) Reservations over MPLS
                TE/DS-TE Tunnels", RFC 4804, February 2007.

   [RFC4920]    Farrel, A., Ed., Satyanarayana, A., Iwata, A., Fujita,
                N., and G. Ash, "Crankback Signaling Extensions for MPLS
                and GMPLS RSVP-TE", RFC 4920, July 2007.

   [RFC5307]    Kompella, K., Ed., and Y. Rekhter, Ed., "IS-IS
                Extensions in Support of Generalized Multi-Protocol
                Label Switching (GMPLS)", RFC 5307, October 2008.

7.  Informative References

   [BGP-TE]     Gredler, H., Farrel, A., Medved, J., and S. Previdi,
                "North-Bound Distribution of Link-State and TE
                Information using BGP", Work in Progress, March 2012.

   [EANTC]      "Multi-vendor Carrier Ethernet Interoperability Event",
                Carrier Ethernet World Congress 2006, Madrid Spain,
                September 2006.

   [FEEDBACK]   Ashwood-Smith, P., Jamoussi, B., Fedyk, D., and D.
                Skalecki, "Improving Topology Data Base Accuracy with
                Label Switched Path Feedback in Constraint Based Label
                Distribution Protocol", Work in Progress, June 2003.

   [OIF-E-NNI]  Optical Interworking Forum (OIF), "External Network-
                Network Interface (E-NNI) OSPFv2-based Routing - 2.0
                (Intra-Carrier) Implementation Agreement", IA # OIF-
                ENNI-OSPF-02.0, July 13, 2011.

   [PNNI]       ATM Forum Technical Committee, "Private Network-Network
                Interface Specification Version 1.1 (PNNI 1.1)",
                af-pnni-0055.002, April 2002.

   [RFC2597]    Heinanen, J., Baker, F., Weiss, W., and J. Wroclawski,
                "Assured Forwarding PHB Group", RFC 2597, June 1999.

   [RFC3246]    Davie, B., Charny, A., Bennet, J., Benson, K., Le
                Boudec, J., Courtney, W., Davari, S., Firoiu, V., and D.
                Stiliadis, "An Expedited Forwarding PHB (Per-Hop
                Behavior)", RFC 3246, March 2002.

   [RFC4090]    Pan, P., Ed., Swallow, G., Ed., and A. Atlas, Ed., "Fast
                Reroute Extensions to RSVP-TE for LSP Tunnels", RFC
                4090, May 2005.

   [RFC4125]    Le Faucheur, F. and W. Lai, "Maximum Allocation
                Bandwidth Constraints Model for Diffserv-aware MPLS
                Traffic Engineering", RFC 4125, June 2005.

   [RFC4126]    Ash, J., "Max Allocation with Reservation Bandwidth
                Constraints Model for Diffserv-aware MPLS Traffic
                Engineering & Performance Comparisons", RFC 4126, June
                2005.

   [RFC4127]    Le Faucheur, F., Ed., "Russian Dolls Bandwidth
                Constraints Model for Diffserv-aware MPLS Traffic
                Engineering", RFC 4127, June 2005.

   [RFC4412]    Schulzrinne, H. and J. Polk, "Communications Resource
                Priority for the Session Initiation Protocol (SIP)", RFC
                4412, February 2006.

   [RFC4655]    Farrel, A., Vasseur, JP., and J. Ash, "A Path
                Computation Element (PCE)-Based Architecture", RFC 4655,
                August 2006.

   [RFC4657]    Ash, J., Ed., and JL. Le Roux, Ed., "Path Computation
                Element (PCE) Communication Protocol Generic
                Requirements", RFC 4657, September 2006.

   [RFC5069]    Taylor, T., Ed., Tschofenig, H., Schulzrinne, H., and M.
                Shanmugam, "Security Threats and Requirements for
                Emergency Call Marking and Mapping", RFC 5069, January
                2008.

   [RFC5152]    Vasseur, JP., Ed., Ayyangar, A., Ed., and R. Zhang, "A
                Per-Domain Path Computation Method for Establishing
                Inter-Domain Traffic Engineering (TE) Label Switched
                Paths (LSPs)", RFC 5152, February 2008.

   [RFC5440]    Vasseur, JP., Ed., and JL. Le Roux, Ed., "Path
                Computation Element (PCE) Communication Protocol
                (PCEP)", RFC 5440, March 2009.

   [RFC5441]    Vasseur, JP., Ed., Zhang, R., Bitar, N., and JL. Le
                Roux, "A Backward-Recursive PCE-Based Computation (BRPC)
                Procedure to Compute Shortest Constrained Inter-Domain
                Traffic Engineering Label Switched Paths", RFC 5441,
                April 2009.

   [RFC5462]    Andersson, L. and R. Asati, "Multiprotocol Label
                Switching (MPLS) Label Stack Entry: "EXP" Field Renamed
                to "Traffic Class" Field", RFC 5462, February 2009.

   [RFC5865]    Baker, F., Polk, J., and M. Dolly, "A Differentiated
                Services Code Point (DSCP) for Capacity-Admitted
                Traffic", RFC 5865, May 2010.

   [RFC5971]    Schulzrinne, H. and R. Hancock, "GIST: General Internet
                Signalling Transport", RFC 5971, October 2010.

   [RFC5974]    Manner, J., Karagiannis, G., and A. McDonald, "NSIS
                Signaling Layer Protocol (NSLP) for Quality-of-Service
                Signaling", RFC 5974, October 2010.

   [RFC5975]    Ash, G., Ed., Bader, A., Ed., Kappler, C., Ed., and D.
                Oran, Ed., "QSPEC Template for the Quality-of-Service
                NSIS Signaling Layer Protocol (NSLP)", RFC 5975, October
                2010.

   [RFC5976]    Ash, G., Morton, A., Dolly, M., Tarapore, P., Dvorak,
                C., and Y. El Mghazli, "Y.1541-QOSM: Model for Networks
                Using Y.1541 Quality-of-Service Classes", RFC 5976,
                October 2010.

   [RFC6401]    Le Faucheur, F., Polk, J., and K. Carlberg, "RSVP
                Extensions for Admission Priority", RFC 6401, October
                2011.

   [TQO]        Ash, G., "Traffic Engineering and QoS Optimization of
                Integrated Voice and Data Networks", Elsevier, 2006.

Appendix A:  Example MPLS GCAC Implementation Including Path Selection,
             Bandwidth Management, QoS Signaling, and Queuing

   Figure 2 illustrates an example of the integrated voice/data MPLS
   GCAC method in which bandwidth is allocated on an aggregated basis to
   the individual DS-TE CTs.  In the example method, CTs have different
   priorities including high-priority, normal-priority, and best-effort-
   priority services CTs.  Bandwidth allocated to each CT is protected
   by bandwidth reservation methods, as needed, but bandwidth is
   otherwise shared among CTs.  Each originating GEF monitors CT
   bandwidth use on each MPLS LSP [RFC3031] for each CT, and determines
   when CT LSP bandwidth needs to be increased or decreased.  In Figure
   2, changes in CT bandwidth capacity are determined by GEFs based on
   an overall aggregated bandwidth demand for CT capacity (not on a per-
   connection/per-flow demand basis).  Based on the aggregated bandwidth
   demand, GEFs make periodic discrete changes in bandwidth allocation,
   that is, they either increase or decrease bandwidth on the LSPs
   constituting the CT bandwidth capacity.  For example, if aggregate
   flow requests are made for CT LSP bandwidth that exceeds the current
   DS-TE tunnel bandwidth allocation, the GEF initiates a bandwidth
   modification request on the appropriate LSP(s).  This may entail
   increasing the current LSP bandwidth allocation by a discrete
   increment of bandwidth denoted here as DBW, where DBW is the
   additional amount needed by the current aggregate flow request.  The
   bandwidth admission control for each link in the path is performed by
   the GCF based on the status of the link using the bandwidth
   allocation procedure described below, where we further describe the
   role of the different parameters (such as the reserved bandwidth
   threshold RBT shown in Figure 2) in the admission control procedure.
   Also, the GEF periodically monitors LSP bandwidth use, and if
   bandwidth use falls below the current LSP allocation, the GEF
   initiates a bandwidth modification request to decrease the LSP
   bandwidth allocation to the current level of bandwidth utilization.

          HIGH-PRIORITY-CT LSP
    +----+======================+----+======================+----+
    |GEF1|NORMAL-PRIORITY-CT LSP| VN |                      |GEF2|
    |    |======================|    |======================|    |
    |    |LOW-PRIORITY/BE-CT LSP|    |                      |    |
    +----+======================+----+======================+----+

   LEGEND
   ------
   BE -  Best Effort
   CT -  Class Type
   GEF - GCAC Edge Function
   LSP - Label Switched Path
   VN -  Via Node

   o  Distributed bandwidth allocation method applied on a
      per-class-type (CT) LSP basis

   o  GEF allocates bandwidth to each CTc LSP based on demand

      - GEF decides CTc LSP bandwidth increase based on

        + aggregate flow sustained bandwidth (SBWi) and variance factor
          VFck

        + routing priority (high, normal, best effort)

        + CTc reserved bandwidth (RBWck) and bandwidth constraint
          (BWCck)

        + link reserved bandwidth threshold (RBTk) and unreserved
          bandwidth (ULBk)

      - GEF periodically decreases CTc LSP bandwidth allocation based on
        bandwidth use

   o  VNs send crankback messages to GEF if DS-TE/MAR bandwidth
      allocation rules not met

   o  Link(s) not meeting request excluded from TE topology database
      before attempting another explicit route computation

           Figure 2:  Per-Class-Type (CT) LSP Bandwidth Management

   GEF uses SBWi, VFck, RBWck, BWCck, RBTk, and ULBk for LSP bandwidth
   allocation decisions and IGP routing and uses RBWcl and UBWcl to
   track per-CT LSP bandwidth allocation and reserved bandwidth.  In
   making a CT bandwidth allocation modification, the GEF determines the
   CT priority (high, normal, or best effort), CT bandwidth-in-use, and
   CT bandwidth allocation thresholds.  These parameters are used to
   determine whether network capacity can be allocated for the CT
   bandwidth modification request.

A.1.  Example of Path Selection and Bandwidth Management Implementation

   In OSPF, link-state flooding is used to make status updates.  This is
   a state-dependent routing (SDR) method where CSPF is typically used
   to alter LSP routing according to the state of the network.  In
   general, SDR methods calculate a path cost for each connection
   request based on various factors such as the load state or congestion
   state of the links in the network.  In contrast, the example MPLS
   GCAC algorithm uses event-dependent routing (EDR), where LSP routing
   is updated locally on the basis of whether connections succeed or
   fail on a given path choice.  In the EDR learning approaches, the
   path that was last tried successfully is tried again until congested,
   at which time another path is selected at random and tried on the
   next connection request.  EDR path choices can also be changed with
   time in accordance with changes in traffic load patterns.  Success-
   to-the-top (STT) EDR path selection, illustrated in Figure 3, uses a
   simplified decentralized learning method to achieve flexible adaptive
   routing.  The primary path (path-p) is used first if available, and a
   currently successful alternate path (path-s) is used until it is
   congested.  In the case that path-s is congested (e.g., bandwidth is
   not available on one or more links), a new alternate path (path-n) is
   selected at random as the alternate path choice for the next
   connection request overflow from the primary path.  Bandwidth
   reservation is used under congestion conditions to protect traffic on
   the primary path.  STT-EDR uses crankback when an alternate path is
   congested at a via node, and the connection request advances to a new
   random path choice.  In STT-EDR, many path choices can be tried by a
   given connection request before the request is rejected.

   Figure 3 illustrates the example MPLS GCAC operation of STT-EDR path
   selection and admission control combined with per-CT bandwidth
   allocation.  GEF A monitors CT bandwidth use on each CT LSP and
   determines when CT LSP bandwidth needs to be increased or decreased.
   Based on the bandwidth demand, GEF A makes periodic discrete changes
   in bandwidth allocation, that is, either increases or decreases
   bandwidth on the LSPs constituting the CT bandwidth capacity.  If
   aggregate flow requests are made for CT LSP bandwidth that exceeds
   the current LSP bandwidth allocation, GEF A initiates a bandwidth
   modification request on the appropriate LSP(s).

                                     |<----- ULBk <= RBTk ---->|
      LSP-p |------------------------|-------------------------|
            A                        B                         E

                            |<-- ULBk <= RBTk -->|
      LSP-s |---------------|--------------------|-------------|
            A               C                    D             E

    Example of STT-EDR routing method:

    1.  If node A to node E bandwidth needs to be modified (say
        increased by DBW), primary LSP-p (e.g., LSP A-B-E) is tried
        first.

    2.  Available bandwidth is tested locally on each link in LSP-p.  If
        bandwidth not available (e.g., unreserved bandwidth on link BE
        is less than the reserved bandwidth threshold and this CT is
        above its bandwidth allocation), crankback to node A.

    3.  If DBW is not available on one or more links of LSP-p, then the
        currently successful LSP-s (e.g., LSP A-C-D-E) is tried next.

    4.  If DBW is not available on one or more links of LSP-s, then a
        new LSP is searched by trying additional candidate paths until a
        new successful LSP-n is found or the candidate paths are
        exhausted.

    5.  LSP-n is then marked as the currently successful path for the
        next time bandwidth needs to be modified.

      Figure 3:  STT-EDR Path Selection and Per-CT Bandwidth Allocation

   For example, in Figure 3, if the LSR-A to LSR-E bandwidth needs to be
   modified, say increased by DBW, the primary LSP-p (A-B-E) is tried
   first.  The bandwidth admission control for each link in the path is
   performed based on the status of the link using the bandwidth
   allocation procedure described below, where we further describe the
   role of the reserved bandwidth RBWck shown in Figure 3 in the
   admission control procedure.  If the first choice LSP cannot admit
   the bandwidth change, node A may then try an alternate LSP.  If DBW
   is not available on one or more links of LSP-p, then the currently
   successful LSP-s A-C-D-E (the 'STT path') is tried next.  If DBW is
   not available on one or more links of LSP-s, then a new LSP is
   searched by trying additional candidate paths (not shown) until a new
   successful LSP-n is found or all of the candidate paths held in the
   cache are exhausted.  LSP-n is then marked as the currently

   successful path for the next time bandwidth needs to be modified.
   DBW is set to the additional amount of bandwidth required by the
   aggregate flow request.

   If all cached candidate paths are tried without success, the search
   then generates a new CSPF path.  If a new CSPF calculation succeeds
   in finding a new path, that path is made the stored path, and the
   bottom cached path falls off the list.  If all cached paths fail and
   a new CSPF path cannot be found, then the original stored LSP is
   retained.  New requests go through the same routing algorithm again,
   since available bandwidth, etc., has changed and new requests might
   be admitted.  Also, GEF A periodically monitors LSP bandwidth use
   (e.g., once each 2-minute interval), and if bandwidth use falls below
   the current LSP allocation, the GEF initiates a bandwidth
   modification request to decrease the LSP bandwidth allocation to the
   currently used bandwidth level.  Bandwidth reservation occurs in STT-
   EDR with PATH/RESV messages per application of [RFC4804].

   In the STT-EDR computation, most of the time the primary path and
   stored path will succeed, and no CSPF calculation needs to be done.
   Therefore, the STT-EDR algorithm achieves good throughput performance
   while significantly reducing link-state flooding control load [TQO].
   An analogous method was proposed in the MPLS working group
   [FEEDBACK], where feedback based on failed path routing attempts is
   kept by the TE database and used when running CSPF.

   In the example GCAC method, bandwidth allocation to the primary and
   alternate LSPs uses the MAR bandwidth allocation procedure, as
   described below.  Path selection uses a topology database that
   includes available bandwidth on each link.  From the topology
   database pruned of links that do not meet the bandwidth constraint,
   the GEF determines a list of shortest paths by using a shortest path
   algorithm (e.g., Bellman-Ford or Dijkstra methods).  This path list
   is determined based on administrative weights of each link, which are
   communicated to all nodes within the routing domain (e.g.,
   administrative weight = 1 + e x distance, where e is a factor giving
   a relatively smaller weight to the distance in comparison to the hop
   count).  Analysis and simulation studies of a large national network
   model show that 6 or more primary and alternate cached paths provide
   the best overall performance.

   PCE [RFC4655][RFC4657][RFC5440] is used to implement
   inter-area/inter-AS/ inter-SP path selection algorithms, including
   alternate path selection, path reoptimization, backup path
   computation to protect DS-TE tunnels, and inter-area/inter-AS/inter-
   SP traffic engineering.  The DS-TE tunnels are protected against

   failure by using MPLS Fast Reroute [RFC4090].  OSPF TE extensions
   [RFC4203] are used to support the TE database (TED) required for
   implementation of the above PCE path selection methods.

   The example MPLS GCAC method incorporates the MAR bandwidth
   constraint model [RFC4126] incorporated within DS-TE [RFC4124].  In
   DS-TE/MAR, a small amount of reserved bandwidth RBTk governs the
   admission control on link k.  Associated with each CTc on link k are
   the allocated bandwidth constraints BWCck to govern bandwidth
   allocation and protection.  The reservation bandwidth on a link,
   RBTk, can be accessed when a given CTc has reserved bandwidth RBWck
   below its allocated bandwidth constraint BWCck.  However, if RBWck
   exceeds its allocated bandwidth constraint BWCck, then the
   reservation bandwidth threshold RBTk cannot be accessed.  In this
   way, bandwidth can be fully shared among CTs if available but is
   otherwise protected by bandwidth reservation methods.  Therefore,
   bandwidth can be accessed for a bandwidth request = DBW for CTc on a
   given link k based on the following rules:

   For an LSP on a high-priority or normal-priority CTc:

   If RBWck = BWCc, admit if DBW = ULBk
   If RBWck > BWCc, admit if DBW = ULBk - RBTk;

   or, equivalently:

   If DBW = ULBCck, admit the LSP.

   where

   ULBCck = idle link bandwidth on link k for CTc = ULBk -
            delta0/1(CTck) x RBWk
   delta0/1(CTck) = 0 if RBWck < BWCck
   delta0/1(CTck) = 1 if RBWck = BWCck

   For an LSP on a best-effort-priority CTc:

   allocated bandwidth BWCc = 0;
   Diffserv queuing serves best-effort packets only if there is
   available link bandwidth.

   In setting the bandwidth constraints for CTck, for a normal-priority
   CTc, the bandwidth constraints (BWCck) on link k are set by
   allocating the maximum reservable link bandwidth (MRBk) in proportion
   to the forecast or measured traffic load bandwidth TRAF_LOAD_BWck for
   CTc on link k.  That is:

   PROPORTIONAL_ BWck =
   TRAF_LOAD_ BWck/[S (c) {TRAF_LOAD_ BWck, c=0, MaxCT-1}] x MRBk

   For a normal-priority CTc:
   BWCck = PROPORTIONAL_ BWck

   For a high-priority CT, the bandwidth constraint BWCck is set to a
   multiple of the proportional bandwidth.  That is:

   For high-priority CTc:
   BWCck = FACTOR * PROPORTIONAL_ BWck

   where FACTOR is set to a multiple of the proportional bandwidth
   (e.g., FACTOR = 2 or 3 is typical).  This results in some over-
   allocation ('overbooking') of the link bandwidth and gives priority
   to the high-priority CTs.  Normally, the bandwidth allocated to high-
   priority CTs should be a relatively small fraction of the total link
   bandwidth, a maximum of 10-15 percent being a reasonable guideline.

   As stated above, the bandwidth allocated to a best-effort-priority
   CTc is set to zero.  That is:

   For a best-effort-priority CTc:
   BWCck = 0

   Analysis and simulation studies show that the level of reserved
   capacity RBTk in the range of 3-5% of link capacity provides the best
   overall performance.

   We give a simple example of the MAR bandwidth allocation method.
   Assume that there are two class types, CT0 and CT1, and a particular
   link with

   MRB = 100

   with the allocated bandwidth constraints set as follows:

   BWC0 = 30
   BWC1 = 50

   These bandwidth constraints are based on the forecasted traffic
   loads, as discussed above.  Either CT is allowed to exceed its
   bandwidth constraint BWCc as long a there is at least RBW units of
   spare bandwidth remaining.  Assume RBT = 10.  So under overload, if

   RBW0 = 20
   RBW1 = 70

   Then, for this loading

   UBW = 100 - 20 - 70 = 10

   If a bandwidth increase request = 5 = DBW arrives for Class Type 0
   (CT0), then accept for CT0 since RBW0 < BWC0 and DBW (= 5) < ILBW (=
   10).

   If a bandwidth increase request = 5 = DBW arrives for Class Type 1
   (CT1), then reject for CT1 since RBW1 > BC1 and DBW (= 5) > ILBW -
   RBT = 10 - 10 = 0.

   Therefore, CT0 can take the additional bandwidth (up to 10 units) if
   the demand arrives, since it is below its BWC value.  CT1, however,
   can no longer increase its bandwidth on the link, since it is above
   its BWC value and there is only RBT=10 units of idle bandwidth left
   on the link.  If best effort traffic is present, it can always seize
   whatever idle bandwidth is available on the link at the moment but is
   subject to being lost at the queues in favor of the higher-priority
   traffic.

   On the other hand, if a request arrives to increase bandwidth for CT1
   by 5 units of bandwidth (i.e., DBW = 5), we need to decide whether or
   not to admit this request.  Since for CT1,

   RBW1 > BWC1 (70 > 50), and
   DBW > UBW - RBT (i.e., 5 > 10 - 10)

   the bandwidth request is rejected by the bandwidth allocation rules
   given above.  Now let's say a request arrives to increase bandwidth
   for CT0 by 5 units of bandwidth (i.e., DBW = 5).  We need to decide
   whether or not to admit this request.  Since for CT0

   RBW0 < BWC0 (20 < 30), and
   DBW < UBW (i.e., 5 < 10)

   The example illustrates that with the current state of the link and
   the current CT loading, CT1 can no longer increase its bandwidth on
   the link, since it is above its BWC1 value and there is only RBW=10
   units of spare bandwidth left on the link.  But CT0 can take the
   additional bandwidth (up to 10 units) if the demand arrives, since it
   is below its BWC0 value.

   For the example GCAC, the method for bandwidth additions and
   deletions to LSPs in is as follows.  The bandwidth constraint
   parameters defined in the MAR method [RFC4126] do not change based on
   traffic conditions.  In particular, these parameters defined in
   [RFC4126], as described above, are configured and do not change until

   reconfigured: MRBk, BWCck, and RBTk.  However, the reserved bandwidth
   variables change based on traffic: RBWck, ULBk, and ULBCck.  The
   RBWck and bandwidth allocated to each DS-TE/MAR tunnel is dynamically
   changed based on traffic: it is increased when the traffic demand
   increases (using RSVP aggregation), and it is periodically decreased
   when the traffic demand decreases.  Furthermore, if tunnel bandwidth
   cannot be increased on the primary path, an alternate LSP path is
   tried.  When LSP tunnel bandwidth needs to be increased to
   accommodate a given aggregate flow request, the bandwidth is
   increased by the amount of the needed additional bandwidth, if
   possible.  The tunnel bandwidth quickly rises to the currently needed
   maximum bandwidth level, wherein no further requests are made to
   increase bandwidth, since departing flows leave a constant amount of
   available or spare bandwidth in the tunnel to use for new requests.
   Tunnel bandwidth is reduced every 120 seconds by 0.5 times the
   difference between the allocated tunnel bandwidth and the current
   level of the actually utilized bandwidth (i.e., the current level of
   spare bandwidth).  Analysis and simulation modeling results show that
   these parameters provide the best performance across a number of
   overload and failure scenarios.

A.2.  Example of QoS Signaling Implementation

   The example GCAC method uses Next Steps in Signaling (NSIS)
   algorithms for signaling MPLS GCAC QoS requirements of individual
   flows.  NSIS QoS signaling has been specified by the IETF NSIS
   working group and extends RSVP signaling by defining a two-layer QoS
   signaling model:

   o  NSIS Transport Layer Protocol (NTLP) [RFC5971]

   o  NSIS Signaling Layer Protocol (NSLP) for Quality-of-Service
      Signaling [RFC5974]

   [RFC5975] defines a QoS specification (QSPEC) object, which contains
   the QoS parameters required by a QoS model (QOSM) [RFC5976].  A QOSM
   specifies the QoS parameters and procedures that govern the resource
   management functions in a QoS-aware router.  Multiple QOSMs can be
   supported by the QoS-NSLP, and the QoS-NSLP allows stacking of QSPEC
   parameters to accommodate different QOSMs being used in different
   domains.  As such, NSIS provides a mechanism for interdomain QoS
   signaling and interworking.

   The QSPEC parameters defined in [RFC5975] include, among others:

   TRAFFIC DESCRIPTION Parameters:

   o  <Traffic Model> Parameters

   CONSTRAINTS Parameters:

   o  <Path Latency>, <Path Jitter>, <Path PLR>, and <Path PER>
      Parameters

   o  <PHB Class> Parameter

   o  <DSTE Class Type> Parameter

   o  <Y.1541 QoS Class> Parameter

   o  <Reservation Priority> Parameter

   o  <Preemption Priority> and <Defending Priority> Parameters

   The ability to achieve end-to-end QoS through multiple Internet
   domains is also an important requirement.  MPLS GCAC end-to-end QoS
   signaling ensures that end-to-end QoS is met by applying the
   Y.1541-QOSM [RFC5976], as now illustrated.

   The QoS GEF initiates an end-to-end, inter-domain QoS RESERVE message
   containing the QoS parameters, including for example, <Traffic
   Model>, <Y.1541 QoS Class>, <Reservation Priority>, and perhaps other
   parameters for the flow.  The RESERVE message may cross multiple
   domains; each node on the data path checks the availability of
   resources and accumulating the delay, delay variation, and loss ratio
   parameters, as described below.  If an intermediate node cannot
   accommodate the new request, the reservation is denied.  If no
   intermediate node has denied the reservation, the RESERVE message is
   forwarded to the next domain.  If any node cannot meet the
   requirements designated by the RESERVE message to support a QoS
   parameter, for example, it cannot support the accumulation of end-to-
   end delay with the <Path Latency> parameter, the node sets a flag
   that will deny the reservation.  Also, parameter negotiation can be
   done, for example, by setting the <Y.1541 QoS Class> to a lower class
   than specified in the RESERVE message.  When the available <Y.1541
   QoS Class> must be reduced from the desired <Y.1541 QoS Class>, say
   because the delay objective has been exceeded, then there is an
   incentive to respond to the GEF with an available value for delay in
   the <Path Latency> parameter.  For example, if the available <Path
   Latency> is 150 ms (still useful for many applications) and the
   desired QoS is 100 ms (according to the desired <Y.1541 QoS Class>

   Class 0), then the response would be that Class 0 cannot be achieved
   and Class 1 is available (with its 400 ms objective).  In addition,
   the response includes an available <Path Latency> = 150 ms, making
   acceptance of the available <Y.1541 QoS Class> more likely.

A.3.  Example of Queuing Implementation

   In this MPLS GCAC example, queuing behaviors for the CT traffic
   priorities incorporates Diffserv mechanisms and assumes separate
   queues based on Traffic Class (TC)/CoS bits.  The queuing
   implementation assumes 3 levels of priority:  high, normal, and best
   effort.  These queues include two EF priority queues
   [RFC3246][RFC5865], with the highest priority assigned to emergency
   traffic (GETS/ETS/E911) and the second priority assigned to normal-
   priority real-time (e.g., VoIP) traffic.  Separate AF queues
   [RFC2597] are used for data services, such as premium private data
   and premium public data traffic, and a separate best-effort queue is
   assumed for the best-effort traffic.  All queues have static
   bandwidth allocation limits applied based on the level of forecast
   traffic on each link, such that the bandwidth limits will not be
   exceeded under normal conditions, allowing for some traffic overload.
   In the MPLS GCAC method, high-priority, normal-priority, and best-
   effort traffic share the same network; under congestion, the Diffserv
   priority-queuing mechanisms push out the best-effort-priority traffic
   at the queues so that the normal-priority and high-priority traffic
   can get through on the MPLS-allocated LSP bandwidth.

Authors' Addresses

   Gerald Ash (editor)
   AT&T

   EMail:  gash5107@yahoo.com

   Dave McDysan
   Verizon
   22001 Loudoun County Pkwy
   Ashburn, VA  20147

   EMail:  dave.mcdysan@verizon.com

 

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