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Versions: (draft-farkas-isis-pcr) 00 01 02 03 04 05 RFC 7813

IS-IS for IP Internets                                    J. Farkas, Ed.
Internet-Draft                                                  Ericsson
Intended status: Standards Track                                N. Bragg
Expires: October 31, 2015                                          Ciena
                                                            P. Unbehagen
                                                                   Avaya
                                                              G. Parsons
                                                                Ericsson
                                                        P. Ashwood-Smith
                                                     Huawei Technologies
                                                               C. Bowers
                                                        Juniper Networks
                                                          April 29, 2015


                 IS-IS Path Computation and Reservation
                         draft-ietf-isis-pcr-00

Abstract

   IEEE 802.1Qca Path Control and Reservation (PCR) specifies explicit
   path control via IS-IS in Layer 2 networks in order to move beyond
   the shortest path capabilities provided by IEEE 802.1aq Shortest Path
   Bridging (SPB).  IS-IS PCR provides capabilities for the
   establishment and control of explicit forwarding trees in a Layer 2
   network domain.  This document specifies the sub-TLVs for IS-IS PCR.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on October 31, 2015.








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Copyright Notice

   Copyright (c) 2015 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
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   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  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Conventions Used in This Document . . . . . . . . . . . . . .   3
   3.  Terminology and Definitions . . . . . . . . . . . . . . . . .   4
   4.  Explicit Trees  . . . . . . . . . . . . . . . . . . . . . . .   6
   5.  Explicit ECT Algorithms . . . . . . . . . . . . . . . . . . .  10
   6.  IS-IS PCR sub-TLVs  . . . . . . . . . . . . . . . . . . . . .  11
     6.1.  Topology sub-TLV  . . . . . . . . . . . . . . . . . . . .  11
     6.2.  Hop sub-TLV . . . . . . . . . . . . . . . . . . . . . . .  15
     6.3.  Bandwidth Constraint sub-TLV  . . . . . . . . . . . . . .  19
     6.4.  Bandwidth Assignment sub-TLV  . . . . . . . . . . . . . .  21
     6.5.  Timestamp sub-TLV . . . . . . . . . . . . . . . . . . . .  23
   7.  MRT-FRR Application . . . . . . . . . . . . . . . . . . . . .  23
   8.  Summary . . . . . . . . . . . . . . . . . . . . . . . . . . .  27
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  28
   10. Security Considerations . . . . . . . . . . . . . . . . . . .  28
   11. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  28
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  28
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  28
     12.2.  Informative References . . . . . . . . . . . . . . . . .  29
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  30

1.  Introduction

   IEEE 802.1Qca Path Control and Reservation (PCR) [IEEE8021Qca]
   specifies extensions to IS-IS for the control of Explicit Trees
   (ETs).  The PCR extensions are compatible with the Shortest Path
   Bridging (SPB) extensions to IS-IS specified by [RFC6329] and
   [IEEE8021aq] (already rolled into [IEEE8021Q]).  Furthermore, IS-IS
   with PCR extensions relies on the SPB architecture and terminology;
   and some of the IS-IS SPB sub-TLVs are also leveraged.  IS-IS PCR
   builds upon IS-IS and uses IS-IS in a similar way to SPB.  IS-IS PCR



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   only addresses point-to-point physical links, although IS-IS also
   supports shared media LANs.

   This document specifies five IS-IS sub-TLVs for the control of
   explicit trees by IS-IS PCR in a Layer 2 network as specified by IEEE
   802.1Qca.  In addition to the sub-TLVs specified here, IS-IS PCR
   relies on the following IS-IS SPB sub-TLVs specified by [RFC6329]:

   o  SPB Link Metric sub-TLV

   o  SPB Base VLAN-Identifiers sub-TLV

   o  SPB Instance sub-TLV

   o  SPBV MAC address sub-TLV

   o  SPBM Service Identifier and Unicast Address sub-TLV

   These sub-TLVs are used to provide the link metric and the
   associations among bridges, MAC addresses, VIDs and I-SIDs within an
   IS-IS domain.  The use of these SPB sub-TLVs for PCR is specified by
   IEEE 802.1Qca.  Note that IS-IS PCR does not require the
   implementation of the full IS-IS SPB protocol but only the support of
   these SPB sub-TLVs.  A bridge can support both IS-IS SPB and IS-IS
   PCR at the same time but when it supports both they are implemented
   by the same IS-IS entity on a per instance basis.

   The sub-TLVs specified here can be also applied for Fast ReRoute
   using Maximally Redundant Trees (MRT-FRR)
   [I-D.ietf-rtgwg-mrt-frr-architecture] in a Layer 2 network.  MRTs are
   computed as specified in [I-D.ietf-rtgwg-mrt-frr-algorithm].  If MRT
   computation is split such that the Generalized Almost Directed
   Acyclic Graph (GADAG) is computed centrally, then these sub-TLVs can
   be used to distribute the GADAG, which is identical for each network
   node throughout a network domain.

   PCR uses IS-IS, the SPB sub-TLVs listed above, and the new sub-TLVs
   defined here.  IS-IS PCR has no impact to IETF protocols.

2.  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 [RFC2119].

   The lowercase forms with an initial capital "Must", "Must Not",
   "Shall", "Shall Not", "Should", "Should Not", "May", and "Optional"
   in this document are to be interpreted in the sense defined in



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   [RFC2119], but are used where the normative behavior is defined in
   documents published by SDOs other than the IETF.

3.  Terminology and Definitions

   ADAG:  Almost Directed Acyclic Graph - a digraph that can be
      transformed into a DAG by removing all arcs incoming to the root.
      [I-D.ietf-rtgwg-mrt-frr-architecture]

   B-VID:  Backbone VID.  [IEEE8021Q]

   Base VID:  The VID used to identify a VLAN in management operations.
      [IEEE8021aq]

   BLCE:  Bridge Local Computation Engine - A computation engine in a
      bridge that performs path and routing computations.  The BLCE
      implements e.g.  SPF, CSPF, or the Maximally Redundant Trees
      Algorithm.  [IEEE8021Qca]

   Constrained tree:  A tree meeting a certain constraint, e.g.
      providing a minimal available bandwidth.  [IEEE8021Qca]

   Cut-node:  A node is a cut-node if removing it partitions the
      network.  [I-D.ietf-rtgwg-mrt-frr-architecture]

   Cut-link:  A link is a cut-link if removing it partitions the
      network.  [I-D.ietf-rtgwg-mrt-frr-architecture]

   DAG:  Directed Acyclic Graph - a digraph containing no directed
      cycle.  [I-D.ietf-rtgwg-mrt-frr-architecture]

   DEI:  Drop Eligible Indicator.  [IEEE8021Q]

   ECT Algorithm:  Equal Cost Tree Algorithm - The algorithm and
      mechanism that is used for the control of the active topology,
      i.e. forwarding trees.  It can be one of the shortest path
      algorithms specified by IEEE 802.1aq.  It can be also one of the
      explicit path control algorithms specified by IEEE 802.1Qca.  Each
      ECT Algorithm has a 32-bit unique ID.  [IEEE8021aq]

   ET:  Explicit Tree - An explicitly defined tree, which is specified
      by its end points and the paths among the end points.  If only the
      end points are specified but the paths are not, then it is a loose
      explicit tree.  If the paths are also specified, then it is a
      strict explicit tree.  [IEEE8021Qca]

   ETDB:  Explicit Tree Database - A database storing explicit trees.
      [IEEE8021Qca]



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   FDB:  Filtering Database.  [IEEE8021Q]

   GADAG:  Generalized ADAG - a digraph, which has only ADAGs as all of
      its topology blocks.  [I-D.ietf-rtgwg-mrt-frr-architecture]

   Hop:  A hop is specified by two nodes.  A strict hop has no
      intermediate nodes, whereas a loose hop can have one or more
      intermediate nodes.  IS-IS PCR specifies an explicit tree by an
      ordered list of hops starting at the root, each successive hop
      being defined by the next element of the list.  [IEEE8021Qca]

   I-SID:  Backbone Service Instance Identifier - A 24-bit ID.
      [IEEE8021Q]

   Maximally Redundant Trees (MRTs):  A pair of trees with a common MRT
      Root where the path from any leaf node to the MRT Root along the
      first tree (MRT-Blue) and the path from the same leaf node along
      the second tree (MRT-Red) share the minimum number of nodes and
      the minimum number of links.  Each such shared node is a cut-node.
      Any shared links are cut-links.
      [I-D.ietf-rtgwg-mrt-frr-architecture]

   MRT-Blue:  MRT-Blue is one of the two MRTs; specifically, MRT-Blue is
      the increasing MRT where links in the GADAG are taken in the
      direction from a lower topologically ordered node to a higher one.
      [I-D.ietf-rtgwg-mrt-frr-architecture]

   MRT-Red:  MRT-Red is one of the two MRTs; specifically, MRT-Red is
      the decreasing MRT where links in the GADAG are taken in the
      direction from a higher topologically ordered node to a lower one.
      [I-D.ietf-rtgwg-mrt-frr-architecture]

   MRT Root:  The common root of the two MRTs: MRT-Blue and MRT-Red.
      [I-D.ietf-rtgwg-mrt-frr-architecture]

   MSRP:  Multiple Stream Registration Protocol, standardized as IEEE
      802.1Qat, already rolled into [IEEE8021Q].

   PCA:  Path Control Agent - The agent that is part of the IS-IS domain
      and thus can perform IS-IS operations on behalf of a PCE, e.g.
      maintain the LSDB and send LSPs.  [IEEE8021Qca]

   PCE:  Path Computation Element - An entity that is capable of
      computing a path through a network based on a representation of
      the topology of the network (obtained by undefined means external
      to the PCE).  [RFC4655]





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   PCP:  Priority Code Point, which identifies a traffic class.
      [IEEE8021Q]

   PTP:  Precision Time Protocol specified by [IEEE1588].

   Redundant trees:  A pair of trees with a common Root where the paths
      from any leaf node to the Root along the first tree and the second
      tree are disjoint.  [I-D.ietf-rtgwg-mrt-frr-architecture]

   SPBM:  SPB MAC - The SPB mode where a MAC or its shorthand
      (SPSourceID: Shortest Path Source ID) is used to identify an SPT.
      [IEEE8021aq]

   SPBV:  SPB VID - The SPB mode where a unique VID is assigned to each
      SPT Root bridge and is used to identify an SPT.  [IEEE8021aq]

   SPF:  Shortest Path First.

   SPT:  Shortest Path Tree.  [IEEE8021aq]

   SRLG:  Shared Risk Link Group - A set of links that share a resource
      whose failure affects each link.  [RFC5307]

   TAI:  Temps Atomique International - International Atomic Time.
      [IEEE1588]

   topology block:  Either a maximally two-connected cluster, a cut-link
      with its endpoints, or an isolated node.
      [I-D.ietf-rtgwg-mrt-frr-architecture]

   TED:  Traffic Engineering Database - A database storing the traffic
      engineering information propagated by IS-IS.  [RFC5305]

   two-connected:  A graph that has no cut-nodes.  This is a graph that
      requires at least two nodes to be removed before gets partitioned.
      [I-D.ietf-rtgwg-mrt-frr-architecture]

   VID:  VLAN ID.  [IEEE8021Q]

   VLAN:  Virtual Local Area Network.  [IEEE8021Q]

4.  Explicit Trees

   An explicit tree is determined by a Path Computation Element (PCE)
   [RFC4655] and is not required to follow the shortest path.  A PCE is
   an entity that is capable of computing a topology for forwarding
   based on a network topology, its corresponding attributes, and
   potential constraints.  A PCE MUST explicitly describe a forwarding



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   tree as described in Section 6.1.  Either a single PCE or multiple
   PCEs determine explicit trees for a domain.  Even if there are
   multiple PCEs in a domain, each explicit tree MUST be only determined
   by one PCE, which is referred to as the owner PCE of the tree.  PCEs
   and IS-IS PCR can be used in combination with IS-IS SPB shortest path
   routing.

   The PCE interacts with the active topology control protocol, i.e.
   with IS-IS.  The collaboration with IS-IS can be provided by a Path
   Control Agent (PCA) on behalf of a PCE.  Either the PCE or the
   corresponding PCA is part of the IS-IS domain.  If the PCE is not
   part of the IS-IS domain, then the PCE MUST be associated with a PCA
   that is part of the IS-IS domain.  The PCE or its PCA MUST establish
   IS-IS adjacency in order to receive all the LSPs transmitted by the
   bridges in the domain.  The PCE, either on its own or via its PCA,
   can control the establishment of explicit trees in that domain by
   injecting an LSP conveying an explicit tree and thus instruct IS-IS
   to set up the explicit tree determined by the PCE.  If instructed to
   do so by a PCE, IS-IS MAY also record and communicate bandwidth
   assignments, which MUST NOT be applied if reservation protocol (e.g.
   Multiple Stream Registration Protocol (MSRP)) is used in the domain.
   Both MSRP and IS-IS MUST NOT be used to make bandwidth assignments in
   the same domain.

   The operation details of the PCE are not specified by this document
   or by IEEE 802.1Qca.  If the PCE is part of the IS-IS domain, then
   the PCE uses IS-IS PDUs to communicate with the IS-IS domain and the
   PCE has a live IS-IS LSDB, (i.e. the PCE implements the PCA functions
   too).  A PCE can instead communicate with the IS-IS domain via a PCA,
   e.g. to retrieve the LSDB or instruct the creation of an explicit
   tree.  However, the means of communication between the PCE and the
   PCA is not specified by this document or by IEEE 802.1Qca.

   An Explicit Tree (ET) is an undirected loop-free topology, whose use
   is under the control of the owner PCE by means of associating VIDs
   and MAC addresses with it.  An ET MUST NOT contain Cycles.  As it is
   undirected, an ET contains no assumptions about the direction of any
   flows that use it; it can be used in either direction as specified by
   the VIDs and MAC addresses associated with it.  It is the
   responsibility of the PCE to ensure reverse path congruency and
   multicast-unicast congruency if that is required.

   An explicit tree is either strict or loose.  A strict explicit tree
   specifies all bridges and paths it comprises.  A loose tree only
   specifies the bridges as a list of hops that have a special role in
   the tree, e.g. a traffic end point, and no path or path segment is
   specified between the bridges, which are therefore loose hops even if
   traffic end points are adjacent neighbors.  The special role of a hop



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   can be: traffic end point, root, leaf, a bridge to be avoided, or a
   transit hop in case of a tree with a single leaf.  The path for a
   loose hop is determined by the Bridge Local Computation Engine (BLCE)
   of the bridges.  The shortest path is used for a loose hop unless
   specified otherwise by the descriptor (Section 6.1) of the tree or by
   the corresponding ECT Algorithm (Section 5).

   A loose explicit tree is constrained if the tree descriptor includes
   one or more constraints, e.g. the administrative group that the links
   of the tree have to belong to.  The BLCE of the bridges then apply
   the Constrained Shortest Path First (CSPF) algorithm, which is
   Shortest Path First (SPF) on the topology that only contains the
   links meeting the constraint(s).

   An explicit tree is specified by a Topology sub-TLV (Section 6.1).
   The Topology sub-TLV associates one or more VIDs with an explicit
   tree.  The Topology sub-TLV includes two or more Hop sub-TLVs
   (Section 6.2), and a hop is specified by an IS-IS System ID.  A Hop
   sub-TLV MAY include a delay constraint for a loose hop.  A Topology
   sub-TLV MAY also include further sub-TLVs to constrain loose hops.
   The bridges involved in an explicit tree store the corresponding
   Topology sub-TLVs in their Explicit Tree Database (ETDB).

   Explicit trees are propagated and set-up by IS-IS PCR in a domain.
   The PCE or its PCA assembles the Topology sub-TLVs (Section 6.1), and
   adds it into an LSP, which is flooded throughout the domain.  The
   Topology sub-TLV is flooded by the same techniques used for the SPB
   LSPs.  The bridges then MUST process the Topology sub-TLV upon
   reception.  If the Topology sub-TLV specifies one or more loose
   trees, then the path for the loose hops is determined by the BLCE of
   the bridges.  The bridges then install the appropriate FDB entries
   for frame forwarding along the tree described by the Topology sub-
   TLV, or the trees computed based on the Topology sub-TLV.  Dynamic
   Filtering Entries are maintained by IS-IS for the VID, MAC address
   tuples associated with an ET.

   Due to the LSP aging of IS-IS, the Topology sub-TLVs (Section 6.1)
   have to be refreshed similar to other IS-IS TLVs in order to keep the
   integrity of the LSDB.  The corresponding Dynamic Filtering Entries
   are also refreshed in the FDB when a Topology sub-TLV is refreshed.
   Refreshing Topology sub-TLVs is the task of the entity being part of
   the IS-IS domain, i.e. either the PCE or the PCA.

   There is no precedence order between Explicit Trees.  Precedence
   order among bandwidth assignments recorded by IS-IS PCR is specified
   in Section 6.4.





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   If it is not possible to install an explicit tree, e.g. constraint(s)
   cannot be met or the Topology sub-TLV is ill-formed, then no tree is
   installed but a management report is generated.

   The bridges MAY support the following IS-IS features for the
   computation of explicit trees.  The Extended IS Reachability TLV
   (type 22) specified in [RFC5305] provides the following link
   attribute IS-IS sub-TLVs:

   o  Administrative Group (color, resource class) (sub-TLV type 3),

   o  Maximum Link Bandwidth (sub-TLV type 9),

   o  Maximum Reservable Link bandwidth (sub-TLV type 10),

   o  Unreserved Bandwidth (sub-TLV type 11),

   o  Traffic Engineering Default Metric (sub-TLV type 18).

   When the Unreserved Bandwidth sub-TLV is used in a Layer 2 bridge
   network, the priority value encoded in the sub-TLV provides the PCP,
   i.e. identifies a traffic class (not a setup priority level).

   Further attributes are provided by the IS-IS TE Metric Extension link
   attribute sub-TLVs specified in [I-D.ietf-isis-te-metric-extensions]:

   o  Unidirectional Link Delay,

   o  Min/Max Unidirectional Link Delay,

   o  Unidirectional Delay Variation,

   o  Unidirectional Link Loss,

   o  Unidirectional Residual Bandwidth,

   o  Unidirectional Available Bandwidth,

   o  Unidirectional Utilized Bandwidth.

   The Shared Risk Link Group (SRLG) information provided by the SRLG
   TLV (type 138) [RFC5307] MAY be also used.  In order to indicate that
   the interface is unnumbered in this case, the corresponding flag
   takes value 0.  The Link Local Identifier is an Extended Local
   Circuit Identifier and the Link Remote Identifier is a Neighbor
   Extended Local Circuit ID.





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5.  Explicit ECT Algorithms

   The exact IS-IS control mode of operation MUST be selected for a VLAN
   by associating its Base VID with the appropriate ECT Algorithm in the
   SPB Base VLAN-Identifiers sub-TLV [RFC6329], in addition to
   allocating the Base VID to IS-IS control.  There are five distinct
   ECT Algorithms for the five explicit path control modes.  The
   operation details of the explicit ECT Algorithms and their
   configuration is specified by IEEE 802.1Qca, a high level overview is
   given here.  An ECT Algorithm value consists of the IEEE 802.1 OUI
   (Organizationally Unique Identifier) value 00-80-C2 concatenated with
   an index [RFC6329].

   The Strict Tree (ST) ECT Algorithm MUST be used for a strict explicit
   tree.  A strict ET is static as no other entity can update it but the
   owner PCE.  In case of a topology change, it is the task of the owner
   PCE to detect the topology change, e.g. based on the changes in the
   LSDB, and to update the strict trees if needed.  That is, the owner
   PCE computes the new tree, assembles its descriptor (Section 6.1),
   and then instructs IS-IS PCR to install it.  The value for the ST ECT
   algorithm is 00-80-C2-17.

   The Loose Tree (LT) ECT Algorithm MAY be also supported.  It is used
   for a single loose explicit tree.  The path for loose hops is
   determined by the BLCE of the bridges; therefore, the Topology sub-
   TLV (Section 6.1) specifying the tree MUST indicate which hop is the
   Root of the tree.  The loose hops are maintained by IS-IS, i.e.
   restored upon a topology change if a loop-free path is available.  If
   the tree computed by the BLCE visits the same bridge twice (implying
   that a loop or hairpin has been created), then that loop or hairpin
   MUST be pruned from the tree even if it contains a hop specified by
   the Topology sub-TLV.  It is a constraint if a bridge is not to be
   included, which can be specified by the Exclude flag of a Hop sub-TLV
   (Section 6.2) conveyed by the Topology sub-TLV specifying the tree.
   The range of values for the LT ECT Algorithms is
   00-80-C2-21...00-80-C2-30.

   The Loose Tree Set (LTS) ECT Algorithm MAY be also supported.  It is
   used if connectivity among the traffic end points specified by the
   Topology sub-TLV (Section 6.1) is to be provided by a set of loose
   trees such that one tree is rooted at each traffic end point.  The
   BLCE of the bridges compute the loose trees, which are maintained by
   IS-IS, i.e. restored upon a topology change.  One constraint can be
   to avoid some bridges in these trees, which can be specified by the
   Exclude flag (item c.6. in Section 6.2).  Further constraints can be
   specified by the Topology sub-TLV.  The range of values for the LT
   ECT Algorithms is 00-80-C2-31...00-80-C2-40.




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   The LT and LTS ECT Algorithms use the shortest paths after pruning
   the topology according to the constraint(s) if any.  The shortest
   path tie-breaking specified by Section 12 of [RFC6329] is applied
   (see also subclauses 28.5 - 28.8 of [IEEE8021aq]), that's why range
   of values are associated with the LT and LTS ECT Algorithms.  In case
   of the LT ECT Algorithm, the indexes are 0x21...0x30, and ECT-
   MASK{index-0x20} is applied to retrieve the ECT-MASK of Section 12 of
   [RFC6329].  In case of the LTS ECT Algorithm, the indexes are
   0x31...0x40, and ECT-MASK{index-0x30} is applied to retrieve the ECT-
   MASK for shortest path tie-breaking.

   The MRT ECT Algorithm MAY be also supported.  It is used for the
   establishment and maintenance of MRTs in a distributed fashion.  The
   MRT Lowpoint Algorithm specified by
   [I-D.ietf-rtgwg-mrt-frr-algorithm] MUST be used for the computation
   of MRTs.  The MRT Lowpoint Algorithm first computes the GADAG then
   produces two MRTs for each MRT Root: MRT-Blue and MRT-Red. If the
   level of redundancy provided by each bridge being an MRT Root is not
   required, then the MRT Roots can be specified by a Topology sub-TLV
   (Section 6.1).  Both the GADAG and the MRT computation steps are
   performed distributed, i.e. by each bridge.  The value for the MRT
   ECT algorithm is 00-80-C2-18.

   The MRT GADAG (MRTG) ECT Algorithm MAY be also supported.  It splits
   the computation into two.  As the GADAG is identical for each MRT
   within a domain, it is computed by a single entity, which is the
   GADAG Computer.  The GADAG is then described in a Topology sub-TLV
   (Section 6.1), which is flooded in the domain.  The bridges then
   compute the MRTs for the MRT Roots based on the GADAG received.
   Section 7 provides more details on the description of the GADAG.  The
   value for the MRTG ECT algorithm is 00-80-C2-19.

   MRTs are loose trees as bridges are involved in their computation and
   restoration.  Thus both the MRT and the MRTG ECT Algorithms provide a
   set of loose trees: two MRTs for each MRT Root.

6.  IS-IS PCR sub-TLVs

   The following sub-TLVs are specified for IS-IS PCR.  The Topology
   sub-TLV MUST be carried in an MT-Capability TLV, the rest of the sub-
   TLVs are conveyed by Topology sub-TLV.

6.1.  Topology sub-TLV

   The variable length Topology sub-TLV MUST be used to describe an
   explicit tree.  The Topology sub-TLV MAY be also used for describing
   a Generalized Almost Directed Acyclic Graph (GADAG) as explained in
   Section 7 in detail.  The Topology sub-TLV MUST be carried in an MT-



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   Capability TLV (type 144) [RFC6329] in a Link State PDU.  A Topology
   sub-TLV specifying an explicit tree conveys one or more Base VIDs,
   two or more Hop sub-TLVs (Section 6.2).  A Topology sub-TLV
   describing a loose tree MAY also convey further sub-TLVs to specify
   constraints.  Figure 1 shows the format of the Topology sub-TLV.

      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
     +-+-+-+-+-+-+-+-+
     |     Type      |                   (1 byte)
     +-+-+-+-+-+-+-+-+
     |    Length     |                   (1 byte)
     +-+-+-+-+-+-+-+-+
     | Num Base VIDs |                   (1 byte)
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |  Res  |  Base VID 1 (12 bits) |   (0 or 2 bytes)
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
            .................
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |  Res  |  Base VID n (12 bits) |   (0 or 2 bytes)
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                        sub-TLV 1  (variable)                  |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                              .................
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                        sub-TLV m  (variable)                  |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                        Figure 1: Topology sub-TLV

   The parameters of explicit trees are encoded by the Topology sub-TLV
   as follows:

   a.  Type (8 bits): The type of the sub-TLV, its value is TBD.

   b.  Length (8 bits): The total number of bytes contained in the Value
       field.

   c.  Number of Base VIDs (8 bits): The number of Base VIDs carried in
       the Topology sub-TLV.  Its minimum value is 1 if the Topology
       sub-TLV specifies one or more explicit trees.  Its value can be 0
       if the Topology sub-TLV specifies a GADAG.

   d.  Reserved (Res) (4 bits): The reserved bits take value 0.

   e.  Base VID (12 bits): The Base VID parameter provides the Base VID
       of the VLAN that is associated with the explicit tree.  Multiple
       Base VIDs can be associated with the same explicit tree.  In



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       addition to the Base VID, some of the explicit ECT Algorithms
       (Section 5) require further VIDs which are associated with the
       VLAN via the SPB Instance sub-TLV [RFC6329].  A Topology sub-TLV
       specifying a GADAG can have zero Base VID parameters.  In this
       case, the given GADAG MUST be applied for each VLAN associated
       with the MRTG ECT Algorithm (Section 5).

   f.  sub TLVs: The rest conveys further sub-TLVs that specify the hops
       of the topology and can also specify constraints as described in
       the following.

   A topology is specified by a list of Hop sub-TLVs (Section 6.2), and
   a hop is specified by an IS-IS System ID.  An ill-formed Topology
   sub-TLV, e.g. specifying an invalid or inconsistent tree is ignored,
   no tree is installed but a management report is generated.

   The Topology sub-TLV specifies a strict tree by decomposing the tree
   to branches.  Each branch is a point-to-point path specified by an
   ordered list of hops where the end of each branch is a leaf.  Each
   element of a branch is the direct link between adjacent neighbor
   bridges whose Hop sub-TLV is next to each other in the Topology sub-
   TLV.  The first hop of the Topology sub-TLV is the root, hence, the
   first branch originates from the root.  The rest of the branches fork
   from another branch.  The first hop of a branch is a bridge that is
   already part of a former branch and the last hop is a leaf bridge.
   Therefore, the hop after a leaf hop is the beginning of a new branch,
   if any.  A hop of a branch is created if and only if the bridge
   specified for that hop is directly connected to the preceding bridge
   of the same branch.  The first branch MUST begin with the root and
   after that the order of the branches does not matter within the
   Topology sub-TLV.  Figure 2 shows an example strict tree and its
   description.



















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                                           +-----------+
                                           |     A     |
                                           +-----------+
                                           |     I     |
                                           +-----------+
                                           |     H     |
                  [B]---[A]---[I]          +-----------+
                   |           |           |     G     |
                   |           |           +-----------+
                   |           |           |     E     |
                  [C]---[F]   [H]          +-----------+
                   |           |           |     A     |
                   |           |           +-----------+
                   |           |           |     B     |
                  [D]   [E]---[G]          +-----------+
                                           |     C     |
                                           +-----------+
                                           |     D     |
                                           +-----------+
                                           |     C     |
                                           +-----------+
                                           |     F     |
                                           +-----------+

        Figure 2: A strict tree and its description; root = Node A

   The Topology sub-TLV of a loose tree does not provide any path or
   path segment, but the hops which are to participate.  The root MUST
   be the first hop.  The leaves of a single loose tree MUST be also
   specified.  Hop sub-TLVs can be included in a Topology sub-TLV to
   specify bridges that have to be avoided.  If the Topology sub-TLV
   only specifies a single leaf, then one or more transit hops can be
   specified by the Topology sub-TLV to direct the path along a sequence
   of bridges, specified by the order of hops.  If bridges whose
   respective Hop sub-TLVs are adjacent to each other in the Topology
   sub-TLV but are not topology neighbors, then it is a loose hop.  If a
   Topology sub-TLV conveys one or more loose hops, then that sub-TLV
   defines a loose explicit tree and each hop is considered as a loose
   hop.  The path of a loose hop MUST be pruned from the tree if the
   path would create a loop or hairpin.

   If the Base VIDs of the Topology sub-TLV are associated with the LTS
   ECT Algorithm or the MRT ECT Algorithm, then the Hop sub-TLVs
   conveyed by the Topology sub-TLV belong to traffic end points or
   bridges to be excluded.  The BLCEs compute the loose trees, e.g.
   MRTs, such that they span the traffic end points and are rooted at a
   traffic end point.




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   The Topology sub-TLV specifies a GADAG if the Base VIDs conveyed by
   the Topology sub-TLV are associated with the MRTG ECT Algorithm.
   Section 7 provides the details on the description of a GADAG by a
   Topology sub-TLV.

   Each traffic end point of an explicit tree MUST be always specified
   in the Topology sub-TLV by the inclusion of the Hop sub-TLVs
   corresponding to the traffic end points.  The traffic end points of a
   tree are identified by setting the Traffic End Point flag (item c.3.
   in Section 6.2) in the appropriate Hop sub-TLVs.

   If the explicit tree is loose, then the Topology sub-TLV MAY convey
   further sub-TLVs to specify constraints, e.g. an Administrative Group
   sub-TLV [RFC5305] or a Bandwidth Constraint (Section 6.3).  If it is
   not possible to meet the constraint(s) specified by the Topology sub-
   TLV, then no tree is installed but a management report is generated.

   If IS-IS PCR is used for recording bandwidth assignment, then the
   Topology sub-TLV conveys Bandwidth Assignment sub-TLV (Section 6.4)
   and it can also convey Timestamp sub-TLV (Section 6.5).  If the
   bandwidth assignment specified by the Topology sub-TLV is not
   possible, e.g. due to overbooking, then bandwidth assignment MUST NOT
   be performed and a management report is generated.  If the Topology
   sub-TLV specifies a new valid explicit tree, then the tree is
   installed without bandwidth assignment.

6.2.  Hop sub-TLV

   The Hop sub-TLV MUST be used to specify a hop of a topology.  Each
   Hop sub-TLV conveys an IS-IS System ID, which specifies a hop.  A Hop
   sub-TLV is conveyed by a Topology sub-TLV (Section 6.1).  A strict
   explicit tree is decomposed to branches where each branch is a point-
   to-point path specified by an ordered list of Hop sub-TLVs as
   specified in Section 6.1.  A hop of a branch is created if and only
   if the bridge specified for that hop is directly connected to the
   preceding bridge in the path.  That is, a point-to-point LAN is
   identified by the two bridges it interconnects; and the LAN is part
   of the strict tree if and only if the Hop sub-TLVs of the two bridges
   are next to each other in the Topology sub-TLV.  A Hop sub-TLV can
   convey a Circuit ID in order to distinguish multiple links between
   adjacent neighbor bridges.  A Hop sub-TLV also specifies the role of
   a bridge, e.g. if it is the root or a traffic end point.  The
   Topology sub-TLV of a loose tree only comprises the Hop sub-TLV of
   the bridges that have special role in the tree.  The Hop sub-TLV MAY
   also specify a delay budget for a loose hop.

   By default, the traffic end points both transmit and receive with
   respect to each VID associated with an explicit tree, except for an



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   LTS (Section 5) associated with a learning VLAN, which uses a
   unidirectional VID per bridge.  The Hop sub-TLV allows different
   configuration by means of the Transmit (T) and Receive (R) flags
   conveyed in the sub-TLV.  The VID and its T/R flags are only present
   in the Hop sub-TLV if the behavior of the traffic end points differs
   from the default.

   Figure 3 shows the format of the variable length Hop sub-TLV, which
   MUST be conveyed by a Topology sub-TLV (Section 6.1).

      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
     +-+-+-+-+-+-+-+-+
     |     Type      |                       (1 byte)
     +-+-+-+-+-+-+-+-+
     |    Length     |                       (1 byte)
     +-+-+-+-+-+-+-+-+
     |C|V|T|R|L|E|Res|                       (1 byte)
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                            System ID                          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |          System ID            |       (6 bytes)
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |            Extended Local Circuit ID  (0 or 4 bytes)          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |  Num of VIDs  |                       (0 or 1 byte)
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |T|R|Res|     VID 1   (12 bits) |       (0 or 2 bytes)
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
            .................
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |T|R|Res|     VID n   (12 bits) |       (0 or 2 bytes)
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                        Delay Constraint                       |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |        Delay Constraint       |       (0 or 6 bytes)
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                           Figure 3: Hop sub-TLV

   The parameters of a hop are encoded as follows:

   a.  Type (8 bits): The type of the sub-TLV, its value is TBD.

   b.  Length (8 bits): The total number of bytes contained in the Value
       field.





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   c.  Hop Flags (8 bits): The Hop sub-TLV conveys six one-bit flags.
       The Circuit and the VID flags influence the length of the Hop
       sub-TLV.  Two bits are reserved for future use, transmitted as 0
       and ignored on receipt.

       1.  Circuit (C) flag (1 bit): The Circuit flag is a one-bit flag
           to indicate whether or not the Extended Local Circuit ID
           parameter is present.  If the flag is set, then an Extended
           Local Circuit ID is also included in the Hop sub-TLV.

       2.  VID (V) flag (1 bit): The VID flag is a one-bit flag to
           indicate whether or not one or more VIDs are conveyed by the
           Hop sub-TLV.  If the flag is set, then the Number of VIDs
           parameter is present and indicates how many VIDs are conveyed
           by the Hop sub-TLV.  If the VID flag is reset, then neither
           the Number of VIDs parameter nor VIDs are present in the Hop
           sub-TLV.

       3.  Traffic End Point (T) flag (1 bit): The Traffic End Point
           flag is a one-bit flag to indicate whether or not the given
           System is a traffic end point, i.e. transmitter and/or
           receiver.  If the System is a traffic end point, then the
           Traffic End Point flag MUSt be set.  (The Traffic End Point
           flag indicates whether FDB entries are to be installed for
           the given hop.)

       4.  Root (R) flag (1 bit): The Root flag is a one-bit flag to
           indicate whether or not the given System is a Root of the
           explicit tree specified by the Topology sub-TLV.  If the
           System is a root of a tree, then the Root flag MUST be set.
           If the Topology sub-TLV specifies a single tree, i.e. the
           Base VIDs conveyed by the Topology sub-TLV are associated
           with either the ST ECT Algorithm or the LT ECT Algorithm
           (Section 5), then the Root flag is only set for one of the
           Systems conveyed by the Topology sub-TLV.  Furthermore, the
           first Hop sub-TLV of the Topology sub-TLV conveys the System
           that is the root of the tree.  If the Topology sub-TLV
           specifies a Loose Tree Set, i.e. the Base VIDs conveyed by
           the Topology sub-TLV are associated with the LTS ECT
           Algorithm (Section 5), then the Root flag is set for each
           traffic end point as each of them roots a tree.  If the
           Topology sub-TLV is used for MRT operations, i.e. the Base
           VIDs conveyed by the Topology sub-TLV are associated with
           either the MRT ECT Algorithm or the MRTG ECT Algorithm
           (Section 5), then the Root flag is set for each MRT Root.  If
           no MRT Root is specified by a Topology sub-TLV specifying a
           GADAG, then each SPT Root is an MRT Root as well.  If the
           Base VIDs conveyed by the Topology sub-TLV are associated



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           with the MRTG ECT Algorithm (Section 5), then the Topology
           sub-TLV specifies a GADAG and the very first Hop sub-TLV
           specifies the GADAG Root.  There is no flag for indicating
           the GADAG Root.

       5.  Leaf (L) flag (1 bit): The Leaf flag is a one-bit flag to
           indicate whether or not the given System is a Leaf of the
           explicit tree specified by the Topology sub-TLV.  If the
           System is a Leaf, then the Leaf flag MUST be set.  The Leaf
           flag is only used to mark a leaf of a tree if the Topology
           sub-TLV specifies a single tree.  The Leaf flag MUST be used
           to indicate the end of a topology block if the Topology sub-
           TLV specifies a GADAG, see Section 7.

       6.  Exclude (E) flag (1 bit): The Exclude flag is a one-bit flag
           to indicate if the given System MUST be excluded from the
           topology.  The Exclude flag and the Root flag cannot be set
           for a given hop at the same time.

       7.  Reserved (Res) (2 bits): The reserved bits take value 0.

   d.  System ID (48 bits): The 6-byte IS-IS System Identifier of the
       bridge that the Hop sub-TLV refers to.

   e.  Extended Local Circuit ID (32 bits): The Extended Local Circuit
       ID [RFC5303] parameter is not necessarily present in the Hop sub-
       TLV.  Its presence is indicated by the Circuit flag.  Parallel
       links corresponding to different IS-IS adjacencies between a pair
       of neighbor bridges can be distinguished by means of the Extended
       Local Circuit ID.  The Extended Local Circuit ID is conveyed by
       the Hop sub-TLV specifying the bridge nearer to the root of the
       tree, and identifies a circuit that attaches the given bridge to
       its neighbor cited by the next Hop sub-TLV of the Topology sub-
       TLV.  The Extended Local Circuit ID can only be used in strict
       trees.

   f.  Number of VIDs (8 bits): The Number of VIDs parameter is not
       present if the Hop sub-TLV does not convey VIDs, which is
       indicated by the VID flag.

   g.  VID and its T/R flags (14 bits): The VID and its T/R flags are
       only present in the Hop sub-TLV if the given bridge is a traffic
       end point and it behaves differently from the default with
       respect to that particular VID.

       1.  T flag (1 bit): This is the Transmit allowed flag for the VID
           following the flag.




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       2.  R flag (1 bit): This is the Receive allowed flag for the VID
           following the flag.

       3.  Reserved (Res) (2 bits): The reserved bits take value 0.

       4.  VID (12 bits): A VID.

   h.  Delay Constraint (48 bits): The last six bytes specify a delay
       constraint if they convey a Unidirectional Link Delay sub-TLV
       [I-D.ietf-isis-te-metric-extensions].  The delay constraint MAY
       be used in a Topology sub-TLV that specifies a single loose tree,
       i.e. the Base VIDs are associated with the LT ECT Algorithm
       (Section 5).  If delay constraint is applied, then the loose hop
       MUST fit in the delay budget specified by the Delay parameter of
       the Unidirectional Link Delay sub-TLV conveyed by the Hop sub-
       TLV.  If the Topology sub-TLV specifies a single leaf, then the
       path between the preceding Hop sub-TLV and the current Hop sub-
       TLV MUST meet the delay budget.  If the Topology sub-TLV
       specifies multiple leaves, then the path between the root and the
       current Hop sub-TLV MUST to meet the delay budget.  If the tree
       is used as a reverse congruent tree, then the delay constraint
       applies in both directions.  If the tree is used as a directed
       tree, then the delay constraint applies in the direction of the
       tree.  If it is not possible to meet the delay constraint
       specified by the Topology sub-TLV, then no tree is installed but
       a management report is generated.

6.3.  Bandwidth Constraint sub-TLV

   The Bandwidth Constraint sub-TLV MAY be included in a Topology sub-
   TLV (Section 6.1) in order to specify how much available bandwidth is
   to be provided by the constrained tree.  Each loose hop MUST meet the
   bandwidth constraint.  The bandwidth value of the constraint is a
   total value or it only refers to a single PCP as specified by the
   sub-TLV.  Figure 4 shows the format of the Bandwidth Constraint sub-
   TLV.















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      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
     +-+-+-+-+-+-+-+-+
     |     Type      |                   (1 byte)
     +-+-+-+-+-+-+-+-+
     |    Length     |                   (1 byte)
     +-+-+-+-+-+-+-+-+
     | PCP |D|P| Res |                   (1 byte)
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |              Available Bandwidth  (4 bytes)                   |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                  Figure 4: Bandwidth Constraint sub-TLV

   The parameters of the bandwidth constraint are encoded as follows:

   a.  Type (8 bits): The type of the sub-TLV, its value is TBD.

   b.  Length (8 bits): The total number of bytes contained in the Value
       field.  The value of the Length field is 5 bytes.

   c.  PCP (4 bits): The Priority Code Point (PCP) parameter identifies
       the traffic class the Available Bandwidth parameter refers to, if
       any.

   d.  DEI (D) (1 bit): This is the Drop Eligible Indicator (DEI)
       parameter.  If the DEI parameter is clear, then the bandwidth
       constraint refers to committed information rate.  If the DEI
       parameter is set, then the bandwidth constraint refers to peak
       information rate.

   e.  PCP (P) flag (1 bit): If this flag is set, then the PCP parameter
       is taken into account.

   f.  Reserved (Res) (3 bits): The reserved bits take value 0.

   g.  Available Bandwidth (32 bits): The Available Bandwidth is
       specific to the traffic class identified by the PCP parameter if
       the PCP flag is set, otherwise, it is total bandwidth.  In-line
       with the bandwidth parameters specified in [RFC5305], the
       Available Bandwidth is encoded as a 32-bit IEEE floating point
       number, and the units are bytes (not bits!) per second.  When the
       Unreserved Bandwidth sub-TLV (sub-TLV type 11 specified by
       [RFC5305]) is used in a Layer 2 bridge network, the priority
       value encoded in the Unreserved Bandwidth sub-TLV provides the
       PCP, i.e. identifies a traffic class (not a setup priority
       level).  Thus, the Available Bandwidth of a traffic class is
       easily comparable with the Unreserved Bandwidth stored in the TED



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       for the given traffic class.  The bandwidth constraint applies
       for both directions in case of symmetric explicit trees.
       Nevertheless, a VID associated with an explicit tree can be made
       unidirectional by means of the T/R flags belonging to the VID in
       the Hop sub-TLV (item g. in Section 6.2) of the traffic end
       points.  If all the VIDs of the Topology sub-TLV (Section 6.1)
       are unidirectional and all belong to the traffic class identified
       by the PCP parameter of the Bandwidth Constraint sub-TLV, then it
       is enough to meet the bandwidth constraint in the direction
       applied for those VIDs.

6.4.  Bandwidth Assignment sub-TLV

   IS-IS PCR MAY be used for recording bandwidth assignment for
   explicitly placed data traffic in a domain if MSRP is not used within
   the domain.  If MSRP is used in a domain, then only MSRP performs
   reservations.  Both MSRP and IS-IS MUST NOT be used to make bandwidth
   assignments in the same domain.

   The Bandwidth Assignment sub-TLV can be used to define the amount of
   bandwidth whose assignment is to be recorded by IS-IS PCR at each hop
   of the explicit tree described by the corresponding Topology sub-TLV
   (Section 6.1).  The Bandwidth Assignment sub-TLV is used by IS-IS PCR
   for the recording of bandwidth assignment for a traffic class
   identified by the PCP parameter of a VLAN tag.  If precedence order
   has to be determined among bandwidth assignments in a domain with
   multiple PCEs, then IS-IS PCR does it as described below.  If the
   bandwidth assignment specified by the Topology sub-TLV is not
   possible, e.g. due to overbooking, then bandwidth recording MUST NOT
   be performed and a management report is generated.  If the Topology
   sub-TLV specifies a new valid explicit tree, then the tree is
   installed without bandwidth assignment.  The Bandwidth Assignment
   sub-TLV is conveyed by a Topology sub-TLV (Section 6.1).  Figure 5
   shows the format of the Bandwidth Assignment sub-TLV.

      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
     +-+-+-+-+-+-+-+-+
     |     Type      |                   (1 byte)
     +-+-+-+-+-+-+-+-+
     |    Length     |                   (1 byte)
     +-+-+-+-+-+-+-+-+
     | PCP |D| Imp |R|                   (1 byte)
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                        Bandwidth  (4 bytes)                   |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                  Figure 5: Bandwidth Assignment sub-TLV



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   The parameters of the bandwidth constraint are encoded as follows:

   a.  Type (8 bits): The type of the sub-TLV, its value is TBD.

   b.  Length (8 bits): The total number of bytes contained in the Value
       field.  The value of the Length field is 5 bytes.

   c.  PCP (3 bits): The PCP parameter identifies the traffic class the
       bandwidth to be assigned for.

   d.  DEI (D) (1 bit): This is the Drop Eligible Indicator (DEI)
       parameter.  If the DEI parameter is clear, then the bandwidth
       assignment is performed for providing committed information rate.
       If the DEI parameter is set, then the bandwidth assignment is
       performed for providing peak information rate.

   e.  Importance (Imp) (3 bits): This is the Importance parameter for
       determining precedence order among bandwidth assignments within a
       PCP as described below.  Lower numerical value indicates more
       important bandwidth assignment within a PCP.  The default value
       of the Importance parameter is 7.

   f.  Reserved (R) (1 bit): The reserved bit takes value 0.

   g.  Bandwidth (32 bits): This is the amount of bandwidth to be
       assigned for the traffic class identified by the PCP parameter.
       In-line with the bandwidth values specified in [RFC5305], the
       Bandwidth parameter is encoded as a 32-bit IEEE floating point
       number, and the units are bytes (not bits!) per second.  The
       bandwidth assignment applies for both directions in case of
       symmetric explicit trees.

   The PCEs are collectively responsible for making a consistent set of
   bandwidth assignments when IS-IS PCR is used for recording bandwidth
   allocations.  If despite of that, precedence ordering is required
   among bandwidth assignments, then ordering based on the following
   parameters MUST be applied:

   1.  PCP parameter of Bandwidth Assignment sub-TLV,

   2.  Importance parameter of Bandwidth Assignment sub-TLV,

   3.  Timestamp sub-TLV (if present in the Topology sub-TLV).

   A bandwidth assignment takes precedence if it has higher PCP, or
   higher Importance within a PCP, or earlier timestamp in case of equal
   Importance within a PCP.  A bandwidth assignment associated with a
   timestamp takes precedence over a bandwidth assignment without



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   timestamp.  If resolution is not possible based on the above
   parameters or they are not available, e.g. each bandwidth assignment
   lacks timestamp or the same VID is called for, then the item is
   granted to the PCE whose LSP has the numerically least LSP ID.

6.5.  Timestamp sub-TLV

   The Timestamp sub-TLV MAY be included in a Topology sub-TLV
   (Section 6.1) in order to provide precedence order among equally
   important bandwidth assignments within a PCP as described in
   Section 6.4.  Figure 6 shows the format of the Timestamp sub-TLV.

      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
     +-+-+-+-+-+-+-+-+
     |     Type      |                   (1 byte)
     +-+-+-+-+-+-+-+-+
     |    Length     |                   (1 byte)
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                        Time       (4 bytes)                   |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                        Figure 6: Timestamp sub-TLV

   The timestamp represents a positive time with respect to the
   Precision Time Protocol (PTP) epoch and it is encoded as follows:

   a.  Type (8 bits): The type of the sub-TLV, its value is TBD.

   b.  Length (8 bits): The total number of bytes contained in the Value
       field.  The value of the Length field is 4 bytes.

   c.  Time (32 bits): This is the time in units of seconds with respect
       to the PTP epoch.

   The Timestamp sub-TLV carries the seconds portion of PTP as specified
   by [IEEE1588].  The epoch is 1970-01-01 00:00:00 TAI (i.e., the PTP
   time does not include leap seconds).

7.  MRT-FRR Application

   The application of MRT by [IEEE8021Qca] is discussed in detail in
   [I-D.bowers-rtgwg-mrt-applicability-to-8021qca].  This section
   describes some special considerations for the use of the MRT Lowpoint
   Algorithm [I-D.ietf-rtgwg-mrt-frr-algorithm], which are applicable
   both to the MRT ECT Algorithm and the MRTG ECT Algorithm.  This
   section also explains details related to the MRTG ECT Algorithm and
   the application of the Topology sub-TLV in particular.



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   The SPB Link Metric sub-TLV [RFC6329] specifies the metric of each
   link for IS-IS PCR including the MRT Algorithms.  If the SPB Link
   Metric values advertised by different ends of an adjacency are
   different, then the maximum value MUST be used.  If equal cost
   (sub)paths are found during the MRT computation, then the default
   tie-breaking specified by Section 11 of [RFC6329] MUST be used, which
   is based on the lower Bridge ID.  (The BridgeID is an 8-byte quantity
   whose upper 2 bytes are the node's BridgePriority and lower 6 bytes
   are the node's SYSID.)  Note also that if MRTs are used for source
   specific multicast (see [IEEE8021Qca] for details), then the bridges
   have to compute the MRTs of the other bridges in addition to their
   own one in order to be able to install the appropriated FDB entries.
   (This is similar to the need for all pairs shortest path computation
   instead of Dijkstra for source specific shortest path multicast
   trees.)

   The GADAG is identical for all the MRTs within a network domain, as a
   consequence of the use of the MRT Lowpoint Algorithm
   [I-D.ietf-rtgwg-mrt-frr-algorithm].  Therefore, it is beneficial to
   compute the GADAG by a single entity, which is referred to as the
   GADAG Computer and is either a PCE or the GADAG Root.  If the MRTG
   ECT Algorithm is applied, then the GADAG MUST be only computed by the
   GADAG Computer, which then MUST flood the descriptor Topology sub-TLV
   of the GADAG.  The bridges then compute the MRTs based on the
   received GADAG.

   The GADAG computation requires the selection of the GADAG Root.  The
   bridge with the best Bridge Identifier MUST be selected as the GADAG
   Root, where the numerically lower value indicates the better
   identifier.  The Bridge Priority component of the Bridge Identifier
   allows the configuration of the GADAG Root by management action.  The
   Bridge Priority is conveyed by the SPB Instance sub-TLV [RFC6329].

   The GADAG Computer MUST perform the GADAG computation as specified by
   the MRT Lowpoint Algorithm [I-D.ietf-rtgwg-mrt-frr-algorithm].  The
   GADAG Computer then MUST encode the GADAG in a Topology sub-TLV
   (Section 6.1), which is then flooded throughout the domain.  A GADAG
   is encoded in a Topology sub-TLV by means of directed ear
   decomposition as follows.  A directed ear is a directed point-to-
   point path whose end points can coincide but no other element of the
   path is repeated in the ear.  Each ear is specified by an ordered
   list of hops such that the order of hops is according to the
   direction of the arcs in the GADAG.  There are no leaves in a GADAG,
   hence, the Leaf flag (item c.5. in Section 6.2) is used to mark the
   end of a topology block.  (A GADAG with multiple blocks is
   illustrated in Figure 8.)  The sequence of ears in the Topology sub-
   TLV is such that the end points of an ear belong to preceding ears.
   The GADAG Root is not marked by any flag but the GADAG Root is the



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   first hop in the Topology sub-TLV, correspondingly the first ear
   starts and ends with the GADAG Root.  MRT Roots MUST be marked by the
   Root flag (item c.4. in Section 6.2) and all other traffic end points
   are leaves of the given MRTs.  If no MRT Root is specified, then each
   SPT Root is also an MRT Root.

   Figure 7 shows an example GADAG.  The figure also illustrates the
   description of the GADAG, it shows the System ID parameter of the Hop
   sub-TLV (Section 6.2) and the order of hops in the Topology sub-TLV
   (Section 6.1).

                                                       Leaf
                                               Hop     flag
                                          +-----------+---+
                                          |     A     |   |
                                          +-----------+---+
                                          |     B     |   |
                                          +-----------+---+
                                          |     C     |   |
                                          +-----------+---+
                                          |     F     |   |
               [B]<---[A]<---[I]          +-----------+---+
                |      ^      ^           |     A     |   |
                |      |      |           +-----------+---+
                V      |      |           |     C     |   |
               [C]--->[F]--->[H]          +-----------+---+
                |             ^           |     D     |   |
                |             |           +-----------+---+
                V             |           |     E     |   |
               [D]--->[E]--->[G]          +-----------+---+
                                          |     G     |   |
                                          +-----------+---+
                                          |     H     |   |
                                          +-----------+---+
                                          |     I     |   |
                                          +-----------+---+
                                          |     A     |   |
                                          +-----------+---+
                                          |     F     |   |
                                          +-----------+---+
                                          |     H     | X |
                                          +-----------+---+

        Figure 7: A GADAG and its description; GADAG root = Node A

   A topology can be comprised of multiple blocks, like the one
   illustrated in Figure 8(a).  This example topology is comprised of
   four blocks as each cut-link is a block.  A-B-C-D-E-F is a block, D-G



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   is another block, G-H, and H-J-K are further blocks.  The GADAG for
   this topology is shown in Figure 8(b).  Note that the GADAG includes
   two arcs for each cut-link and the direction of each arc is
   different, e.g.  D->G and G->D.  The encoding starts with the Block
   (ADAG) involving the GADAG Root as illustrated in Figure 8.  The
   first hop in the Topology sub-TLV is the GADAG Root (node A in this
   example.)  The ADAG of the first block is then described using the
   ear decomposition, as described above.  In this example, the first
   block has been completely traversed at the second occurrence of node
   A in the GADAG descriptor.  The end of a block is indicated by
   setting the Leaf flag for the last hop of the block, e.g. for the
   second occurrence of node A in the example GADAG descriptor.  The
   next node that appears in the GADAG descriptor (D in this case) is
   the localroot for the nodes in the next block.  Continuing this
   process, the Leaf flag is set for the third occurrence of D, the
   third occurrence of G, and the third occurrence of H, each indicating
   the end of a block.  The first hop of the first block is the GADAG
   Root, the fist hop in the rest of the blocks is the localroot.  The
   position of the set Leaf flags helps to determine the localroot,
   which is the next hop.  In the example GADAG descriptor, one can
   determine that A is the localroot for B,C,D,E,F (and A is the GADAG
   Root).  D is the localroot for G.  G is the localroot for H.  And H
   is the localroot for J and K.  The GADAG Root is assigned a localroot
   of None.

   Block IDs are reconstructed while parsing a Topology sub-TLV
   specifying a GADAG.  The current Block ID starts at 0 and is assigned
   to the GADAG Root.  A node appearing in the GADAG descriptor without
   a previously-assigned Block ID value is assigned the current Block
   ID.  And the current Block ID is incremented by 1 after processing
   the localroot of a block.  Note that the localroot of a block will
   keep the Block ID of the first block in which it is assigned a Block
   ID.  In the example in Figure 8, A has Block ID=0.  B, C, D, E, and F
   have Block ID=1.  G has Block ID=2.  H has Block ID=3.  J and K have
   Block ID=4.
















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                                                       Leaf
                                               Hop     flag
               [F]--[E]        |--[K]     +-----------+---+
                |    |         |   |      |     A     |   |
                |    |         |   |      +-----------+---+
               [A]  [D]--[G]--[H]  |      |     B     |   |
                |    |         |   |      +-----------+---+
                |    |         |   |      |     C     |   |
               [B]--[C]        |--[J]     +-----------+---+
                                          |     D     |   |
                    (a) topology          +-----------+---+
                                          |     E     |   |
                                          +-----------+---+
                                          |     F     |   |
                                          +-----------+---+
                                          |     A     | X |
                                          +-----------+---+
               [F]<-[E]        |--[K]     |     D     |   |
                |    ^         |   ^      +-----------+---+
                V    |         V   |      |     G     |   |
               [A]  [D]->[G]->[H]  |      +-----------+---+
                |   [D]<-[G]<-[H]  |      |     D     | X |
                |    ^         |   |      +-----------+---+
                V    |         |   |      |     G     |   |
               [B]->[C]        |->[J]     +-----------+---+
                                          |     H     |   |
                     (b) GADAG            +-----------+---+
                                          |     G     | X |
                                          +-----------+---+
                                          |     H     |   |
                                          +-----------+---+
                                          |     J     |   |
                                          +-----------+---+
                                          |     K     |   |
                                          +-----------+---+
                                          |     H     | X |
                                          +-----------+---+

                                       (c) GADAG descriptor

    Figure 8: A GADAG with cut-links and its description; GADAG root =
                                  Node A

8.  Summary

   This document specifies IS-IS sub-TLVs for the control of explicit
   trees in Layer 2 networks.  These sub-TLVs can be also used for the




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   distribution of a centrally computed GADAG or MRTs if MFT-FRR is
   used.

9.  IANA Considerations

   Five new code points are required within MT-Capability [RFC6329] for
   the five new sub-TLVs:

   o  Topology sub-TLV

   o  Hop sub-TLV

   o  Bandwidth Constraint sub-TLV

   o  Bandwidth Assignment sub-TLV

   o  Timestamp sub-TLV

10.  Security Considerations

   This document adds no additional security risks to IS-IS, nor does it
   provide any additional security for IS-IS when used in a configured
   environment or a single-operator domain such as a data center.  IS-IS
   PCR is not for zero configuration environments.

   However, if IS-IS PCR is used to record bandwidth assignments in a
   network with multiple PCEs, then race conditions can appear and the
   precedence can be resolved by Importance parameter of the Bandwidth
   Assignment sub-TLV and the Time parameter of the Timestamp sub-TLV,
   especially if the different PCEs are administered by different
   entities.

11.  Acknowledgements

   The authors would like to thank Don Fedyk and Eric Gray for their
   comments and suggestions.

12.  References

12.1.  Normative References

   [I-D.ietf-isis-te-metric-extensions]
              Previdi, S., Giacalone, S., Ward, D., Drake, J., Atlas,
              A., Filsfils, C., and W. Wu, "IS-IS Traffic Engineering
              (TE) Metric Extensions", draft-ietf-isis-te-metric-
              extensions-06 (work in progress), April 2015.





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   [I-D.ietf-rtgwg-mrt-frr-algorithm]
              Envedi, G., Csaszar, A., Atlas, A., Bowers, C., and A.
              Gopalan, "Algorithms for computing Maximally Redundant
              Trees for IP/LDP Fast- Reroute", draft-ietf-rtgwg-mrt-frr-
              algorithm-03 (work in progress), March 2015.

   [IEEE8021Qca]
              IEEE 802.1, "IEEE 802.1Qca Bridges and Bridged Networks -
              Amendment: Path Control and Reservation - Draft 1.4",
              (work in progress), February 12, 2015,
              <http://www.ieee802.org/1/pages/802.1ca.html>.

   [IEEE8021aq]
              IEEE 802.1, "IEEE 802.1aq: IEEE Standard for Local and
              metropolitan area networks - Media Access Control (MAC)
              Bridges and Virtual Bridged Local Area Networks -
              Amendment 20: Shortest Path Bridging", 2012,
              <http://standards.ieee.org/getieee802/
              download/802.1aq-2012.pdf>.

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

   [RFC5303]  Katz, D., Saluja, R., and D. Eastlake, "Three-Way
              Handshake for IS-IS Point-to-Point Adjacencies", RFC 5303,
              October 2008.

   [RFC5305]  Li, T. and H. Smit, "IS-IS Extensions for Traffic
              Engineering", RFC 5305, October 2008.

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

   [RFC6329]  Fedyk, D., Ashwood-Smith, P., Allan, D., Bragg, A., and P.
              Unbehagen, "IS-IS Extensions Supporting IEEE 802.1aq
              Shortest Path Bridging", RFC 6329, April 2012.

12.2.  Informative References

   [I-D.bowers-rtgwg-mrt-applicability-to-8021qca]
              Bowers, C. and J. Farkas, "Applicability of Maximally
              Redundant Trees to IEEE 802.1Qca Path Control and
              Reservation", draft-bowers-rtgwg-mrt-applicability-to-
              8021qca-00 (work in progress), February 2015.






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   [I-D.ietf-rtgwg-mrt-frr-architecture]
              Atlas, A., Kebler, R., Bowers, C., Envedi, G., Csaszar,
              A., Tantsura, J., and R. White, "An Architecture for IP/
              LDP Fast-Reroute Using Maximally Redundant Trees", draft-
              ietf-rtgwg-mrt-frr-architecture-05 (work in progress),
              January 2015.

   [IEEE1588]
              IEEE 1588, "IEEE 1588: IEEE Standard for a Precision Clock
              Synchronization Protocol for Networked Measurement and
              Control Systems", 2008,
              <http://standards.ieee.org/findstds/
              standard/1588-2008.html>.

   [IEEE8021Q]
              IEEE 802.1, "IEEE 802.1Q-2014: IEEE Standard for Local and
              metropolitan area networks - Bridges and Bridged
              Networks", 2014, <http://standards.ieee.org/getieee802/
              download/802.1aq-2012.pdf>.

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

Authors' Addresses

   Janos Farkas (editor)
   Ericsson
   Konyves Kalman krt. 11/B
   Budapest  1097
   Hungary

   Email: janos.farkas@ericsson.com


   Nigel Bragg
   Ciena
   43-51 Worship Street
   London  EC2A 2DX
   UK

   Email: nbragg@ciena.com










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   Paul Unbehagen Jr
   Avaya
   1300 W. 120th Avenue
   Westminster  CO 80234
   USA

   Email: unbehagen@avaya.com


   Glenn Parsons
   Ericsson
   349 Terry Fox Drive
   Ottawa  ON, K2K 2V6
   Canada

   Email: glenn.parsons@ericsson.com


   Peter Ashwood-Smith
   Huawei Technologies
   303 Terry Fox Drive, Suite 400
   Ottawa  ON, K2K 3J1
   Canada

   Email: Peter.AshwoodSmith@huawei.com


   Chris Bowers
   Juniper Networks
   1194 N. Mathilda Ave.
   Sunnyvale, CA  94089
   US

   Email: cbowers@juniper.net

















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