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INTERNET-DRAFT                                              Mingui Zhang
Intended Status: Proposed Standard                                Huawei
Updates: 6325                                         Tissa Senevirathne
                                                              Consultant
                                                    Janardhanan Pathangi
                                                                 Gigamon
                                                           Ayan Banerjee
                                                                   Cisco
                                                          Anoop Ghanwani
                                                                    DELL
Expires: July 23, 2018                                  January 19, 2018

                  TRILL: Resilient Distribution Trees
                draft-ietf-trill-resilient-trees-09.txt

Abstract

   The TRILL (Transparent Interconnection of Lots of Links) protocol
   provides multicast data forwarding based on IS-IS link state routing.
   Distribution trees are computed based on the link state information
   through Shortest Path First calculation. When a link on the
   distribution tree fails, a campus-wide re-convergence of this
   distribution tree will take place, which can be time consuming and
   may cause considerable disruption to the ongoing multicast service.

   This document specifies how to build backup distribution trees to
   protect links on the primary distribution tree. Since the backup
   distribution tree is built up ahead of the link failure, when a link
   on the primary distribution tree fails, the pre-installed backup
   forwarding table will be utilized to deliver multicast packets
   without waiting for the campus-wide re-convergence. This minimizes
   the service disruption. This document updates RFC 6325.

Status of this Memo

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

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

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

   The list of current Internet-Drafts can be accessed at



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   http://www.ietf.org/1id-abstracts.html

   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html


Copyright and License Notice

   Copyright (c) 2018 IETF Trust and the persons identified as the
   document authors. All rights reserved.

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


Table of Contents

   1. Introduction  . . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.1. Conventions used in this document . . . . . . . . . . . . .  5
     1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . .  5
   2. Usage of the Affinity Sub-TLV . . . . . . . . . . . . . . . . .  6
     2.1. Indicating Affinity Links . . . . . . . . . . . . . . . . .  6
     2.2. Distribution Tree Calculation with Affinity Links . . . . .  7
   3. Distribution Tree Calculation . . . . . . . . . . . . . . . . .  8
     3.1. Designating Roots for Backup Distribution Trees . . . . . .  8
     3.2. Backup DT Calculation with Affinity Links . . . . . . . . .  9
       3.2.1. The Algorithm for Choosing Affinity Links . . . . . . .  9
       3.2.2. Affinity Links Advertisement  . . . . . . . . . . . . . 10
   4. Resilient Distribution Trees Installation . . . . . . . . . . . 10
     4.1. Pruning the Backup Distribution Tree  . . . . . . . . . . . 11
     4.2. RPF Filters Preparation . . . . . . . . . . . . . . . . . . 12
   5. Protection Mechanisms with Resilient Distribution Trees . . . . 12
     5.1. Global 1:1 Protection . . . . . . . . . . . . . . . . . . . 13
     5.2. Global 1+1 Protection . . . . . . . . . . . . . . . . . . . 14
       5.2.1. Failure Detection . . . . . . . . . . . . . . . . . . . 14
       5.2.2. Traffic Forking and Merging . . . . . . . . . . . . . . 14
     5.3. Local Protection  . . . . . . . . . . . . . . . . . . . . . 15
       5.3.1. Starting to Use the Backup Distribution Tree  . . . . . 15
       5.3.2. Duplication Suppression . . . . . . . . . . . . . . . . 16
       5.3.3. An Example to Walk Through  . . . . . . . . . . . . . . 16
     5.4. Protection Mode Signaling . . . . . . . . . . . . . . . . . 16



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     5.5. Updating the Primary and the Backup Distribution Trees  . . 17
   6. TRILL IS-IS Extensions  . . . . . . . . . . . . . . . . . . . . 18
     6.1. Resilient Trees Extended Capability Field . . . . . . . . . 18
     6.2 Backup Tree Root APPsub-TLV  . . . . . . . . . . . . . . . . 18
   7. Security Considerations . . . . . . . . . . . . . . . . . . . . 19
   8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 19
     8.1. Resilient Tree Extended Capability Field  . . . . . . . . . 19
     8.2. Backup Tree Root APPsub-TLV . . . . . . . . . . . . . . . . 19
   Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . 19
   9. References  . . . . . . . . . . . . . . . . . . . . . . . . . . 20
     9.1. Normative References  . . . . . . . . . . . . . . . . . . . 20
     9.2. Informative References  . . . . . . . . . . . . . . . . . . 21
   Author's Addresses . . . . . . . . . . . . . . . . . . . . . . . . 22






































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1. Introduction

   Lots of multicast traffic is generated by interrupt latency sensitive
   applications, e.g., video distribution including IPTV, video
   conference and so on. Normally, a network fault will be recovered
   through a network wide re-convergence of the forwarding states, but
   this process is too slow to meet tight Service Level Agreement (SLA)
   requirements on the duration of service disruption.

   Protection mechanisms are commonly used to reduce the service
   disruption caused by network faults. With backup forwarding states
   installed in advance, a protection mechanism can restore an
   interrupted multicast stream in a much shorter time than the normal
   network wide re-convergence, which can meet stringent SLAs on service
   disruption. A protection mechanism for multicast traffic has been
   developed for IP/MPLS networks [RFC7431]. However, TRILL constructs
   distribution trees (DT) in a different way from IP/MPLS; therefore a
   multicast protection mechanism suitable for TRILL is developed in
   this document.

   This document specifies "Resilient Distribution Trees" in which
   backup trees are installed in advance for the purpose of fast failure
   repair. Three types of protection mechanisms are specified.

   o  Global 1:1 protection refers to the mechanism where the multicast
      source RBridge normally injects one multicast stream onto the
      primary DT. When an interruption of this stream is detected, the
      source RBridge switches to the backup DT to inject subsequent
      multicast streams until the primary DT is recovered.

   o  Global 1+1 protection refers to the mechanism where the multicast
      source RBridge always injects two copies of multicast streams, one
      onto the primary DT and one onto the backup DT respectively. In
      the normal case, multicast receivers pick the stream sent along
      the primary DT and egress it to its local link. When a link
      failure interrupts the primary stream, the backup stream will be
      picked until the primary DT is recovered.

   o  Local protection refers to the mechanism where the RBridge
      attached to the failed link locally repairs the failure.

   Resilient Distribution Trees can greatly reduce the service
   disruption caused by link failures. In the global 1:1 protection, the
   time cost for DT recalculation and installation can be saved. The
   global 1+1 protection and local protection further saves the time
   spent on the propagation of failure indication. Routing can be
   repaired for a failed link in tens of milliseconds.




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   Protection mechanisms to handle node failures are out the scope of
   this document. Although it's possible to use Resilient Distribution
   Trees to achieve load balancing of multicast traffic, this document
   leaves that for future study.

   [RFC7176] specifies the Affinity Sub-TLV. An "Affinity Link" can be
   explicitly assigned to a distribution tree or trees as discussed in
   Section 2.1. This offers a way to manipulate the calculation of
   distribution trees. With intentional assignment of Affinity Links, a
   backup distribution tree can be set up to protect links on a primary
   distribution tree.

   This document updates [RFC6325] as specified in Section 5.3.1.

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].

1.2. Terminology

   BFD: Bidirectional Forwarding Detection [RFC7175] [RBmBFD]

   CMT: Coordinated Multicast Trees [RFC7783]

   Child: A directly connected node further from the Root.

   DT: Distribution Tree [RFC6325]

   IS-IS: Intermediate System to Intermediate System [RFC7176]

   LSP: IS-IS Link State PDU

   mLDP: Multipoint Label Distribution Protocol [RFC6388]

   MPLS: Multi-Protocol Label Switching

   Parent: A directly connected node closer to the Root.

   PDU: Protocol Data Unit

   Root: The top node in a tree.

   PIM: Protocol Independent Multicast [RFC7761]

   PLR: Point of Local Repair. In this document, PLR is the multicast
     upstream RBridge connecting to the failed link. It's valid only for



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     local protection (Section 5.3).

   RBridge: A device implementing the TRILL protocol [RFC6325] [RFC7780]

   RPF: Reverse Path Forwarding

   SLA: Service Level Agreement

   Td: failure detection timer

   TRILL: TRansparent Interconnection of Lots of Links or Tunneled
     Routing in the Link Layer [RFC6325] [RFC7780]

2. Usage of the Affinity Sub-TLV

   This document uses the already existing Affinity Sub-TLV [RFC7176] to
   assign a parent to an RBridge in a tree as discussed below. Support
   of the Affinity Sub-TLV by an RBridge is indicated by a capability
   bit in the TRILL-VER Sub-TLV [RFC7783].

2.1. Indicating Affinity Links

   The Affinity Sub-TLV explicitly assigns parents for RBridges on
   distribution trees. It is distributed in an LSP and can be recognized
   by each RBridge in the campus. The originating RBridge becomes the
   parent and the nickname contained in the Affinity Record identifies
   the child. This explicitly provides an "Affinity Link" on a
   distribution tree or trees. The "Tree-num of roots" in the Affinity
   Record(s) in the Affinity Sub-TLV identify the distribution trees
   that adopt this Affinity Link [RFC7176].

   Suppose the link between RBridge RB2 and RBridge RB3 is chosen as an
   Affinity Link on the distribution tree rooted at RB1 in Figure 2.1.
   RB2 sends out the Affinity Sub-TLV with an Affinity Record that says
   {Nickname=RB3, Num of Trees=1, Tree-num of roots=RB1}. Different from
   the Affinity Link usage in [RFC7783], RB3 does not have to be a leaf
   node on a distribution tree. Therefore an Affinity Link can be used
   to identify any link on a distribution tree. This kind of assignment
   offers a flexibility of control to RBridges in distribution tree
   calculation: they can be directed to choose a child for which they
   are not on the shortest paths from the root. This flexibility is used
   to construct back-up trees that can be used to increase the
   reliability of distribution trees. Affinity Links may be configured
   or automatically determined according to an algorithm as described in
   this document.

   Affinity Link SHOULD NOT be misused to declare connection of two
   RBridges that are not adjacent. If it is, the Affinity Link is



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   ignored and has no effect on tree building.

2.2. Distribution Tree Calculation with Affinity Links

          Root                         Root
          +---+ -> +---+ -> +---+      +---+ -> +---+ -> +---+
          |RB1|    |RB2|    |RB3|      |RB1|    |RB2|    |RB3|
          +---+ <- +---+ <- +---+      +---+ <- +---+ <- +---+
           ^ |      ^ |      ^ |        ^ |      ^        ^ |
           | v      | v      | v        | v      |        | v
          +---+ -> +---+ -> +---+      +---+ -> +---+ -> +---+
          |RB4|    |RB5|    |RB6|      |RB4|    |RB5|    |RB6|
          +---+ <- +---+ <- +---+      +---+ <- +---+    +---+

                 Full Graph                    Sub Graph


                Root 1                       Root 1
                    / \                          / \
                   /   \                        /   \
                  4     2                      4     2
                       / \                     |     |
                      /   \                    |     |
                     5     3                   5     3
                     |                         |
                     |                         |
                     6                         6

   Shortest Path Tree of Full Graph   Shortest Path Tree of Sub Graph

       Figure 2.1: DT Calculation with the Affinity Link RB4-RB5

   When RBridges receive an Affinity Sub-TLV declaring an Affinity Link
   that is an incoming link of an RBridge (i.e., this RBridge is the
   child on this Affinity Link) for a particular distribution tree, this
   RBridge's incoming links/adjacencies other than the Affinity Link are
   removed from the full graph of the campus to get a sub graph to
   compute that tree. RBridges perform the Shortest Path First
   calculation to compute the tree based on the resulting sub graph.
   This assures that the Affinity Link appears in the distribution tree
   being calculated.

   Take Figure 2.1 as an example. Suppose RB1 is the root and link RB4-
   RB5 is the Affinity Link. RB5's other incoming links RB2-RB5 and RB6-
   RB5 are removed from the Full Graph to get the Sub Graph. Since RB4-
   RB5 is the unique link to reach RB5, the Shortest Path Tree
   inevitably contains this link.




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   Note that outgoing links/adjacencies are not affected by the Affinity
   Link. When two RBridges, say RB4 and RB5, are adjacent, the
   adjacency/link from RB4 to RB5 and the adjacency/link from RB5 to RB4
   are separate and, for example, might have different costs.

3. Distribution Tree Calculation

   RBridges use IS-IS to advertise  adjacencies and thus advertise
   network faults through the withdrawal of such adjacencies. A node or
   link failure will trigger a campus-wide re-convergence of all TRILL
   distribution trees. The re-convergence generally includes the
   following sequence of procedures:

   1. Failure (loss of adjacency) detected through IS-IS control
      messages (HELLO) not getting through or some other link test such
      as BFD [RFC7175] [RBmBFD];

   2. IS-IS state flooding so each RBridge learns about the failure;

   3. Each RBridge recalculates affected distribution trees
      independently;

   4. RPF filters are updated according to the new distribution trees.
      The recomputed distribution trees are pruned and installed into
      the multicast forwarding tables.

   The re-convergence time to go through these four steps disrupts
   ongoing multicast traffic. In protection mechanisms, alternative
   paths prepared ahead of potential node or link failures are available
   to detour around the failures upon the failure detection; thus
   service disruption can be minimized.

   This document focuses only on link failure protection. The
   construction of backup DTs (distribution trees) for the purpose of
   node protection is out of scope. (The usual way to protect from a
   node failure on the primary tree, is to have a backup tree setup
   without this node. When this node fails, the backup tree can be
   safely used to forward multicast traffic to make a detour. However,
   TRILL distribution trees are shared among all VLANs and Fine Grained
   Labels [RFC7172] and they have to cover all RBridge nodes in the
   campus [RFC6325]. A DT that does not span all RBridges in the campus
   may not cover all receivers of many multicast groups. (This is
   different from the multicast trees construction signaled by PIM
   (protocol independent multicast [RFC7761]) or mLDP (multicast label
   distribution protocol [RFC6388].))

3.1. Designating Roots for Backup Distribution Trees




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   The RBridge, say, RB1, having the highest root priority nickname
   controls the creation of backup DTs and specifies their roots. It
   explicitly advertises a list of nicknames identifying the roots of
   primary and their backup DTs using the Backup Tree APPsub-TLV as
   specified in Section 6.2 (See also Section 4.5 of [RFC6325]). It's
   possible that a backup DT and a primary DT have the same root RBridge
   but this is not required. In that case, to distinguish the primary DT
   and the backup DT for the common root case, the root RBridge MUST own
   at least two nicknames so a different nickname can be used to name
   each tree.

   The method by which the highest priority root RBridge determines
   which primary distribution trees to protect with a backup and what
   the root of each such back up will be is out of scope for this
   document.

3.2. Backup DT Calculation with Affinity Links

                          2                  1
                         /                    \
                   Root 1___                ___2 Root
                       /|\  \              /  /|\
                      / | \  \            /  / | \
                     3  4  5  6          3  4  5  6
                     |  |  |  |           \/    \/
                     |  |  |  |           /\    /\
                     7  8  9  10         7  8  9  10

                      Primary DT          Backup DT

        Figure 3.1: An Example of a Primary DT and its Backup DT

   TRILL supports the computation of multiple distribution trees by
   RBridges. With the intentional assignment of Affinity Links in DT
   calculation, this document specifies a method to construct Resilient
   Distribution Trees. For example, in Figure 3.1, the backup DT is set
   up to be maximally disjoint to the primary DT. (The full topology is
   a combination of these two DTs, which is not shown in the figure.)
   Except for the link between RB1 and RB2, all other links on the
   primary DT do not overlap with any link on the backup DT. Thus every
   link on the primary DT, except link RB1-RB2, is protected by the
   backup DT.

3.2.1. The Algorithm for Choosing Affinity Links

   Operators MAY configure Affinity Links, for example, to intentionally
   protect a specific link such as the link connected to a gateway. But
   it is desirable that every RBridge independently computes Affinity



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   Links for a backup DT across the whole campus. This enables a
   distributed deployment and also minimizes configuration.

   Compared to the algorithms for Maximally Redundant Trees in
   [RFC7811], TRILL has both an advantage and a disadvantage. An
   advantage of TRILL is that Resilient Distribution Tree does not
   restrict the root of the backup DT to be the same as that of the
   primary DT. Two disjoint (or maximally disjoint) trees may have
   different root nodes, which significantly augments the solution
   space.

   A disadvantage of TRILL, when using the algorithm specified below in
   this section is that the backup DT is computed with reference to the
   primary tree but there may be a pair of tree that is more disjoint
   than any backup tree can be with the particular primary tree.

   This document RECOMMENDS achieving the independent backup tree
   determination method through a change to the conventional DT
   calculation process of TRILL. After the primary DT is calculated,
   every RBridge will be aware of which links are used in that primary
   tree. When the backup DT is calculated, each RBridge increases the
   metric of these links by the summation of all original link metrics
   in the campus but not more than 2**23, which gives these links a
   lower priority of being chosen for the backup DT by the Shortest Path
   First calculation. All links on this backup DT can be assigned as
   Affinity Links but this may not be necessary. In order to reduce the
   amount of Affinity Sub-TLVs flooded across the campus, only those NOT
   picked by the conventional DT calculation process SHOULD be announced
   as Affinity Links.

3.2.2. Affinity Links Advertisement

   Similar to [RFC7783], every parent RBridge of an Affinity Link takes
   charge of announcing this link in an Affinity Sub-TLV. When this
   RBridge plays the role of parent RBridge for several Affinity Links,
   it is natural to have them advertised together in the same Affinity
   Sub-TLV, and each Affinity Link is structured as one Affinity Record
   [RFC7176].

   Affinity Links are announced in the Affinity Sub-TLV that is
   recognized by every RBridge. Since each RBridge computes distribution
   trees as the Affinity Sub-TLV requires, the backup DT will be built
   consistently by all RBridges in the campus.

4. Resilient Distribution Trees Installation

   As specified in Section 4.5.2 of [RFC6325], an ingress RBridge MUST
   announce the distribution trees it may choose to ingress multicast



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   frames. Thus other RBridges in the campus can limit the amount of
   state necessary for RPF checks. Also, [RFC6325] recommends that an
   ingress RBridge by default chooses the DT or DTs whose root or roots
   are least cost from the ingress RBridge. To sum up, RBridges do pre-
   compute all the trees that might be used so they can properly forward
   multi-destination packets, but only install RPF state for some
   combinations of ingress and tree.

   This document specifies that the backup DT MUST be included in an
   ingress RBridge's DT announcement list in this ingress RBridge's LSP
   if the corresponding primary tree is included. In order to reduce the
   service disruption time, RBridges SHOULD install backup DTs in
   advance, which also includes the RPF filters that need to be set up
   for RPF Checks.

   Since the backup DT is intentionally built highly disjoint to the
   primary DT, when a link fails and interrupts the ongoing multicast
   traffic sent along the primary DT, it is probable that the backup DT
   is not affected. Therefore, the backup DT installed in advance can be
   used to deliver multicast packets immediately.

4.1. Pruning the Backup Distribution Tree

   The way that a backup DT is pruned is different from the way that the
   primary DT is pruned. To enable protection it is possible that a
   branch should not be pruned (see Section 4.5.3 of [RFC6325]), even
   though it does not have any downstream receivers for a particular
   data label. The rule for backup DT pruning is that the backup DT
   should be pruned, eliminating branches that have no potential
   downstream RBridges which appear on the pruned primary DT.

   Even though the primary DT may not be optimally pruned in practice,
   the backup DT SHOULD always be pruned as if the primary DT is
   optimally pruned. Those redundant links that ought to be pruned on
   the primary DT will not be protected.

                                              1
                                               \
                    Root 1___                ___2 Root
                        / \  \              /  /|\
                       /   \  \            /  / | \
                      3     5  6          3  4  5  6
                      |     |  |            /    \/
                      |     |  |           /     /\
                      7     9  10         7     9  10
                    Pruned Primary DT   Pruned Backup DT

  Figure 4.1: The Backup DT is Pruned Based on the Pruned Primary DT.



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   Suppose RB7, RB9 and RB10 constitute a multicast group MGx. The
   pruned primary DT and backup DT are shown in Figure 4.1. Referring
   back to Figure 3.1, branches RB2-RB1 and RB4-RB1 on the primary DT
   are pruned for the distribution of MGx traffic since there are no
   potential receivers on these two branches. Although branches RB1-RB2
   and RB3-RB2 on the backup DT have no potential multicast receivers,
   they appear on the pruned primary DT and may be used to repair link
   failures of the primary DT. Therefore they are not pruned from the
   backup DT. Branch RB8-RB3 can be safely pruned because it does not
   appear on the pruned primary DT.

4.2. RPF Filters Preparation

   RB2 announces in its LSP the trees RB2 might choose when RB2
   ingresses a multicast packet [RFC6325]. When RB2 specifies such
   trees, it SHOULD include the backup DT. Other RBridges will prepare
   the RPF check states for both the primary DT and backup DT. When a
   multicast packet is sent along either the primary DT or the backup
   DT, it will be subject to the RPF Check. This works when global 1:1
   protection is used. However, when global 1+1 protection or local
   protection is applied, traffic duplication will happen if multicast
   receivers accept both copies of the multicast packets from two RPF
   filters. In order to avoid such duplication, egress RBridge multicast
   receivers MUST act as merge points to activate a single RPF filter
   and discard the duplicated packets from the other RPF filter. In the
   normal case, the RPF state is set up according to the primary DT.
   When a link failure on the primary DT is detected, the egress node
   RPF filter based on the backup DT should be activated.

5. Protection Mechanisms with Resilient Distribution Trees

   Protection mechanisms make use of the backup DT installed in advance.
   Protection mechanisms developed using PIM or mLDP for multicast in
   IP/MPLS networks are not applicable to TRILL due to the following
   fundamental differences in their distribution tree calculation.

   o  The link on a TRILL distribution tree is always bidirectional
      while the link on a distribution tree in IP/MPLS networks may be
      unidirectional.

   o  In TRILL, a multicast source node does not have to be the root of
      the distribution tree. It is just the opposite in IP/MPLS
      networks.

   o  In IP/MPLS networks, distribution trees are constructed for each
      multicast source node as well as their backup distribution trees.
      In TRILL, a small number of core distribution trees are shared
      among multicast groups. A backup DT does not have to share the



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      same root as the primary DT.

   Therefore a TRILL specific multicast protection mechanism is needed.

   Global 1:1 protection, global 1+1 protection and local protection are
   described in this section. In Figure 4.1, assume RB7 is the ingress
   RBridge of the multicast stream while RB9 and RB10 are the multicast
   receivers. Suppose link RB1-RB5 fails during the multicast
   forwarding. The backup DT rooted at RB2 does not include link RB1-
   RB5, therefore it can be used to protect this link. In global 1:1
   protection, RB7 will switch the subsequent multicast traffic to this
   backup DT when it's notified of the link failure. In the global 1+1
   protection, RB7 will inject two copies of the multicast stream and
   let multicast receivers RB9 and RB10 choose which copy would be
   delivered. In the local protection, when link RB1-RB5 fails, RB1 will
   locally replicate the multicast traffic and send it on the backup DT.

   The type of protection in use at an RBridge is indicated by a two-bit
   field in that RBridge's Extended Capability TLV as discussed in
   Section 5.4.

5.1. Global 1:1 Protection

   In the global 1:1 protection, the ingress RBridge of the multicast
   traffic is responsible for switching the failure affected traffic
   from the primary DT over to the backup DT. Since the backup DT has
   been installed in advance, the global protection need not wait for
   the DT recalculation and installation. When the ingress RBridge is
   notified about the failure, it immediately makes this switch over.

   This type of protection is simple and duplication safe. However,
   depending on the topology of the RBridge campus, the time spent on
   the failure detection and propagation through the IS-IS control plane
   may still cause a considerable service disruption.

   BFD (Bidirectional Forwarding Detection) protocol can be used to
   reduce the failure detection time. Link failures can be rapidly
   detected with one-hop BFD [RFC7175]. [RBmBFD] introduces the fast
   failure detection of multicast paths. It can be used to reduce both
   the failure detection and the propagation time for global protection.
   In [RBmBFD], the ingress RBridge needs to send BFD control packets to
   poll each receiver, and receivers return BFD control packets to the
   ingress as the response. If no response is received from a specific
   receiver for a detection time, the ingress can judge that the
   connectivity to this receiver is broken. Therefore, [RBmBFD] is used
   to detect the connectivity of a path rather than a link. The ingress
   RBridge will determine a minimum failed branch that contains this
   receiver. The ingress RBridge will switch ongoing multicast traffic



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   based on this judgment. For example, in Figure 4.1, if RB9 does not
   respond while RB10 still responds, RB7 will presume that link RB1-RB5
   and RB5-RB9 are failed. Multicast traffic will be switched to a
   backup DT that can protect these two links. More accurate link
   failure detection might help ingress RBridges make smarter decision
   but it's out of the scope of this document.

5.2. Global 1+1 Protection

   In the global 1+1 protection, the multicast source RBridge always
   replicates the multicast packets and sends them onto both the primary
   and backup DT. This may sacrifice the capacity efficiency but given
   there is much connection redundancy and inexpensive bandwidth in Data
   Center Networks, such kind of protection can be popular [RFC7431].

5.2.1. Failure Detection

   Egress RBridges (merge points) SHOULD realize the link failure as
   early as practical and update their RPF filters quickly to minimize
   the traffic disruption. Three options are provided as follows.

   1. If you had a very reliable and steady data stream, egress RBridges
      assume a minimum known packet rate for that data stream [RFC7431].
      A failure detection timer (say Td) is set as the interval between
      two continuous packets. Td is reinitialized each time a packet is
      received. If Td expires and packets are arriving at the egress
      RBridge on the backup DT (within the time frame Td), it updates
      the RPF filters and starts to receive packets forwarded on the
      backup DT.  This method requires configuration at the egress
      RBridge of Td and of some method (filter) to determine if a packet
      is part of the reliable data stream. Since the filtering
      capabilities of various fast path logic differs greatly, specifics
      of such configuration are outside the scope of this document.

   2. With multi-point BFD [RBmBFD], when a link failure happens,
      affected egress RBridges can detect a lack of connectivity from
      the ingress. Therefore these egress RBridges are able to update
      their RPF filters promptly.

   3. Egress RBridges can always rely on the IS-IS control plane to
      learn the failure and determine whether their RPF filters should
      be updated.

5.2.2. Traffic Forking and Merging

   For the sake of protection, transit RBridges SHOULD activate both
   primary and backup RPF filters, therefore both copies of the
   multicast packets will pass through transit RBridges.



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   Multicast receivers (egress RBridges) MUST act as "merge points" to
   egress only one copy of each multicast packet. This is achieved by
   the activation of only a single RPF filter. In the normal case,
   egress RBridges activate the primary RPF filter. When a link on the
   pruned primary DT fails, the ingress RBridge cannot reach some of the
   receivers. When these unreachable receivers realize the link failed,
   they SHOULD update their RPF filters to receive packets sent on the
   backup DT.

   Note that the egress RBridge need not be a literal merge point, that
   is receiving the primary and backup DT versions over different links.
   Even if the egress RBridge receives both copies over the same link,
   because disjoint links are not available, it can still filter out one
   copy because the RFP filtering logic is designed to test which tree
   the packet is on as indicated by a field in the TRILL Header
   [RFC6325].

5.3. Local Protection

   In the local protection, the Point of Local Repair (PLR) happens at
   the upstream RBridge connected to the failed link. It is this RBridge
   that makes the decision to replicate the multicast traffic to recover
   from this link failure. Local protection can further save the time
   spent on failure notification through the flooding of LSPs across the
   TRILL campus. In addition, the failure detection can be sped up using
   BFD [RFC7175], therefore local protection can minimize the service
   disruption, typically reducing it to less than 50 milliseconds.

   Since the ingress RBridge is not necessarily the root of the
   distribution tree in TRILL, a multicast downstream point may not be
   the descendant of the ingress point on the distribution tree.

   Due to the multi-destination RPF check in TRILL, local protection can
   only be used at a fork point where the primary and backup trees
   diverge and the set of nodes downstream is identical for both paths.
   If these conditions do not apply, local protection MUST NOT be used.

5.3.1. Starting to Use the Backup Distribution Tree

   The egress nickname TRILL Header field of the replicated multicast
   TRILL data packets specifies the tree on which they are being
   distributed. This field will be rewritten to the backup DT's root
   nickname by the PLR. But the ingress nickname field of the multicast
   TRILL Data packet MUST remain unchanged. The PLR forwards all
   multicast traffic with the backup DT egress nickname along the backup
   DT. This updates [RFC6325] which specifies that the egress nickname
   in the TRILL header of a multi-destination TRILL data packet must not
   be changed by transit RBridges.



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   In the above example, the PLR RB1 locally decides to send replicated
   multicast packets according to the backup DT. It will send them to
   the next hop RB2.

5.3.2. Duplication Suppression

   When a PLR starts to send replicated multicast packets on the backup
   DT, some multicast packets are still being sent along the primary DT.
   Some egress RBridges might receive duplicated multicast packets. The
   traffic forking and merging method in the global 1+1 protection can
   be adopted to suppress the duplication.

5.3.3. An Example to Walk Through

   The example used to illustrate the above local protection is put
   together to get a whole "walk through" below.

   In the normal case, multicast frames ingressed by RB7 in Figure 4.1
   with pruned distribution on the primary DT rooted at RB1 are being
   received by RB9 and RB10. When the link RB1-RB5 fails, the PLR RB1
   begins to replicate and forward subsequent multicast packets using
   the pruned backup DT rooted at RB2. When RB2 gets the multicast
   packets from the link RB1-RB2, it accepts them since the RPF filter
   {DT=RB2, ingress=RB7, receiving links=RB1-RB2, RB3-RB2, RB4-RB2, RB5-
   RB2 and RB6-RB2} is installed on RB2. RB2 forwards the replicated
   multicast packets to its neighbors except RB1. The multicast packets
   reach RB6 where both RPF filters {DT=RB1, ingress=RB7, receiving
   link=RB1-RB6} and {DT=RB2, ingress=RB7, receiving links=RB2-RB6 and
   RB9-RB6} are active. RB6 will let both multicast streams through.
   Multicast packets will finally reach RB9 where the RPF filter is
   updated from {DT=RB1, ingress=RB7, receiving link=RB5-RB9} to
   {DT=RB2, ingress=RB7, receiving link=RB6-RB9}. RB9 will egress the
   multicast packets from the Backup Distribution Tree on to the local
   link and drop those from the Primary Distribution Tree based on the
   reverse path forwarding filter.

5.4. Protection Mode Signaling

   The desired mode of resilient tree operation for each RBridge is
   chosen by the network operator and configured on that RBridge. This
   mode is announced by each RBridge is a two-bit Resilient Tree Mode
   field in their Extended Capabilities TLV (see Sections 6.1, 8.1). The
   values of this field have the following meanings:

     Value  Short Name   Effect
     -----  ----------   ------
       00   No support   If any RBridge does not support Resilient
                         Trees, then the Resilient Tree mechanism is



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                         disabled in all RBridges. This also applies if
                         any RBridge does not announce an Extended
                         Capabilities TLV.
       01   Global 1:1   An RBridge advertising this value will, when it
                         ingresses a multi-destination frames, send them
                         on only one of the primary and backup DTs. All
                         other RBridges set their RPF filters to accept
                         traffic on both trees from this ingress.
       10   Global 1+1   An RBridge advertising this value will, when it
                         ingresses a multi-destination frames, send them
                         on both the primary and backup DTs. All other
                         RBridges MUST set their RPF filters to accept
                         Traffic only on the primary or backup DT.
       11   1+1 & Local  An RBridge advertising this value acts as an
                         for the value 01 above when it is the ingress
                         RBridge. In addition, if it is a transit
                         RBridge at a fork point between the primary and
                         backup tress and detects that an adjacency has
                         failed, it diverts multi-destination TRILL data
                         packts on the primary tree to the backup tree,
                         changing the tree id in the packet to the
                         backup tree.

5.5. Updating the Primary and the Backup Distribution Trees

   Assume an RBridge receives the LSP that indicates a link failure.
   This RBridge starts to calculate the new primary DT based on the new
   topology with the failed link excluded. Suppose the new primary DT is
   installed at t1.

   The propagation of LSPs around the campus will take some time. For
   safety, we assume all RBridges in the campus will have converged to
   the new primary DT at t1+Ts. By default, Ts (the "settling time") is
   set to 30 seconds but it is configurable in seconds from 1 to 100. At
   t1+Ts, the ingress RBridge switches the traffic from the backup DT
   back to the new primary DT.

   After another Ts (at t1+2*Ts), no multicast packets are being
   forwarded along the old primary DT. The backup DT should be updated
   (recalculated and reinstalled) after the new primary DT. The process
   of this update under different protection types are discussed as
   follows.

   a) For the global 1:1 protection, the backup DT is simply updated at
      t1+2*Ts.

   b) For the global 1+1 protection, the ingress RBridge stops
      replicating the multicast packets onto the old backup DT at t1+Ts.



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      The backup DT is updated at t1+2*Ts. The ingress RBridge MUST wait
      for another Ts, during which time period all RBridges converge to
      the new backup DT. At t1+3*Ts, it's safe for the ingress RBridge
      to start to replicate multicast packets onto the new backup DT.

   c) For the local protection, the PLR stops replicating and sending
      packets on the old backup DT at t1+Ts. It is safe for RBridges to
      start updating the backup DT at t1+2*Ts.

6. TRILL IS-IS Extensions

   This section lists extensions to TRILL IS-IS to support resilient
   trees.

6.1. Resilient Trees Extended Capability Field

   An RBridge that supports the facilities specified in this document
   MUST announce the Extended RBridge Capabilities APPsub-TLV [RFC7782]
   with a non-zero value in the Resilient Trees field. If there are
   RBridges that do not announce field set to a non-zero value, all
   RBridges of the campus MUST disable the Resilient Distribution Tree
   mechanism as defined in this document and fall back to the
   distribution tree calculation algorithm as specified in [RFC6325].

6.2 Backup Tree Root APPsub-TLV

   The structure of the Backup Tree Root APPsub-TLV is shown below.

      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | Type = tbd2                   |  (2 bytes)
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | Length                        |  (2 bytes)
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | Primary Tree Root Nickname    |  (2 bytes)
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | Backup Tree Root Nickname     |  (2 bytes)
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      o  Type = Backup Tree Root APPsubTLV type, set to tbd2

      o  Length = 4, if the length is any other value, the APPsub-TLV is
         corrupt and MUST be ignored.

      o  Primary Tree Root Nickname = the nickname of the root RBridge
         of the primary tree for which a resilient backup tree is being
         created

      o  Backup Tree Root Nickname = the nickname of the root RBridge of



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         the backup tree

   If either nickname is not the nickname of a tree whose calculation is
   being directed by the highest priority tree root RBridge, the APPsub-
   TLV is ignored. This APPsub-TLV MUST be advertised by the highest
   priority RBridge to be a tree root. Backup Tree Root APPsub-TLVs
   advertised by other RBridges are ignored. If there are two or more
   Backup Tree Root APPsub-TLVs for the same primary tree specifying
   different backup trees, then the one specifying the lowest magnitude
   backup tree root nickname is used, treating nicknames as unsigned 16-
   bit quantities.

7. Security Considerations

   This document raises no new security issues for TRILL. The IS-IS PDUs
   used to transmit the information specified in Section 6 can be
   secured with IS-IS security [RFC5310].

   For general TRILL Security Considerations, see [RFC6325].

8. IANA Considerations

   The Affinity Sub-TLV has already been defined in [RFC7176]. This
   document does not change its definition. See below for IANA Actions.

8.1. Resilient Tree Extended Capability Field

   IANA will assign two adjacent bits (Sections 5.4, 6.1) in the
   Extended RBridge Capabilities subregistry on the TRILL Parameters
   page  to form the Resilient Tree Extended Capability field and change
   the heading of the "Bit" column to be "Bit(s)", adding the following
   to the registry [for example, tbd1 could be "2-3"]:

   Bit   Mnemonic Description             Reference
   ----  -------- -----------             ---------
   tbd1   RT      Resilient Tree Support  [this document]

8.2. Backup Tree Root APPsub-TLV

   IANA will assign and APPsub-TLV type under IS-IS TLV 251 Application
   Identifier 1 on the TRILL Parameters page from the range below 255
   for the Backup Tree Root APPsub-TLV (Section 6.2) as follows:

   Type   Name              Reference
   ----  ----------------  ---------------
   tbd2  Backup Tree Root  [this document]

Acknowledgements



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   The careful review from Gayle Noble is gracefully acknowledged. The
   authors would like to thank the comments and suggestions from Donald
   Eastlake, Erik Nordmark, Fangwei Hu, Gayle Noble, Hongjun Zhai and
   Xudong Zhang.

9. References

9.1. Normative References


   [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
             Requirement Levels", BCP 14, RFC 2119, DOI
             10.17487/RFC2119, March 1997, <http://www.rfc-
             editor.org/info/rfc2119>.

   [RFC7176] Eastlake 3rd, D., Senevirathne, T., Ghanwani, A., Dutt, D.,
             and A. Banerjee, "Transparent Interconnection of Lots of
             Links (TRILL) Use of IS-IS", RFC 7176, DOI
             10.17487/RFC7176, May 2014, <http://www.rfc-
             editor.org/info/rfc7176>.

   [RFC7783] Senevirathne, T., Pathangi, J., and J. Hudson, "Coordinated
             Multicast Trees (CMT) for Transparent Interconnection of
             Lots of Links (TRILL)", RFC 7783, DOI 10.17487/RFC7783,
             February 2016, <http://www.rfc-editor.org/info/rfc7783>.

   [RFC6325] Perlman, R., Eastlake 3rd, D., Dutt, D., Gai, S., and A.
             Ghanwani, "Routing Bridges (RBridges): Base Protocol
             Specification", RFC 6325, DOI 10.17487/RFC6325, July 2011,
             <http://www.rfc-editor.org/info/rfc6325>.

   [RFC7761] Fenner, B., Handley, M., Holbrook, H., Kouvelas, I.,
             Parekh, R., Zhang, Z., and L. Zheng, "Protocol Independent
             Multicast - Sparse Mode (PIM-SM): Protocol Specification
             (Revised)", STD 83, RFC 7761, DOI 10.17487/RFC7761, March
             2016, <http://www.rfc-editor.org/info/rfc7761>.

   [RFC6388] Wijnands, IJ., Ed., Minei, I., Ed., Kompella, K., and B.
             Thomas, "Label Distribution Protocol Extensions for Point-
             to-Multipoint and Multipoint-to-Multipoint Label Switched
             Paths", RFC 6388, DOI 10.17487/RFC6388, November 2011,
             <http://www.rfc-editor.org/info/rfc6388>.

   [RBmBFD]  M. Zhang, S. Pallagatti and V. Govindan, "TRILL Support of
             Point to Multipoint BFD", draft-ietf-trill-p2mp-bfd, work
             in progress.

   [RFC7175] Manral, V., Eastlake 3rd, D., Ward, D., and A. Banerjee,



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             "Transparent Interconnection of Lots of Links (TRILL):
             Bidirectional Forwarding Detection (BFD) Support", RFC
             7175, DOI 10.17487/RFC7175, May 2014, <http://www.rfc-
             editor.org/info/rfc7175>.

   [RFC7780] Eastlake 3rd, D., Zhang, M., Perlman, R., Banerjee, A.,
             Ghanwani, A., and S. Gupta, "Transparent Interconnection of
             Lots of Links (TRILL): Clarifications, Corrections, and
             Updates", RFC 7780, DOI 10.17487/RFC7780, February 2016,
             <http://www.rfc-editor.org/info/rfc7780>.

   [RFC7782] Zhang, M., Perlman, R., Zhai, H., Durrani, M., and S.
             Gupta, "Transparent Interconnection of Lots of Links
             (TRILL) Active-Active Edge Using Multiple MAC Attachments",
             RFC 7782, DOI 10.17487/RFC7782, February 2016,
             <http://www.rfc-editor.org/info/rfc7782>.

   [RFC5310] Bhatia, M., Manral, V., Li, T., Atkinson, R., White, R.,
             and M. Fanto, "IS-IS Generic Cryptographic Authentication",
             RFC 5310, DOI 10.17487/RFC5310, February 2009,
             <http://www.rfc-editor.org/info/rfc5310>.

9.2. Informative References

   [RFC7811] Enyedi, G., Csaszar, A., Atlas, A., Bowers, C., and A.
             Gopalan, "An Algorithm for Computing IP/LDP Fast Reroute
             Using Maximally Redundant Trees (MRT-FRR)", RFC 7811, DOI
             10.17487/RFC7811, June 2016, <http://www.rfc-
             editor.org/info/rfc7811>.

   [RFC7431] Karan, A., Filsfils, C., Wijnands, IJ., Ed., and B.
             Decraene, "Multicast-Only Fast Reroute", RFC 7431, DOI
             10.17487/RFC7431, August 2015, <http://www.rfc-
             editor.org/info/rfc7431>.

   [mBFD]    D. Katz, D. Ward, "BFD for Multipoint Networks", draft-
             ietf-bfd-multipoint, work in progress.

   [RFC7172] Eastlake 3rd, D., Zhang, M., Agarwal, P., Perlman, R., and
             D. Dutt, "Transparent Interconnection of Lots of Links
             (TRILL): Fine-Grained Labeling", RFC 7172, DOI
             10.17487/RFC7172, May 2014, <http://www.rfc-
             editor.org/info/rfc7172>.








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Author's Addresses

   Mingui Zhang
   Huawei Technologies Co.,Ltd
   Huawei Building, No.156 Beiqing Rd.
   Beijing 100095 P.R. China

   Email: zhangmingui@huawei.com


   Tissa Senevirathne
   Consultant

   Email: tsenevir@gmail.com


   Janardhanan Pathangi
   Gigamon

   Email: path.jana@gmail.com


   Ayan Banerjee
   Cisco
   170 West Tasman Drive
   San Jose, CA 95134 USA

   Email: ayabaner@cisco.com


   Anoop Ghanwani
   Dell
   350 Holger Way
   San Jose, CA 95134

   Phone: +1-408-571-3500
   Email: Anoop@alumni.duke.edu














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