INTERNET-DRAFT Mingui Zhang Intended Status: Proposed Standard Huawei Expires: January 1, 2013 Tissa Senevirathne CISCO Janardhanan Pathangi DELL Ayan Banerjee June 30, 2012 TRILL Resilient Distribution Trees draft-zhang-trill-resilient-trees-00.txt Abstract TRILL protocol provides layer 2 multicast data forwarding using IS-IS link state routing. Distribution trees are computed based on the link state information through Shortest Path First calculation and shared among VLANs across the campus. When a link on the distribution tree fails, a campus-wide recovergence of this distribution tree will take place, which can be time consuming and may cause considerable disruption to the ongoing multicast service. This document makes use of Affinity TLVs to build the backup distribution tree 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 recovergence, which minimizes the service disruption. 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 http://www.ietf.org/1id-abstracts.html The list of Internet-Draft Shadow Directories can be accessed at Zhang, et al. Expires January 1, 2013 [Page 1] INTERNET-DRAFT Resilient Distribution Trees June 30, 2012 http://www.ietf.org/shadow.html Copyright and License Notice Copyright (c) 2012 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal 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 Affinity TLV . . . . . . . . . . . . . . . . . . . . . 5 2.1. Allocating Affinity Links . . . . . . . . . . . . . . . . . 5 2.2. Distribution Tree Calculation with Affinity Links . . . . . 6 3. Resilient Distribution Trees Calculation . . . . . . . . . . . 7 3.1. Algorithm for Choosing Affinity Links . . . . . . . . . . . 8 3.2. Affinity Links Advertisement . . . . . . . . . . . . . . . 9 4. Resilient Distribution Trees Installation . . . . . . . . . . . 9 4.1. Pruning the Backup Distribution Tree . . . . . . . . . . . 10 4.2. RPF Filters Preparation . . . . . . . . . . . . . . . . . . 10 5. Protection Mechanisms with Resilient Distribution Trees . . . . 11 5.1. Global 1:1 Protection . . . . . . . . . . . . . . . . . . . 11 5.2. Global 1+1 Protection . . . . . . . . . . . . . . . . . . . 12 5.2.1. Failure Detection . . . . . . . . . . . . . . . . . . . 12 5.2.2. Traffic Forking and Merging . . . . . . . . . . . . . . 13 5.3. Local Protection . . . . . . . . . . . . . . . . . . . . . 13 5.3.1. Start Using Backup Distribution Tree . . . . . . . . . 14 5.3.2. Duplication Suppression . . . . . . . . . . . . . . . . 14 5.3.3. An Example to Walk Through . . . . . . . . . . . . . . 14 5.4. Back to the Primary Distribution Tree . . . . . . . . . . . 15 6. Security Considerations . . . . . . . . . . . . . . . . . . . . 15 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 15 8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 16 8.1. Normative References . . . . . . . . . . . . . . . . . . . 16 8.2. Informative References . . . . . . . . . . . . . . . . . . 16 Zhang, et al. Expires January 1, 2013 [Page 2] INTERNET-DRAFT Resilient Distribution Trees June 30, 2012 Author's Addresses . . . . . . . . . . . . . . . . . . . . . . . . 18 Zhang, et al. Expires January 1, 2013 [Page 3] INTERNET-DRAFT Resilient Distribution Trees June 30, 2012 1. Introduction Lots of multicast traffic is generated by interrupt latency sensitive applications, e.g., video distribution, including IP-TV, video conference and so on. Normally, network fault will be recovered through a network wide reconvergence of the forwarding states but this process is too slow to meet the tight SLA requirements on the service disruption duration. What is worse, updating multicast forwarding states may take significantly longer than unicast convergence since multicast states are updated based on control-plane signaling [mMRT]. Protection mechanisms are commonly used to reduce the service disruption caused by network fault. With backup forwarding states installed in advance, a protection mechanism is possible to restore a interrupted multicast stream in tens of milliseconds which guarantees the stringent SLA on service disruption. Several protection mechanisms for multicast traffic have been developed for IP/MPLS networks [mMRT] [MoFRR]. However, the way TRILL constructs distribution trees (DT) is different from the way multicast trees are computed under IP/MPLS therefore a multicast protection mechanism suitable for TRILL is required. [6326bis] defines the Affinity TLV. An "Affinity Link" can be explicitly assigned to a distribution tree or trees. 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. Based on this, this document proposes "Resilient Distribution Trees (RDT)" in which backup trees are installed in advance for the purpose of fast failure repair. In this way, resilience is built into the distribution tree calculation. This document makes use of RDT to realize link protection while node protection is out of its scope. Three types of protection mechanisms are proposed. Global 1:1 protection is used to refer to the mechanism having the multicast source RBridge normally injects one multicast stream onto the primary DT. When this stream is detected to be interrupted, the source RBridge switches to the backup DT to inject subsequent multicast stream until the primary DT is recovered. Global 1+1 protection is used to refer to the mechanism having the multicast source RBridge always injects two copies of multicast streams onto the primary DT and backup DT respectively. In 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 one will be picked until the primary DT is recovered. Local protection refers to the mechanism having the RBridge attached to the failed link to locally repair the failure. Zhang, et al. Expires January 1, 2013 [Page 4] INTERNET-DRAFT Resilient Distribution Trees June 30, 2012 RDT may greatly reduce the service disruption caused by link failures. In the global 1:1 protection, the time cost by DT recalculation and installation can be saved. The global 1+1 protection and local protection further save the time spent on failure propagation. A failed link can be repaired in tens of milliseconds. Although it's possible to make use of RDT to achieve load balance of multicast traffic, this document leaves it behind for future study. 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 IS-IS: Intermediate System to Intermediate System TRILL: TRansparent Interconnection of Lots of Links DT: Distribution Tree RPF: Reverse Path Forwarding RDT: Resilient Distribution Tree SPF: Shortest Path First SPT: Shortest Path Tree PRB: the Parent RBridge attached to a link on a distribution tree PLR: Point of Local Repair, in this document, it is the multicast upstream RBridge connecting the failed link. It's valid only for local protection. 2. Usage of Affinity TLV The Affinity TLV is currently only used to assign parents for leaf nodes [6326bis]. This document expands the scope of its usage to assign a parent to a non-leaf RBridge without changing the definition of this TLV. 2.1. Allocating Affinity Links Affinity TLV explicitly assigns parents for RBridges on distribution trees. They are advertised in the Affinity TLV and recognized by each Zhang, et al. Expires January 1, 2013 [Page 5] INTERNET-DRAFT Resilient Distribution Trees June 30, 2012 RBridge in the campus. The originating RBridge becomes the parent and the nickname contained in the Affinity Record identifies the child, which explicitly provides an "Affinity Link" on a distribution tree or trees. The "Tree-num of roots" of the Affinity Record identify the distribution trees that adopt this Affinity Link [6326bis]. Affinity Links may be configured or automatically determined using a certain algorithm [CMT]. Suppose link RB2-RB3 is chosen as an Affinity Link on the distribution tree rooted at RB1. RB2 should send out the Affinity TLV with an Affinity Record like {Nickname=RB3, Num of Trees=1, Tree-num of roots=RB1}. In this document, 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 to RBridges in distribution tree calculation: they are allowed to choose parents not on the shortest paths to the root. This flexibility is leveraged to increase the reliability of distribution trees in this document. 2.2. Distribution Tree Calculation with Affinity Links When RBridges receive an Affinity Link which is an incoming link of RB2. RB2's incoming links other than the Affinity Link are removed from the full graph of the campus to get a sub graph. RBridges perform Shortest Path First (SPF) calculation to compute the distribution tree based on the sub graph. In this way, the Affinity Link will appear on the distribution tree. Zhang, et al. Expires January 1, 2013 [Page 6] INTERNET-DRAFT Resilient Distribution Trees June 30, 2012 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 SPT of Full Graph SPT of Sub Graph Figure 2.1: DT Calculation with the Affinity Link RB4-RB5 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 (SPT) inevitably contain this link. 3. Resilient Distribution Trees Calculation RBridges leverage IS-IS to detect and advertise network fault. A node or link failure will trigger a campus-wide recovergence of distribution trees. The reconvergence generally includes the following procedures: 1. Failure detected through IS-IS control messages (HELLO) exchanging; 2. IS-IS flooding and each RBridge recognizes the failure; 3. Each RBridge recalculates affected distribution trees independently; Zhang, et al. Expires January 1, 2013 [Page 7] INTERNET-DRAFT Resilient Distribution Trees June 30, 2012 4. RPF filters are updated according to the new distribution trees. The recomputed distribution trees are pruned per VLAN and installed into the multicast forwarding tables. The slow reconvergence can be as long as tens of seconds or even minutes, which will cause disruption to ongoing multicast traffic. In protection mechanisms, alternative paths prepared ahead of potential node or link failures are leveraged to detour the failures upon the failure detection, therefore service disruption can be minimized. In order to protect a node on the primary tree, a backup tree can be set up as lack of this node [mMRT]. 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 they have to cover all RBridge nodes in the campus [RFC6325]. A DT does not span all RBridges in the campus may not cover all receivers of many a multicast group (This is different from the multicast trees construction signaled by PIM [RFC4601] or mLDP [RFC6388].). Therefore, the construction of backup DT for the purpose of node protection in TRILL does not make sense. This document will focus only on link protection from now on. 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 allows RBridges to compute multiple distribution trees. With the intentional assignment of Affinity Links in DT calculation, this document proposes the method to construct Resilient Distribution Trees (RDT). For example, in Figure 3.1, the backup DT is set up maximally disjoint to the primary DT (The full topology is an combination of these two DTs, which is not shown in the figure.). Except the link between RB1 and RB2, all other links on the primary DT do not overlap with links on the backup DT. It means that every link on the primary DT except link RB1-RB2 can be protected by the backup DT. 3.1. Algorithm for Choosing Affinity Links Zhang, et al. Expires January 1, 2013 [Page 8] INTERNET-DRAFT Resilient Distribution Trees June 30, 2012 Operators MAY configure Affinity Links to intentionally protect a specific link, such as the link connected to a gateway. But it is desirable that each RBridge independently computes Affinity Links for a backup DT while the same result is got across the whole campus, which enables a distributed deployment. Algorithms for MRT [mMRT] may be used to figure out Affinity Links on a backup DT which is maximally disjoint to the primary DT but it only provides a subset of all possible solutions. In TRILL, RDT does not restrict that the root of the backup DT is the same as that of the primary DT. Two disjoint (or maximally disjoint) trees may root from different nodes, which significantly augments the solution space. This document RECOMMENDS to achieve the independent method through a slight change to the conventional DT calculation process of TRILL. Basically, after the primary DT is calculated, the RBridge will be aware of which links will be used. When the backup DT is calculated, each RBridge increases the metric of these links by a proper value (for safety, the summation of all original link metrics in the campus is RECOMMENDED), which gives these links a lower priority being chosen by the backup DT by performing SPF calculation. All links on this backup DT can be assigned as Affinity Links but this is unnecessary. In order to reduce the amount of Affinity TLVs flooded across the campus, only those will not picked by conventional DT calculation process ought to be recognized as Affinity Links. 3.2. Affinity Links Advertisement Similar as [CMT], every Parent RBridge (PRB) of an Affinity Link take charge of announcing this link in the Affinity TLV. When this RBridge plays the role of PRB for several Affinity Links, it is natural to have them advertised together in the same Affinity TLV and each Affinity Link is structured as one Affinity Record. Affinity Links are announced in the Affinity TLV which is recognized by every RBridge. Since each RBridge computes distribution trees as the Affinity TLV requires, the backup DT will built up naturally. 4. Resilient Distribution Trees Installation In order to reduce the service disruption time, RBridges SHOULD install backup DTs in advance. This also includes the RPF filters that need to be set up for RPF Check. Since the backup DT is intentionally built up maximally disjoint to the primary DT, when a link fails and interrupts the ongoing multicast traffic sent along the primary DT, it is probably that the backup DT is not affected. Therefore, the backup DT installed in Zhang, et al. Expires January 1, 2013 [Page 9] INTERNET-DRAFT Resilient Distribution Trees June 30, 2012 advance can be used to deliver multicast frames immediately. 4.1. Pruning the Backup Distribution Tree Backup DT should be pruned per-VLAN. But the way backup DT being pruned is different from the way that the primary DT is pruned. Even though a branch contains no downstream receivers, it is probably that it should not be pruned for the purpose of protection. Therefore, a branch on the backup DT should be pruned per-VLAN, eliminating branches that have no potential downstream RBridges which appear on the pruned primary DT. It is probably that the primary DT is not optimally pruned in practice. In this case, the backup DT SHOULD be pruned presuming that the primary DT is optimally pruned. Those redundant links ought to be pruned 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. Suppose RB7, RB9 and RB10 constitute a multicast group. The pruned primary DT and backup DT are shown in Figure 4.1. Branches RB2 and RB4 on the primary DT are pruned since there are no potential receivers on these two branches. Although branches RB1 and RB3 on the backup DT have no potential multicast receivers, they may be used to repair link failures of the primary DT. Therefore they are not pruned from the backup DT. Branch RB8 can be safely pruned because it does not appear on the pruned primary DT. 4.2. RPF Filters Preparation RB2 includes in its LSP the information to indicate which trees RB2 might choose to ingress multicast frames [RFC6325]. When RB2 specifies the trees it might choose to ingress multicast traffic, 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 pass the RPF Check. This works when global 1:1 protection is used. Zhang, et al. Expires January 1, 2013 [Page 10] INTERNET-DRAFT Resilient Distribution Trees June 30, 2012 However, when global 1+1 protection or local protection is applied, traffic duplication will happen if multicast receivers accept both copies of multicast frame from two RPF filters. In order to avoid such duplication, multicast receivers (egress RBridge) MUST act as merge points to active a single RPF filter and discard the duplicated frames from the other RPF filter. In normal case, the RPF state is set up according to the primary DT. When a link fails, the RPF filter should be updated instantly according to the backup DT. 5. Protection Mechanisms with Resilient Distribution Trees Protection mechanisms can be developed to make use of the backup DT installed in advance. But protection mechanisms already developed using PIM or mLDP for multicast of 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 bidirectional while the link on a distribution tree in IP/MPLS networks is unidirectional. o In TRILL, an multicast source node does not have to be the root of the distribution tree. It goes 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 same root as the primary DT. Therefore TRILL needs dedicated multicast protection mechanisms. Global 1:1 protection, global 1+1 protection and local protection are developed 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 multicasting. The backup DT rooted at RB2 does not include the link RB1-RB5, therefore it can be used to protect this link. In the global 1:1 protection, RB7 will switch the subsequent multicast traffic to this backup DT when it's notified about 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 merge them. In the local protection, when link RB1-RB5 fails, RB1 will locally replicate the multicast traffic and send it on the backup DT. 5.1. Global 1:1 Protection In the global 1:1 protection, the ingress of the multicast traffic is Zhang, et al. Expires January 1, 2013 [Page 11] INTERNET-DRAFT Resilient Distribution Trees June 30, 2012 responsible to switch 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 does not need to wait for the DT recalculation and installation. Upon 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 considerable service disruption. BFD (Bidirectional Forwarding Detection) protocol can be used to reduce the failure detection time [rbBFD]. Multi-destination BFD extends BFD mechanism to include the fast failure detection of multicast paths [mBFD]. It can be used to reduce both the failure detection and propagation time in the global protection. In multi- destination BFD, ingress RBridge need to send BFD control packets to poll each receiver, and receivers return BFD control packets to the ingress as 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. In this way, multi- destination BFD detects the connectivity of a path rather than a link. The ingress RBridge will determine a minimum failed branch which contains this receiver. Ingress RBridge will switch ongoing multicast traffic based on this judgment. For example, if RB9 does not response while RB10 still responses, 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. Accurate link failure detection might help ingress RBridge to make smarter decision but it's out of the scope of this document. RBridges may make use of RBridge Channel to speed up the failure propagation [RBch]. LSPs for the purpose of failure notification may be sent to the ingress RBridge as unicast TRILL Data using RBridge Channel. 5.2. Global 1+1 Protection In the global 1+1 protection, the multicast source RBridge always replicate the multicast frames and send 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 [MoFRR]. 5.2.1. Failure Detection Egress RBridges (merge points) SHOULD realize the link failure as early as possible so that failure affected egress RBridges may update Zhang, et al. Expires January 1, 2013 [Page 12] INTERNET-DRAFT Resilient Distribution Trees June 30, 2012 their RPF filters quickly to minimize the traffic disruption. Three options are provided as follows. 1. Egress RBridges assume a minimum known packet rate for a given data stream [MoFRR]. A failure detection timer Td are 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. 2. With multi-destination BFD, when a link failure happens, affected egress RBridges can detect a lack of connectivity from the ingress [mBFD]. 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 active both primary and backup RPF filters, therefore both copies of the multicast frames will pass through transit RBridges. Multicast receivers (egress RBridges) MUST act as "merge points" to egress only one copy of these multicast frames. This is achieved by the activation of only a single RPF filter. In normal case, egress RBridges will activate the primary RPF filter. When a link on the pruned primary DT fails, ingress RBridge cannot reach some of the receivers. When these unreachable receivers realize it, they SHOULD update their RPF filters to receive packets sent on the backup DT. 5.3. Local Protection In the local protection, the Point of Local Repair (PLR) happens at the upstream RBridge connecting the failed link who makes the decision to replicate the multicast traffic to recover this link failure. Local protection can further save the time spent on failure notification through the flooding of LSPs across the campus. In addition, the failure detection can be speeded up using BFD [RFC5880], therefore local protection can minimize the service disruption within 50 milliseconds. Since the ingress RBridge is not necessarily the root of the distribution tree in TRILL, a multicast downstream point may be not the descendants of the ingress point on the distribution tree. Zhang, et al. Expires January 1, 2013 [Page 13] INTERNET-DRAFT Resilient Distribution Trees June 30, 2012 Moreover, distribution trees in TRILL are bidirectional and do not share the same root. There are fundamental differences between the distribution tree calculation of TRILL and those used in PIM and mLDP, therefore local protection mechanisms used for PIM and mLDP, such as [mMRT] and [MoFRR], are not applicable to TRILL. 5.3.1. Start Using Backup Distribution Tree The egress nickname of the replicated multicast TRILL data frames will be rewritten to the backup DT's root nickname by the PLR. But the ingress of the multicast frame MUST be remained unchanged. This is a halfway change of the DT for multicast frames. Then the PLR begins to forward multicast traffic along the backup DT (same ingress but different egress). In the above example, if PLR RB1 decides to send replicated multicast frames according to the backup DT, it will send it to the next hop RB2. However, according to the RPF filter built up from the backup DT, multicast frames ingressed by RB7 should only be received from the link RB4-RB2. So RB2 will discard these frames. In fact, any RBridge should receive multicast frames from any ingress, through a single link. The halfway change of DT must modify this rule in order to be valid. When RB20 computes the RPF filter for each ingress RB30 for the backup DT, RB20 believes any link on the backup DT connecting RB20 may be the link on which RB20 may receive a packet from RB30. In this way, in the above example RB2 will not discard the multicast frames sent from RB1. 5.3.2. Duplication Suppression When a PLR starts to sent replicated multicast frames on the backup DT, multicast frames sent along the primary DT are still going on. Some RBridges on the primary DT might receive two copies of these multicast frames, filled with two different egress nicknames. Local protection MUST adopt duplication suppression mechanism such as the traffic forking and merging method in the global 1+1 protection. 5.3.3. An Example to Walk Through The example used in the above local protection is put together to get a whole "walk through" below. In the normal case, multicast frames ingressed by RB7 using the pruned 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 frames using the pruned backup DT rooted at RB2. When RB2 gets the multicast frames from the link RB1-RB2, it accepts them since the RPF filter {DT=RB2, ingress=RB7, receiving Zhang, et al. Expires January 1, 2013 [Page 14] INTERNET-DRAFT Resilient Distribution Trees June 30, 2012 links=RB1-RB2, RB3-RB2, RB4-RB2, RB5-RB2 and RB6-RB2} is installed on RB2. RB2 forwards the replicated multicast frames to its neighbors except RB1. When the multicast frames 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 frames 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 frames on to the local link. From the above explanation, we can find that we have to change the data plane with egress rewriting and relax the RPF Checking for the local protection. 5.4. Back to the Primary Distribution Tree After the reconvergence finishes, the new primary DT and RPF filters will be installed. The link failure will be recovered. Then the backup DT and its RPF filters will be updated as well. Before the primary DT is successfully installed and come into use, the backup DT SHOULD not be updated since repaired multicast stream may be still being forwarded on it. For the global 1:1 protection, the ingress RBridge SHOULD switch to the new primary DT immediately after it's installed. Then the backup DT is updated. For the global 1+1 protection, the ingress RBridge SHOULD start to use the installed primary DT, and at the same time MUST stop replicating multicast frames onto the backup DT. After the backup DT is updated, the ingress RBridge starts to replicate multicast frames onto the backup DT. For the local protection, the ingress RBridge starts to use the new primary DT immediately after it's installed. The PLR MUST stop replicating and sending packets on the old backup DT. Then the backup DT will be updated. 6. Security Considerations This document raises no new security issues for IS-IS. 7. IANA Considerations No new registry is requested to be assigned by IANA. The Affinity TLV has already been defined in [6326bis]. This document does not change its definition. RFC Editor: please remove this section before Zhang, et al. Expires January 1, 2013 [Page 15] INTERNET-DRAFT Resilient Distribution Trees June 30, 2012 publication. 8. References 8.1. Normative References [6326bis] D. Eastlake, A. Banerjee, et al., "Transparent Interconnection of Lots of Links (TRILL) Use of IS-IS", draft-eastlake-isis-rfc6326bis-07.txt, work in Progress. [CMT] T. Senevirathne, J. Pathangi, et al, "Coordinated Multicast Trees (CMT)for TRILL", draft-ietf-trill-cmt-00.txt, working in progress. [RFC6325] R. Perlman, D. Eastlake, et al, "RBridges: Base Protocol Specification", RFC 6325, July 2011. [RFC4601] Fenner, B., Handley, M., Holbrook, H., and I. Kouvelas, "Protocol Independent Multicast - Sparse Mode (PIM-SM): Protocol Specification (Revised)", RFC 4601, August 2006. [RFC6388] Wijnands, IJ., Minei, I., Kompella, K., and B. Thomas, "Label Distribution Protocol Extensions for Point-to- Multipoint and Multipoint-to-Multipoint Label Switched Paths", RFC 6388, November 2011. [rbBFD] V. Manral, D. Eastlake, et al, "TRILL (Transparent Interconnetion of Lots of Links): Bidirectional Forwarding Detection (BFD) Support", draft-ietf-trill-rbridge-bfd- 06.txt, work in progress. [mBFD] D. Katz, D. Ward, "BFD for Multipoint Networks", draft- ietf-bfd-multipoint-00.txt, work in progress. [RFC5880] D. Katz, D. Ward, "Bidirectional Forwarding Detection (BFD)", RFC 5880, June 2010. 8.2. Informative References [mMRT] A. Atlas, R. Kebler, et al., "An Architecture for Multicast Protection Using Maximally Redundant Trees", draft-atlas- rtgwg-mrt-mc-arch-00.txt, work in progress. [MoFRR] A. Karan, C. Filsfils, et al., "Multicast only Fast Re- Route", draft-karan-mofrr-02.txt, work in progress. [RBch] D. Eastlake, V. Manral, et al, "TRILL: RBridge Channel Zhang, et al. Expires January 1, 2013 [Page 16] INTERNET-DRAFT Resilient Distribution Trees June 30, 2012 Support", draft-ietf-trill-rbridge-channel-06.txt, work in progress. Zhang, et al. Expires January 1, 2013 [Page 17] INTERNET-DRAFT Resilient Distribution Trees June 30, 2012 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 Cisco Systems 375 East Tasman Drive, San Jose, CA 95134 Phone: +1-408-853-2291 Email: tsenevir@cisco.com Janardhanan Pathangi Dell/Force10 Networks Olympia Technology Park, Guindy Chennai 600 032 Phone: +91 44 4220 8400 Email: Pathangi_Janardhanan@Dell.com Ayan Banerjee Email: ayabaner@gmail.com Zhang, et al. Expires January 1, 2013 [Page 18]