INTERNET-DRAFT Mingui Zhang Intended Status: Proposed Standard Huawei Updates: RFC 6325 Tissa Senevirathne Cisco Janardhanan Pathangi DELL Ayan Banerjee Cisco Anoop Ghanwani DELL Expires: January 3, 2016 July 2, 2015 TRILL Resilient Distribution Trees draft-ietf-trill-resilient-trees-03.txt Abstract TRILL 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 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 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 recovergence. 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 Zhang, et al. Expires January 3, 2016 [Page 1] INTERNET-DRAFT Resilient Distribution Trees July 2, 2015 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) 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 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 Sub-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. Designating Roots for Backup Trees . . . . . . . . . . . . 8 3.1.1. Conjugate Trees . . . . . . . . . . . . . . . . . . . . 8 3.1.2. Explicitly Advertising Tree Roots . . . . . . . . . . . 8 3.2. Backup DT Calculation . . . . . . . . . . . . . . . . . . . 8 3.2.1. Backup DT Calculation with Affinity Links . . . . . . . 8 3.2.1.1. Algorithm for Choosing Affinity Links . . . . . . . 9 3.2.1.2. Affinity Links Advertisement . . . . . . . . . . . 10 3.2.2. Backup DT Calculation without Affinity Links . . . . . 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 . . . . . . . . . . . . . . . . . . . 13 5.2.1. Failure Detection . . . . . . . . . . . . . . . . . . . 14 5.2.2. Traffic Forking and Merging . . . . . . . . . . . . . . 14 5.3. Local Protection . . . . . . . . . . . . . . . . . . . . . 14 Zhang, et al. Expires January 3, 2016 [Page 2] INTERNET-DRAFT Resilient Distribution Trees July 2, 2015 5.3.1. Start Using the Backup Distribution Tree . . . . . . . 15 5.3.2. Duplication Suppression . . . . . . . . . . . . . . . . 15 5.3.3. An Example to Walk Through . . . . . . . . . . . . . . 15 5.4. Switching Back to the Primary Distribution Tree . . . . . . 16 6. Security Considerations . . . . . . . . . . . . . . . . . . . . 16 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 17 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . 17 8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 17 8.1. Normative References . . . . . . . . . . . . . . . . . . . 17 8.2. Informative References . . . . . . . . . . . . . . . . . . 18 Author's Addresses . . . . . . . . . . . . . . . . . . . . . . . . 19 Zhang, et al. Expires January 3, 2016 [Page 3] INTERNET-DRAFT Resilient Distribution Trees July 2, 2015 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, a network fault will be recovered through a network wide reconvergence of the forwarding states, but this process is too slow to meet the tight Service Level Agreement (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 faults. With backup forwarding states installed in advance, a protection mechanism can restore an interrupted multicast stream in tens of milliseconds which meets stringent SLAs on service disruption. Several protection mechanisms for multicast traffic have been developed for IP/MPLS networks [mMRT] [MoFRR]. However, the way that TRILL constructs distribution trees (DT) is different from the way that multicast trees are computed under IP/MPLS, therefore a multicast protection mechanism suitable for TRILL is required. This document proposes "Resilient Distribution Trees" (RDT) in which backup trees are installed in advance for the purpose of fast failure repair. Three types of protection mechanisms are proposed. o Global 1:1 protection is used to refer 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 is used to refer 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 one 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. 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 Zhang, et al. Expires January 3, 2016 [Page 4] INTERNET-DRAFT Resilient Distribution Trees July 2, 2015 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 that for future study. [RFC7176] specifies the Affinity Sub-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. 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 DT: Distribution Tree IS-IS: Intermediate System to Intermediate System PLR: Point of Local Repair. In this document, PLR is the multicast upstream RBridge connecting the failed link. It's valid only for local protection. RDT: Resilient Distribution Tree RPF: Reverse Path Forwarding SLA: Service Level Agreement TRILL: TRansparent Interconnection of Lots of Links 2. Usage of Affinity Sub-TLV This document uses the Affinity Sub-TLV [RFC7176] to assign a parent to an RBridge in a tree as discussed below. 2.1. Allocating Affinity Links The Affinity Sub-TLV explicitly assigns parents for RBridges on distribution trees. It 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" of the Affinity Record identify the Zhang, et al. Expires January 3, 2016 [Page 5] INTERNET-DRAFT Resilient Distribution Trees July 2, 2015 distribution trees that adopt this Affinity Link [RFC7176]. Affinity Links may be configured or automatically determined using an 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 Sub-TLV with an Affinity Record that is 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 child for which they are not on the shortest paths from the root. This flexibility is used to increase the reliability of distribution trees in this document. Note that Affinity Link MUST NOT be misused to connect two RBridges which are not adjacent. If it is, the Affinity Link is ignored and has no effect on tree building. 2.2. Distribution Tree Calculation with Affinity Links When RBridges receive an Affinity Sub-TLV with Affinity Link that is an incoming link of RB2 (i.e., RB2 is the child on this Affinity Link), 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 the Shortest Path First calculation to compute the distribution tree based on the sub graph. In this way, the Affinity Link will surely appear on the distribution tree. Zhang, et al. Expires January 3, 2016 [Page 6] INTERNET-DRAFT Resilient Distribution Trees July 2, 2015 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 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. 3. Resilient Distribution Trees Calculation RBridges use IS-IS to detect and advertise network faults. A node or link failure will trigger a campus-wide reconvergence of distribution trees. The reconvergence generally includes the following procedures: 1. Failure detected through IS-IS control messages (HELLO) exchanging or some other method 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; Zhang, et al. Expires January 3, 2016 [Page 7] INTERNET-DRAFT Resilient Distribution Trees July 2, 2015 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 reconvergence can be slow, which will disrupt ongoing multicast traffic. In protection mechanisms, alternative paths prepared ahead of potential node or link failures are used to detour the failures upon the failure detection, therefore service disruption can be minimized. This document focuses only on link protection. The construction of backup DT for the purpose of node protection is out the scope of this document. In order to protect a node on the primary tree, a backup tree can be 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 [RFC4601] or mLDP [RFC6388].) 3.1. Designating Roots for Backup Trees Operators MAY manually configure the roots for the backup DTs. Nevertheless, this document aims to provide a mechanism with minimum configuration. Two options are offered as follows. 3.1.1. Conjugate Trees [RFC6325] and [RFC7180] specify how distribution tree roots are selected. When a backup DT is computed for a primary DT, its root is set to be the root of this primary DT. In order to distinguish the primary DT and the backup DT, the root RBridge MUST own multiple nicknames. 3.1.2. Explicitly Advertising Tree Roots RBridge RB1 having the highest root priority nickname might explicitly advertise a list of nicknames to identify the roots of the primary and backup tree roots (See Section 4.5 of [RFC6325]). 3.2. Backup DT Calculation 3.2.1. Backup DT Calculation with Affinity Links Zhang, et al. Expires January 3, 2016 [Page 8] INTERNET-DRAFT Resilient Distribution Trees July 2, 2015 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 proposes a method to construct RDTs. For example, in Figure 3.1, the backup DT is set up 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 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. 3.2.1.1. Algorithm for Choosing Affinity Links Operators MAY configure Affinity Links to intentionally protect a specific link, such as the link connected to a gateway. But it is desirable that every RBridge independently computes Affinity Links for a backup DT across the whole campus. This enables a distributed deployment and also minimizes configuration. Algorithms for Maximally Redundant Trees [MRT] may be used to figure out Affinity Links on a backup DT which is maximally disjointed to the primary DT but it only provides a subset of all possible solutions, i.e., the conjugate trees described in Section 3.1.1. In TRILL, RDT does not restrict the root of the backup DT to be the same as that of the primary DT. Two disjoint (or maximally disjointed) trees may root from different nodes, which significantly augments the solution space. This document RECOMMENDS achieving 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, it's recommended to used the summation of all original link metrics in the campus but not more than 2**23), which gives Zhang, et al. Expires January 3, 2016 [Page 9] INTERNET-DRAFT Resilient Distribution Trees July 2, 2015 these links a lower priority being chosen by the backup DT by performing Shortest Path First 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 Sub-TLVs flooded across the campus, only those NOT picked by conventional DT calculation process ought to be recognized as Affinity Links. 3.2.1.2. Affinity Links Advertisement Similar to [CMT], 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. 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 up consistently. 3.2.2. Backup DT Calculation without Affinity Links This section provides an alternative method to set up a disjoint backup DT. After the primary DT is calculated, each RBridge increases the cost of those links which are already in the primary DT by a multiplier (For safety, 64x is RECOMMENDED.). It would ensure that a link appears in both trees if and only if there is no other way to reach the node (i.e. the graph would become disconnected if it were pruned of the links in the first tree.). In other words, the two trees will be maximally disjoint. The above algorithm is similar as that defined in Section 3.2.1.1. All RBridges MUST agree on the same algorithm, then the backup DT can be calculated by each RBridge consistently and configuration is unnecessary. 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 frames. Thus other RBridges in the campus can limit the amount of states which are necessary for RPF check. 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 Zhang, et al. Expires January 3, 2016 [Page 10] INTERNET-DRAFT Resilient Distribution Trees July 2, 2015 some combinations of ingress and tree. This document states that the backup DT MUST be contained in an ingress RBridge's DT announcement list and included in this ingress RBridge's LSP. 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 Check. Since the backup DT is intentionally built maximally 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. Even though a branch contains no downstream receivers, it is probable that it should not be pruned for the purpose of protection. 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. 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 that 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 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 Zhang, et al. Expires January 3, 2016 [Page 11] INTERNET-DRAFT Resilient Distribution Trees July 2, 2015 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 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. 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 normal case, the RPF state is set up according to the primary DT. When a link fails, the RPF filter based on the backup DT should be activated. 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, 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 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 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 Zhang, et al. Expires January 3, 2016 [Page 12] INTERNET-DRAFT Resilient Distribution Trees July 2, 2015 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 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 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 propagation time in the global protection. In [RBmBFD], 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. Therefore, [RBmBFD] is used to detect the connectivity of a path rather than a link. The ingress RBridge will determine a minimum failed branch which contains this receiver. The ingress RBridge will switch ongoing multicast traffic based on this judgment. For example, on Figure 4.1, if RB9 does not response 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. Accurate link failure detection might help ingress RBridges to 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 Zhang, et al. Expires January 3, 2016 [Page 13] INTERNET-DRAFT Resilient Distribution Trees July 2, 2015 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 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 [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. 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 normal case, egress RBridges 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. It is this RBridge that makes the decision to replicate the multicast traffic to recover this link failure. Local protection can further save the time spent Zhang, et al. Expires January 3, 2016 [Page 14] INTERNET-DRAFT Resilient Distribution Trees July 2, 2015 on failure notification through the flooding of LSPs across the campus. In addition, the failure detection can be speeded up using [RFC7175], 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 not be the descendants of the ingress point on the distribution tree. 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 here. 5.3.1. Start Using 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 of the multicast frame MUST remain unchanged. This is a halfway change of the DT for multicast packets. Afterwards, the PLR begins to forward multicast traffic 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. In the above example, the PLR RB1 locally determines to send replicated multicast packets according to the backup DT. It will send it 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 in the above local protection is put together to get a whole "walk through" below. In the normal case, multicast frames ingressed by RB7 with pruned distribution on 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 Zhang, et al. Expires January 3, 2016 [Page 15] INTERNET-DRAFT Resilient Distribution Trees July 2, 2015 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 on to the local link. 5.4. Switching Back to the Primary Distribution Tree 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 without the failed link. 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 30s but it is configurable. 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) according to 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. The backup DT is updated at t1+2*Ts. It 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 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. Security Considerations This document raises no new security issues for TRILL. Zhang, et al. Expires January 3, 2016 [Page 16] INTERNET-DRAFT Resilient Distribution Trees July 2, 2015 For general TRILL Security Considerations, see [RFC6325]. 7. IANA Considerations No new registry or registry entries are requested to be assigned by IANA. The Affinity Sub-TLV has already been defined in [RFC7176]. This document does not change its definition. RFC Editor: please remove this section before publication. Acknowledgements 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, Hongjun Zhai and Xudong Zhang. 8. References 8.1. Normative References [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, May 2014. [CMT] T. Senevirathne, J. Pathangi, et al, "Coordinated Multicast Trees (CMT) for TRILL", draft-ietf-trill-cmt, work 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. [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, "Transparent Interconnection of Lots of Links (TRILL): Bidirectional Forwarding Detection (BFD) Support", RFC 7175, May 2014. Zhang, et al. Expires January 3, 2016 [Page 17] INTERNET-DRAFT Resilient Distribution Trees July 2, 2015 [RFC7180] Eastlake 3rd, D., Zhang, M., Ghanwani, A., Manral, V., and A. Banerjee, "Transparent Interconnection of Lots of Links (TRILL): Clarifications, Corrections, and Updates", RFC 7180, May 2014. 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, work in progress. [MRT] A. Atlas, Ed., R. Kebler, et al., "An Architecture for IP/LDP Fast-Reroute Using Maximally Redundant Trees", draft-ietf-rtgwg-mrt-frr-architecture, work in progress. [MoFRR] A. Karan, C. Filsfils, et al., "Multicast only Fast Re- Route", draft-ietf-rtgwg-mofrr, work in progress. [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, May 2014. Zhang, et al. Expires January 3, 2016 [Page 18] INTERNET-DRAFT Resilient Distribution Trees July 2, 2015 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 Cisco Email: ayabaner@cisco.com Anoop Ghanwani Dell 350 Holger Way San Jose, CA 95134 Phone: +1-408-571-3500 Email: Anoop@alumni.duke.edu Zhang, et al. Expires January 3, 2016 [Page 19]