Network Working Group H. Chen Internet-Draft D. Cheng Intended status: Standards Track Huawei Technologies Expires: January 3, 2019 M. Toy Verizon Y. Yang IBM July 2, 2018 OSPF Flooding Reduction draft-cc-ospf-flooding-reduction-02 Abstract This document proposes an approach to flood OSPF link state advertisements on a topology that is a subgraph of the complete OSPF topology per underline physical network, so that the amount of flooding traffic in the network is greatly reduced, and it would reduce convergence time with a more stable and optimized routing environment. The approach can be applied to any network topology in a single OSPF area, and can be used in both OSPFv2 ([RFC2328]) network and OSPFv3 ([RFC5340]) network. Requirements Language 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]. Status of This Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at https://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." This Internet-Draft will expire on January 3, 2019. Chen, et al. Expires January 3, 2019 [Page 1] Internet-Draft OSPF Flooding Reduction July 2018 Copyright 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 (https://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 . . . . . . . . . . . . . . . . . . . . . . . . 2 2. Problem Statement . . . . . . . . . . . . . . . . . . . . . . 3 3. Flooding Topology . . . . . . . . . . . . . . . . . . . . . . 4 4. Extensions to OSPF . . . . . . . . . . . . . . . . . . . . . 6 5. Flooding Behavior . . . . . . . . . . . . . . . . . . . . . . 8 5.1. Nodes Perform Flooding Reduction . . . . . . . . . . . . 8 5.1.1. Receiving an OSPF LSA . . . . . . . . . . . . . . . . 8 5.1.2. Originating an OSPF LSA . . . . . . . . . . . . . . . 9 5.1.3. An Exception Case . . . . . . . . . . . . . . . . . . 9 5.1.4. One More Note . . . . . . . . . . . . . . . . . . . . 9 5.2. Nodes Not Support Flooding Reduction . . . . . . . . . . 10 6. Security Considerations . . . . . . . . . . . . . . . . . . . 10 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10 8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 10 9. References . . . . . . . . . . . . . . . . . . . . . . . . . 10 9.1. Normative References . . . . . . . . . . . . . . . . . . 10 9.2. Informative References . . . . . . . . . . . . . . . . . 11 Appendix A. Algorithms to Build Flooding Topology . . . . . . . 11 A.1. Algorithms to Build Tree without Considering Others . . . 11 A.2. Algorithms to Build Tree Considering Others . . . . . . . 12 A.3. Connecting Leaves . . . . . . . . . . . . . . . . . . . . 15 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 16 1. Introduction For some networks such as dense Data Center (DC) networks, the existing OSPF Link State Advertisement (LSA) flooding mechanism is not efficient and may have some issues. The extra LSA flooding consumes network bandwidth. Processing the extra LSA flooding, including receiving, buffering and decoding the extra LSAs, wastes Chen, et al. Expires January 3, 2019 [Page 2] Internet-Draft OSPF Flooding Reduction July 2018 memory space and processor time. This may cause scalability issues and affect the network convergence negatively. A flooding reduction method between spines and leaves is proposed in [I-D.shen-isis-spine-leaf-ext]. A flooding reduction focusing on central computation of flooding topology is discussed in [I-D.li-dynamic-flooding]. This document proposes an approach to flood OSPF LSAs on a topology that is a subgraph of the entire OSPF topology per underline physical network, so that the amount of flooding traffic in the network is greatly reduced. The workload for processing the extra LSA flooding is decreased significantly. This would improve the scalability and speed up the network convergence, stable and optimize the routing environment. This approach is flexible. It has multiple modes for computation of flooding topology. Users can select a mode they prefer, and smoothly switch from one current mode to another. The approach proposed is applicable to any network topology in a single OSPF area. It can be used in both a OSPFv2 network and a OSPFv3 network. The approach is backward compatible. 2. Problem Statement OSPF, like other link-state routing protocols, deploys a so-called reliable flooding mechanism, where a node must transmit a received or self-originated LSA to all its OSPF interfaces (except the interface where a LSA is received) in the defined context. While this mechanism assures each LSA being distributed to every OSPF node in the relevant routing area or domain, the side-effect is that the mechanism often causes redundant LSAs in individual network segments (e.g., on an OSPF point-to-point link or a broadcast subnet), which in turn forces OSPF nodes to process identical LSAs more than once. This results waste of OSPF link bandwidth and OSPF nodes' computing resources, and the delay of OSPF topology convergence. The problem explained above becomes more serious in OSPF networks with large number of nodes and links, and in particular, higher degree of interconnection (e.g., meshed topology, spine-leaf topology, etc,). In some environment such as in data centers, the drawback of the existing flooding mechanism has already caused operational problems, including repeated and waves of flooding storms, chock of computing resources, slow convergence, oscillating topology changes, instability of routing environment. One example is as shown in Figure 1 (a), where Node 1, Node 2 and Node 3 are interconnected in a mesh. When Node 1 receives a new or updated OSPF LSA on its interface I11, it by default would forward to its interface Il2 and I13 towards Node 2 and Node 3, respectively, Chen, et al. Expires January 3, 2019 [Page 3] Internet-Draft OSPF Flooding Reduction July 2018 after processing. Node 2 and Node 3 upon reception of the LSA and after processing, would potentially flood the same LSA over their respective interface I23 and I32 toward each other, which is obviously not necessary and at the cost of link bandwidth as well as both nodes' computing resource. In example Figure 1 (b), Node 2 and Node 3 both connect to a LAN where Node 4, Node 5 and Node 6 also connect to. When Node 1 receives a LSA as in (a) and floods it to Node 2 and Node 3 respectively, the two nodes would in turn both (instead of one) flood to the LAN, which is unnecessary and at the cost of link bandwidth as well as computing resource of all nodes connected to the LAN. | | |I11 |I11 +--o---+ +--o---+ |Node 1| |Node 1| +-o--o-+ +-o--o-+ I12 / \ I13 / \ / \ I12/ \I13 I21/ \I31 / \ +----o-+ I32+-o----+ +----o-+ +-o----+ |Node 2|------|Node 3| |Node 2| |Node 3| +------+I23 +------+ +--o---+ +---o--+ I2L| LAN |I3L (a) -----o--------o-----o--o----- I4L| I5L| I6L| +---o--+ +--o---+ +--o---+ |Node 4| |Node 5| |Node 6| +------+ +------+ +------+ (b) Figure 1 3. Flooding Topology It is a norm that an OSPF node sending a received LSA and self- originated LSA to all its OSPF interfaces (except that where a LSA is received), as the reliable-flooding mechanism requires, i.e., any OSPF LSA would potentially traverses on each OSPF link in a given OSPF network topology, sometimes both directions. As demonstrated in Section 2, dissemination over the entire OSPF network topology has drawbacks. Chen, et al. Expires January 3, 2019 [Page 4] Internet-Draft OSPF Flooding Reduction July 2018 To change OSPF's aggressive flooding behavior, a flooding topology is introduced. For a given OSPF network topology, a flooding topology is a sub-graph or sub-network of the given network topology that has the same reachability to every node as the given network topology. Thus all the nodes in the given network topology MUST be in the flooding topology. All the nodes MUST be inter-connected directly or indirectly. As a result, OSPF flooding will in most cases occur only on the flooding topology, that includes all OSPF nodes but a subset of OSPF links. Note even the flooding topology is a sub-graph of the original OSPF topology, any single LSA MUST still be disseminated in the entire OSPF network. There are many different flooding topologies for a given OSPF network topology. A chain connecting all the nodes in the given network topology is a flooding topology. A circle connecting all the nodes is another flooding topology. A tree connecting all the nodes is a flooding topology. In addition, the tree plus the connections between some leaves of the tree and branch nodes of the tree is a flooding topology. There are many different ways to construct a flooding topology for a given OSPF network topology. A few of them are listed below: o Central Mode: One node in the network builds a flooding topology and floods the flooding topology to all the other nodes in the network (This seems not very good. Flooding the flooding topology may increase the flooding.); o Distributed Mode: Each node in the network automatically calculates a flooding topology by using the same algorithm (No flooding for flooding topology); o Static Mode: Links on the flooding topology are configured statically. The minimum requirement for a flooding topology is all OSPF nodes are interconnected (directly or indirectly), but there is only one path from any node to any other node. While this lean-and-mean type of flooding topology degrades OSPF flooding traffic volume to the least, it may introduce some delay of topology convergence in the network with some network topologies. To compensate convergence efficiency, additional OSPF links may be added as part of the flooding topology. There is a trade-off between the density of the flooding topology and the convergence efficiency. Note that the flooding topology constructed by an OSPF node is dynamic in nature, that means when the OSPF's base topology (the entire topology graph) changes, the flooding topology (the sub-graph) Chen, et al. Expires January 3, 2019 [Page 5] Internet-Draft OSPF Flooding Reduction July 2018 MUST be re-computed/re-constructed to ensure that any node that is reachable on the base topology MUST also be reachable on the flooding topology. For reference purpose, some algorithms that allow OSPF nodes to automatically compute flooding topology are elaborated in Appendix A. However, this document does not attempt to standardize how a flooding topology is established. 4. Extensions to OSPF A couple of TLVs are defined in OSPF RI LSA [RFC7770]. One TLV contains instructions about flooding reduction, which is called Flooding Reduction Instruction TLV or Instruction TLV for short. This TLV is originated from only one node at any time. Another TLV includes the information on flooding reduction of a node, which is called Flooding Reduction Information TLV or Information TLV for short. This TLV is generated by every node that supports flooding reduction in general. The format of a Flooding Reduction Instruction TLV is as follows. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | INST-TLV-Type (TBD1) | TLV-Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | OP | MOD | Algorithm | Reserved (MUST be zero) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ~ sub TLVs (optional) ~ | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ A OP field of three bits is defined in the TLV. It may have a value of the followings. o 0x001 (R): Perform flooding Reduction, which instructs the nodes in a network to perform flooding reduction. o 0x010 (N): Roll back to Normal flooding, which instructs the nodes in a network to roll back to perform normal flooding. When any of the other values is received, it is ignored. A MOD field of three bits is defined in the TLV and may have a value of the followings. Chen, et al. Expires January 3, 2019 [Page 6] Internet-Draft OSPF Flooding Reduction July 2018 o 0x001 (C): Central Mode, which instructs 1) the nodes in a network to select a leader node and a backup leader node; 2) the leader node in a network to compute a flooding topology and flood the flooding topology to all the other nodes in the network; 3) every node in the network to receive and use the flooding topology originated by the leader node. o 0x010 (D): Distributed Mode, which instructs every node in a network to compute and use its own flooding topology. o 0x010 (S): Static Mode, which instructs every node in a network to use the flooding topology statically configured on the node. When any of the other values is received, it is ignored. An Algorithm field of eight bits is defined in the TLV to instruct the leader node in central mode or every node in distributed mode to use the algorithm indicated in this field for computing a flooding topology. Some optional sub TLVs may be defined in the future, but none is defined now. The format of a Flooding Reduction Information TLV is as follows. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | INFO-TLV-Type (TBD2) | TLV-Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Priority | Reserved (MUST be zero) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ~ sub TLVs (optional) ~ | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ A Priority field of eight bits is defined in the TLV to indicate the priority of the node originating the TLV to become the leader node in central mode. Some optional sub TLVs may be defined in the future, but none is defined now. Chen, et al. Expires January 3, 2019 [Page 7] Internet-Draft OSPF Flooding Reduction July 2018 5. Flooding Behavior 5.1. Nodes Perform Flooding Reduction This section describes OSPF flooding behavior for OSPF nodes that perform flooding reduction described in this document. The flooding behavior for these nodes differs from that as specified in OSPFv2 ([RFC2328]) and OSPFv3 ([RFC5340]). Section 5.1.1 describes the flooding behavior when an OSPF node receives an OSPF LSA from one of its interfaces, and Section 5.1.2 describes the flooding behavior for LSA originated by itself. The revised flooding procedure MUST flood LSAs to every node in the network in any case, as the standard OSPF flooding procedure does. It assumes that the OSPF node of which the flooding behavior is described below is on the flooding topology, i.e., the node and at least one of its OSPF interface are on the flooding topology, where: 1. When the node has only one interface on the flooding topology, the node is a leaf on the topology. 2. When the node has two interfaces on the flooding topology, the node is a transit node on the topology. 3. A flooding topology with nodes having one or two interfaces on the topology is a lean graph, i.e., there is only one path from any node to any other node on the graph. For flooding efficiency, there could be extra OSPF interfaces that are on the flooding topology, i.e., a node may have more than two interfaces that belong to the flooding topology. 5.1.1. Receiving an OSPF LSA The flooding behavior when an OSPF node receives a newer OSPF LSA that is not originated by itself from one of its OSPF interfaces is as follows: 1. The LSA is received on a link that is on the flooding topology. The LSA is flooded only to all the other interfaces that are on the flooding topology. 2. The LSA is received on a link that is not on the flooding topology. This situation can happen when a neighboring node on a point-to-point link newly forms adjacency with the receiving node, or is not currently on the flooding topology; it can happen when the LSA sending neighbor does not support the OSPF flooding reduction; it can also happen as the receiving link is a Chen, et al. Expires January 3, 2019 [Page 8] Internet-Draft OSPF Flooding Reduction July 2018 broadcast-type interface. The LSA is flooded only to all other interfaces that are on the flooding topology. In any case, the LSA must not be transmitted back to the receiving interface. Note before forwarding a received LSA, the OSPF node would do the normal processing as usual. 5.1.2. Originating an OSPF LSA The flooding behavior when an OSPF node originates an OSPF LSA is as follows: 1. If it is a refresh LSA, i.e., there is no significant change contained in the LSA comparing to the previous LSA, the LSA is transmitted over links on the flooding topology. 2. Otherwise, the LSA is transmitted to all OSPF interfaces. Choosing this action instead of limiting to links on flooding topology would speed up the synchronization around the advertising node's neighbors, which could then disseminate the new LSA quickly. 5.1.3. An Exception Case In Section 5.1.1 and Section 5.1.2, there are times when an OSPF node sending out a LSA to an interface on the flooding topology detects a critical interface or node failure. A critical interface is an interface on the flooding topology and is the only connection among some nodes on the flooding topology. When this interface goes down, the flooding topology will be split. Note the flooding topology was pre-computed/pre-constructed; but if at the time a critical interface or a node goes down before a re-newed flooding topology can be computed/constructed, the OSPF node MUST send out the LSA to all interfaces (except where it is received from) as a traditional OSPF node would do. This handling is also taking place if there are more than one interfaces or nodes on the existing flooding topology fail, i.e., if more than one interfaces or nodes on the flooding topology fail, the OSPF node does traditional flooding before the flooding topology is re-built. 5.1.4. One More Note The destination address that is used when an OSPF node sends out a LSA on an interface on its flooding topology follows the specification in OSPFv2 ([RFC2328]) and OSPFv3 ([RFC5340]). This means on a local LAN, all other OSPF nodes will receive the LSA. Chen, et al. Expires January 3, 2019 [Page 9] Internet-Draft OSPF Flooding Reduction July 2018 5.2. Nodes Not Support Flooding Reduction The LSA flooding behavior of OSPF nodes that do not support flooding reduction as described in this document MUST follow that as specified in OSPFv2 ([RFC2328]) and OSPFv3 ([RFC5340]). 6. Security Considerations This document does not introduce any security issue. 7. IANA Considerations Under Registry Name: OSPF Router Information (RI) TLVs [RFC7770], IANA is requested to assign two new TLV values for OSPF flooding reduction as follows: +===============+==================+=====================+ | TLV Value | TLV Name | reference | +===============+==================+=====================+ | TBD1 | Instruction TLV | This document | +---------------+------------------+---------------------+ | TBD2 | Information TLV | This document | +---------------+------------------+---------------------+ 8. Acknowledgements The authors would like to thank Acee Lindem, Zhibo Hu, Robin Li, Stephane Litkowski and Alvaro Retana for their valuable suggestions and comments on this draft. 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, . [RFC2328] Moy, J., "OSPF Version 2", STD 54, RFC 2328, DOI 10.17487/RFC2328, April 1998, . [RFC5340] Coltun, R., Ferguson, D., Moy, J., and A. Lindem, "OSPF for IPv6", RFC 5340, DOI 10.17487/RFC5340, July 2008, . Chen, et al. Expires January 3, 2019 [Page 10] Internet-Draft OSPF Flooding Reduction July 2018 [RFC7770] Lindem, A., Ed., Shen, N., Vasseur, JP., Aggarwal, R., and S. Shaffer, "Extensions to OSPF for Advertising Optional Router Capabilities", RFC 7770, DOI 10.17487/RFC7770, February 2016, . 9.2. Informative References [I-D.li-dynamic-flooding] Li, T. and P. Psenak, "Dynamic Flooding on Dense Graphs", draft-li-dynamic-flooding-05 (work in progress), June 2018. [I-D.shen-isis-spine-leaf-ext] Shen, N., Ginsberg, L., and S. Thyamagundalu, "IS-IS Routing for Spine-Leaf Topology", draft-shen-isis-spine- leaf-ext-06 (work in progress), June 2018. Appendix A. Algorithms to Build Flooding Topology There are many algorithms to build a flooding topology. A simple and efficient one is briefed below. o Select a node R according to a rule such as the node with the biggest/smallest node ID; o Build a tree using R as root of the tree (details below); and then o Connect k (k>=0) leaves to the tree to have a flooding topology (details follow). A.1. Algorithms to Build Tree without Considering Others An algorithm for building a tree from node R as root starts with a candidate queue Cq containing R and an empty flooding topology Ft: 1. Remove the first node A from Cq and add A into Ft 2. If Cq is empty, then return with Ft 3. Suppose that node Xi (i = 1, 2, ..., n) is connected to node A and not in Ft and X1, X2, ..., Xn are in a special order. For example, X1, X2, ..., Xn are ordered by the cost of the link between A and Xi. The cost of the link between A and Xi is less than the cost of the link between A and Xj (j = i + 1). If two costs are the same, Xi's ID is less than Xj's ID. In another example, X1, X2, ..., Xn are ordered by their IDs. If they are not ordered, then make them in the order. Chen, et al. Expires January 3, 2019 [Page 11] Internet-Draft OSPF Flooding Reduction July 2018 4. Add Xi (i = 1, 2, ..., n) into the end of Cq, goto step 1. Another algorithm for building a tree from node R as root starts with a candidate queue Cq containing R and an empty flooding topology Ft: 1. Remove the first node A from Cq and add A into Ft 2. If Cq is empty, then return with Ft 3. Suppose that node Xi (i = 1, 2, ..., n) is connected to node A and not in Ft and X1, X2, ..., Xn are in a special order. For example, X1, X2, ..., Xn are ordered by the cost of the link between A and Xi. The cost of the link between A and Xi is less than the cost of the link between A and Xj (j = i + 1). If two costs are the same, Xi's ID is less than Xj's ID. In another example, X1, X2, ..., Xn are ordered by their IDs. If they are not ordered, then make them in the order. 4. Add Xi (i = 1, 2, ..., n) into the front of Cq and goto step 1. A third algorithm for building a tree from node R as root starts with a candidate list Cq containing R associated with cost 0 and an empty flooding topology Ft: 1. Remove the first node A from Cq and add A into Ft 2. If all the nodes are on Ft, then return with Ft 3. Suppose that node A is associated with a cost Ca which is the cost from root R to node A, node Xi (i = 1, 2, ..., n) is connected to node A and not in Ft and the cost of the link between A and Xi is LCi (i=1, 2, ..., n). Compute Ci = Ca + LCi, check if Xi is in Cq and if Cxi (cost from R to Xi) < Ci. If Xi is not in Cq, then add Xi with cost Ci into Cq; If Xi is in Cq, then If Cxi > Ci then replace Xi with cost Cxi by Xi with Ci in Cq; If Cxi == Ci then add Xi with cost Ci into Cq. 4. Make sure Cq is in a special order. Suppose that Ai (i=1, 2, ..., m) are the nodes in Cq, Cai is the cost associated with Ai, and IDi is the ID of Ai. One order is that for any k = 1, 2, ..., m-1, Cak < Caj (j = k+1) or Cak = Caj and IDk < IDj. Goto step 1. A.2. Algorithms to Build Tree Considering Others An algorithm for building a tree from node R as root with consideration of others's support for flooding reduction starts with Chen, et al. Expires January 3, 2019 [Page 12] Internet-Draft OSPF Flooding Reduction July 2018 a candidate queue Cq containing R associated with previous hop PH=0 and an empty flooding topology Ft: 1. Remove the first node A that supports flooding reduction from the candidate queue Cq if there is such a node A; otherwise (i.e., if there is not such node A in Cq), then remove the first node A from Cq. Add A into the flooding topology Ft. 2. If Cq is empty or all nodes are on Ft, then return with Ft 3. Suppose that node Xi (i = 1, 2, ..., n) is connected to node A and not in the flooding topology Ft and X1, X2, ..., Xn are in a special order considering whether some of them that support flooding reduction (. For example, X1, X2, ..., Xn are ordered by the cost of the link between A and Xi. The cost of the link between A and Xi is less than that of the link between A and Xj (j = i + 1). If two costs are the same, Xi's ID is less than Xj's ID. The cost of a link is redefined such that 1) the cost of a link between A and Xi both support flooding reduction is much less than the cost of any link between A and Xk where Xk with F=0; 2) the real metric of a link between A and Xi and the real metric of a link between A and Xk are used as their costs for determining the order of Xi and Xk if they all (i.e., A, Xi and Xk) support flooding reduction or none of Xi and Xk support flooding reduction. 4. Add Xi (i = 1, 2, ..., n) associated with previous hop PH=A into the end of the candidate queue Cq, and goto step 1. Another algorithm for building a tree from node R as root with consideration of others' support for flooding reduction starts with a candidate queue Cq containing R associated with previous hop PH=0 and an empty flooding topology Ft: 1. Remove the first node A that surports flooding reduction from the candidate queue Cq if there is such a node A; otherwise (i.e., if there is not such node A in Cq), then remove the first node A from Cq. Add A into the flooding topology Ft. 2. If Cq is empty or all nodes are on Ft, then return with Ft. 3. Suppose that node Xi (i = 1, 2, ..., n) is connected to node A and not in the flooding topology Ft and X1, X2, ..., Xn are in a special order considering whether some of them support flooding reduction. For example, X1, X2, ..., Xn are ordered by the cost of the link between A and Xi. The cost of the link between A and Xi is less than the cost of the link between A and Xj (j = i + 1). If two costs are the same, Xi's ID is less than Xj's ID. Chen, et al. Expires January 3, 2019 [Page 13] Internet-Draft OSPF Flooding Reduction July 2018 The cost of a link is redefined such that 1) the cost of a link between A and Xi both surport flooding reduction is much less than the cost of any link between A and Xk where Xk does not surport flooding reduction; 2) the real metric of a link between A and Xi and the real metric of a link between A and Xk are used as their costs for determining the order of Xi and Xk if they all (i.e., A, Xi and Xk) surport flooding reduction or none of Xi and Xk supports flooding reduction. 4. Add Xi (i = 1, 2, ..., n) associated with previous hop PH=A into the front of the candidate queue Cq, and goto step 1. A third algorithm for building a tree from node R as root with consideration of others' surport for flooding reduction (using flag F = 1 for surport, and F = 0 for not surport in the following) starts with a candidate list Cq containing R associated with low order cost Lc=0, high order cost Hc=0 and previous hop ID PH=0, and an empty flooding topology Ft: 1. Remove the first node A from Cq and add A into Ft. 2. If all the nodes are on Ft, then return with Ft 3. Suppose that node A is associated with a cost Ca which is the cost from root R to node A, node Xi (i = 1, 2, ..., n) is connected to node A and not in Ft and the cost of the link between A and Xi is LCi (i=1, 2, ..., n). Compute Ci = Ca + LCi, check if Xi is in Cq and if Cxi (cost from R to Xi) < Ci. If Xi is not in Cq, then add Xi with cost Ci into Cq; If Xi is in Cq, then If Cxi > Ci then replace Xi with cost Cxi by Xi with Ci in Cq; If Cxi == Ci then add Xi with cost Ci into Cq. 4. Suppose that node A is associated with a low order cost LCa which is the low order cost from root R to node A and a high order cost HCa which is the high order cost from R to A, node Xi (i = 1, 2, ..., n) is connected to node A and not in the flooding topology Ft and the real cost of the link between A and Xi is Ci (i=1, 2, ..., n). Compute LCxi and HCxi: LCxi = LCa + Ci if both A and Xi have flag F set to one, otherwise LCxi = LCa HCxi = HCa + Ci if A or Xi does not have flag F set to one, otherwise HCxi = HCa If Xi is not in Cq, then add Xi associated with LCxi, HCxi and PH = A into Cq; If Xi associated with LCxi' and HCxi' and PHxi' is in Cq, then If HCxi' > HCxi then replace Xi with HCxi', LCxi' and PHxi' by Xi with HCxi, LCxi and PH=A in Cq; otherwise (i.e., HCxi' == HCxi) if LCxi' > LCxi , then replace Xi with HCxi', LCxi' and PHxi' by Xi with HCxi, LCxi and PH=A in Cq; otherwise (i.e., HCxi' == HCxi and LCxi' == LCxi) if PHxi' > PH, then Chen, et al. Expires January 3, 2019 [Page 14] Internet-Draft OSPF Flooding Reduction July 2018 replace Xi with HCxi', LCxi' and PHxi' by Xi with HCxi, LCxi and PH=A in Cq. 5. Make sure Cq is in a special order. Suppose that Ai (i=1, 2, ..., m) are the nodes in Cq, HCai and LCai are low order cost and high order cost associated with Ai, and IDi is the ID of Ai. One order is that for any k = 1, 2, ..., m-1, HCak < HCaj (j = k+1) or HCak = HCaj and LCak < LCaj or HCak = HCaj and LCak = LCaj and IDk < IDj. Goto step 1. A.3. Connecting Leaves Suppose that we have a flooding topology Ft built by one of the algorithms described above. Ft is like a tree. We may connect k (k >=0) leaves to the tree to have a enhanced flooding topology with more connectivity. Suppose that there are m (0 < m) leaves directly connected to a node X on the flooding topology Ft. Select k (k <= m) leaves through using a deterministic algorithm or rule. One algorithm or rule is to select k leaves that have smaller or larger IDs (i.e., the IDs of these k leaves are smaller/bigger than the IDs of the other leaves directly connected to node X). Since every node has a unique ID, selecting k leaves with smaller or larger IDs is deterministic. If k = 1, the leaf selected has the smallest/largest node ID among the IDs of all the leaves directly connected to node X. For a selected leaf L directly connected to a node N in the flooding topology Ft, select a connection/adjacency to another node from node L in Ft through using a deterministic algorithm or rule. Suppose that leaf node L is directly connected to nodes Ni (i = 1,2,...,s) in the flooding topology Ft via adjacencies and node Ni is not node N, IDi is the ID of node Ni, and Hi (i = 1,2,...,s) is the number of hops from node L to node Ni in the flooding topology Ft. One Algorithm or rule is to select the connection to node Nj (1 <= j <= s) such that Hj is the largest among H1, H2, ..., Hs. If there is another node Na ( 1 <= a <= s) and Hj = Ha, then select the one with smaller (or larger) node ID. That is that if Hj == Ha and IDj < IDa then select the connection to Nj for selecting the one with smaller node ID (or if Hj == Ha and IDj < IDa then select the connection to Na for selecting the one with larger node ID). Suppose that the number of connections in total between leaves selected and the nodes in the flooding topology Ft to be added is NLc. We may have a limit to NLc. Chen, et al. Expires January 3, 2019 [Page 15] Internet-Draft OSPF Flooding Reduction July 2018 Authors' Addresses Huaimo Chen Huawei Technologies Email: huaimo.chen@huawei.com Dean Cheng Huawei Technologies Email: dean.cheng@huawei.com Mehmet Toy Verizon USA Email: mehmet.toy@verizon.com Yi Yang IBM Cary, NC United States of America Email: yyietf@gmail.com Chen, et al. 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