Region Division in Networks using BGP-LS SPF Routing

Introduction To tackle issues such as slow convergence and excessive flooding overhead, large-scale routing networks generally adopt the method of region division to improve the efficiency of routing convergence and network management. The existing region division method typically divides a network into a backbone region and multiple non-backbone regions. The backbone region is unique and holds routing information for all regions. The non-backbone regions establish fixed connections around the backbone region. Thus, the nodes in the backbone region are responsible for traffic forwarding between all regions, and require higher performance. However, some networks (e.g., satellite networks) have the characteristics of a flat design, which means that no backbone region exists in the network. For these networks, the existing region division method does not work. This document describes a routing framework for region division in large-scale flat routing networks. This framework can be implemented on the basis of the existing BGP Link State (BGP-LS) Shortest Path First (SPF) routing , with some potential minor extensions defined in other documents. This document is the successor of , which described a region-based routing framework for satellite networks on the basis of existing Interior Gateway Protocols; this document re-targets that framework at BGP-LS SPF. describes an alternative, hierarchical routing architecture for satellite networks based on two-level IS-IS. describes use cases and requirements for multi-domain BGP-LS SPF, to which the framework in this document is applicable, and lists routing requirements for large satellite networks that motivate it.

Region Model Taking into account both of the generality and the practicability, this document adopts the quadrangle as the basic model of region division. That is, a region (e.g., R19) has four contiguous neighbor regions (R14, R18, R24, and R20), one in each type refers to a direction, as shown in Figure 1. In the figure, h is the number of nodes a region spans in one direction (for a satellite network, the number of satellites of one orbit plane that belong to the region) and l is the number of nodes it spans in the other direction (for a satellite network, the number of orbit planes); both are planned region parameters.

Basic Model of Region Division

Two Types of Network Node According to the relative position of a network node in the region, a network node can be classified as one of two types: the internal node or the border node. An internal node does not connect to nodes outside of the region it belongs to. A border node is part of its region and has connections to nodes outside of its region. Internal nodes and border nodes are collectively referred to as intra-region nodes. A node within a neighbor region that has a connection to a border node is called a cross-region neighbor node.

Border Node Capability The region border node should have the following features: (1) The capability of establishing connections with cross-region neighbor nodes. The border node acts as the agent for external connections of the local region, and is responsible for establishing connections with cross-region neighbor nodes. During the connection establishment process, these nodes build an adjacency relationship with each other. (2) The capability of maintaining connections with cross-region neighbor nodes. Changes in the connection relationship between a border node and a cross-region neighbor node need to be advertised to the nodes in the same region. Figure 2 presents a link failure between R19 adn R24; Node 8 is responsible for informing the other nodes in region R19.

Inter-region Link Failure (3) The capability of preventing routing information advertisement loops between regions. When a border node advertises routing information of its region to an adjacent region, there should be a mechanism deployed that prevents the advertised routing information from being looped back to the region the border node belongs to.

Internal Node Capability In addition to the above features required of the border node, both the border node and the internal node share the following feature: (1) The capability of maintaining intra-region network information. When a region is initially created, each node within the region establishes connections with its neighbor nodes, determines the nodes of the region based on the planned region parameters, advertises its own network information to the other nodes within the region, and continuously monitors its own direct links. When a sudden failure occurs, the node advertises it and updates its local network information and the intra-region topology. Figure 3 presents a link failure between Node 5 and Node 8; Node 5 and Node 8 are responsible for informing the other nodes in region R19.

Intra-region Link Failure

Data Packet Forwarding

Scenario 1: Intra-region Data Forwarding As shown in Figure 4, N2 and N4 are located in the same region, R19. N2 serves as the source node and sends data traffic to the destination node N4. The data forwarding path between N2 and N4 passes through nodes N2, N5, and N4 in turn. The routing process is consistent with the existing BGP-LS SPF routing .

Intra-region Data Forwarding ...-| N6 |-... | | \__/ \__/ | | ^ R18 ^ | -------------+--------+-------------------+---------+------------- R14 | v R19 v | R24 .--. | .--. .--. | .--. ..-| N4 |-... <+> ...-| N1 |-... <--> ...-| N4 |-... <+> ...-| N1 |-. \__/ | \__/ \__/ | \__/ ^ | ^ ^ ^ | ^ | | | | | | | v | v --------------->| v | v .--. | .--. .--. | .--. ..-| N5 |-... <+> ...-| N2 |-... <--> ...-| N5 |-... <+> ...-| N2 |-. \__/ | \__/ \__/ | \__/ ^ | ^ ^ | ^ | | | | | | v | v v | v .--. | .--. .--. | .--. ..-| N6 |-... <+> ...-| N3 |-... <--> ...-| N6 |-... <+> ...-| N3 |-. \__/ | \__/ \__/ | \__/ | ^ ^ | -------------+--------+-------------------+---------+------------- | v R20 v | | .--. .--. | | ...-| N1 |-... <--> ...-| N4 |-... | | \__/ \__/ | ]]>

Scenario 2: Inter-region Data Forwarding As shown in Figure 5, the BGP-LS SPF network is divided into 30 regions (R1 to R30). Source node S is located in region R4 and destination node D is located in region R22. S sends data to D. The data forwarding path between S and D passes through regions R4, R9, R14, R19, R18, R17, and R22 in turn.

End-to-end Inter-region Path > | | +-----+-----+-----+----^+-----+-----+ | R3 | R8 | R13 | R18^| R23 | R28 | | | | | ^| | | +-----+-----+-----+----^+-----+-----+ | R4 | R9 | R14 | R19^| R24 | R29 | | >>>>>>>>>>>>>>>>>>>>| | | +-----+-----+-----+-----+-----+-----+ | R5 | R10 | R15 | R20 | R25 | R30 | | | | | | | | +-----+-----+-----+-----+-----+-----+ ]]> As shown in Figure 6, the data is first forwarded from Node N6 in region R14 to Node N3 in region R19. Node N3 resolves the destination address and looks up the intra-region and inter-region routing tables to forward the data. According to the inter-region routing table, if the destination node is located in region R22, the data packet is forwarded via region R18, and the next hop node is Node N6 within region R19. Each node within region R19 repeats the above operations, and finally forwards the data packet is forwarded to Node N6 within region R18. For a border node, the next hop address in the inter-region routing table is the port address of the cross-region neighbor node.

Inter-region Data Forwarding ...-| N6 |-... | | \__/ \__/ | | ^ R18 ^ ^ | -------------+--------+-----------------|-+---------+------------- R14 | v R19 | v | R24 .--. | .--. .--. | .--. ..-| N4 |-... <+> ...-| N1 |-... <--> ...-| N4 |-... <+> ...-| N1 |-. \__/ | \__/ \__/ | \__/ ^ | ^ ^ ^ | ^ | | | | | | | v | v | v | v .--. | .--. .--. | .--. ..-| N5 |-... <+> ...-| N2 |-... <--> ...-| N5 |-... <+> ...-| N2 |-. \__/ | \__/ \__/ | \__/ ^ | ^ ^ ^ | ^ | | | | | | | v ---------------> v --------------->| v | v .--. | .--. .--. | .--. ..-| N6 |-... <+> ...-| N3 |-... <--> ...-| N6 |-... <+> ...-| N3 |-. \__/ | \__/ \__/ | \__/ | ^ ^ | -------------+--------+-------------------+---------+------------- | v R20 v | | .--. .--. | | ...-| N1 |-... <--> ...-| N4 |-... | | \__/ \__/ | ]]>

Security Considerations Security issues discussed in apply to this document.

Acknowledgements The authors would like to acknowledge Fenlin Zhou for his contribution to the predecessor of this document.

IANA Considerations This document has no IANA actions.