# Patent

## Title
Label distribution protocol over segment routing

## Abstract

A method and apparatus for forwarding packets through a network domain that contains nodes that are label distribution protocol (LDP) enabled and nodes that are segment routing (SR) enabled. In one embodiment, the method may include a network node receiving a packet with a label attached thereto. The node swaps the label with a segment identifier (ID). The node then forwards the packet to an SR node. In another embodiment, the method may include a network node receiving a packet with a segment ID attached thereto. The node swaps the segment ID with a label. The node then forwards the packet to an LDP enabled node.

## Background
RELATED APPLICATIONS
This application claims the domestic benefit under Title 35 of the United States Code §119(e) of U.S. Provisional Patent Application Ser. No. 61/791,242 entitled “Segment Routing,” filed Mar. 15, 2013, which is hereby incorporated by reference in its entirety and for all purposes as if completely and fully set forth herein.


BACKGROUND
Network nodes forward packets using forwarding tables. Network nodes may take form in one or more routers, one or more bridges, one or more switches, one or more servers, or any other suitable communications processing device. A packet is a formatted unit of data that typically contains control information and payload data. Control information may include: source and destination IP addresses, error detection codes like checksums, sequencing information, etc. Control information is typically found in packet headers and trailers, with payload data in between.
Packet forwarding requires a decision process that, while simple in concept, can be complex. Since packet forwarding decisions are handled by network nodes, the total time required for this can become a major limiting factor in overall network performance.
Multiprotocol Label Switching (MPLS) is one packet forwarding mechanism. MPLS Nodes make packet forwarding decisions based on Label Distribution Protocol (LDP) distributed labels attached to packets and LDP forwarding tables. LDP is a process in which network nodes capable of MPLS exchange LDP labels (hereinafter labels). Packet forwarding based on labels stands in stark contrast to traditional Internet Protocol (IP) routing in which packet forwarding decisions are made by nodes using IP addresses contained within the packet.



BRIEF DESCRIPTION OF THE DRAWINGS
The present disclosure may be better understood, and its numerous objects, features, and advantages made apparent to those skilled in the art by referencing the accompanying drawings.
FIG. 1 is a block diagram illustrating certain components of an example network.
FIG. 2 is a block diagram illustrating certain components of an example network.
FIG. 3 is a flow chart illustrating an example process employed by a node of FIG. 2.
FIG. 4 is a block diagram illustrating certain components of an example network.
FIG. 5 is a block diagram illustrating certain components of an example network.
FIG. 6 is a block diagram illustrating certain components of an example network.
FIG. 7 is a block diagram illustrating certain components of an example node that can be employed in the network of FIG. 1, 2, 4, 5 or 6.



DETAILED DESCRIPTION
1. Overview
A method and apparatus is disclosed for forwarding packets through a network domain that contains nodes that are LDP enabled and nodes that are segment routing (SR) enabled. In one embodiment, the method may include a network node receiving a packet with a label attached thereto. The node swaps the label with a segment identifier (ID). The node then forwards the packet to an SR node. In another embodiment, the method may include a network node receiving a packet with a segment ID attached thereto. The node swaps the segment ID with a label. The node then forwards the packet to an LDP enabled node.
2. Packet Forwarding Mechanisms
IP routing and MPLS are distinct packet forwarding mechanisms. IP routing uses IP addresses inside packet headers to make packet forwarding decisions. In contrast, MPLS implements packet forwarding decisions based on short path identifiers called labels attached to packets. Segment routing (SR) is yet another packet forwarding mechanism and can be seen as a modification of MPLS. SR is similar to MPLS in many regards and employs many of the data plane functions thereof. For example, like MPLS packet forwarding decisions in SR can be based on short path identifiers called segment IDs attached to packets. While similarities exist between MPLS and SR, substantial differences exist between SR and MPLS as will be more fully described below.
2.1 IP Packet Routing
IP packet routing uses IP forwarding tables, which are created at nodes using routing information distributed between nodes via one or more protocols like the internal gateway protocol (IGP) and/or the border gateway protocol (BGP). In simple terms, IP forwarding tables map destination addresses to the next hops that packets take to reach their destinations. When a node receives a packet, the node can access a forwarding table using the packet's destination IP address and lookup a corresponding egress interface to the next hop. The node then forwards the packet through the egress interface. The next hop that receives the packet performs its own forwarding table lookup using the same destination IP address in the packet, and so on.
2.2 MPLS and LDP
MPLS is commonly employed in provider networks consisting of interconnected LDP nodes. For the purposes of explanation, LDP nodes take form in MPLS enabled nodes that also implement LDP in the control plane. Packets enter an MPLS network via an ingress edge LDP node, travel hop-by-hop along a label-switched path (LSP) that typically includes one or more core LDP nodes, and exit via an egress edge LDP node.
Packets are forwarded along an LSP based on labels and LDP forwarding tables. Labels allow for the use of very fast and simple forwarding engines in the data planes of nodes. Another benefit of MPLS is the elimination of dependence on a particular Open Systems Interconnection (OSI) model data link layer technology to forward packets.
A label is a short, fixed-length, locally significant identifier that can be associated with a forwarding equivalence class (FEC). Packets associated with the same FEC should follow the same LSP through the network. LSPs can be established for a variety of purposes, such as to guarantee a certain level of performance when transmitting packets, to forward packets around network congestion, to create tunnels for network-based virtual private networks, etc. In many ways, LSPs are no different than circuit-switched paths in ATM or Frame Relay networks, except that they are not dependent on a particular Layer 2 technology.
LDP is employed in the control planes of nodes. For purpose of explanation, LDP nodes are those nodes that employ only LDP in their control plane. Two LDP nodes, called LDP peers, can bi-directionally exchange labels on a FEC by FEC basis. LDP can be used in a process of building and maintaining LDP forwarding tables that map labels and next hop egress interfaces. These forwarding tables can be used to forward packets through MPLS networks as more fully described below.
When a packet is received by an ingress edge LDP node of an MPLS network, the ingress node may use information in the packet to determine a FEC corresponding to an LSP the packet should take across the network to reach the packet's destination IP address. In one embodiment, the FEC is an identifier of the egress edge node that is closest to the packet's destination IP address. In this embodiment, the FEC may take form in the egress edge node's loopback address.
Characteristics for determining the FEC for a packet can vary, but typically the determination is based on the packet's destination IP address. Quality of Service for the packet or other information may also be used to determine the FEC. Once determined, the ingress edge LDP node can access a table to select a label that is mapped to the FEC. The table may also map a next hop egress interface to the FEC. Before the ingress edge LDP node forwards the packet to the next hop via, the ingress node attaches the label.
When an LDP node receives a packet with an attached label (i.e., the incoming label), the node accesses an LDP forwarding table to read a next hop egress interface and another label (i.e., an outgoing label), both which are mapped to the incoming label. Before the packet is forwarded via the egress interface, the LDP node swaps the incoming label with the outgoing label. The next hop receives the packet with label and may perform the same process. This process is often called hop-by-hop forwarding along a non-explicit path (i.e., the LSP). The penultimate node in the LSP may pop or remove the incoming label before forwarding the packet to an egress edge LDP node in the network, which in turn may forward the packet towards its destination using the packet's destination address and an IP forwarding table. In another embodiment, the egress edge LDP node may pop the incoming label before forwarding the packet using the destination address and an IP forwarding table.
To illustrate MPLS aspects, FIG. 1 shows a portion of an example MPLS network 100 that includes LDP nodes 102-122 coupled together via communication links. An LSP from node 102 to node 122 can be created so that all packets of a stream associated with a particular FEC sent from node 102 to node 122 will travel through the same set of nodes. Each LDP node maintains information for the LSP established through it in an LDP forwarding table. Thus, if node 110 knows that node 114 is the next hop along the LSP for all packets received from node 102 that are destined for node 122, node 110 can forward the packets to node 114.
2.3 Segment Routing
Segment routing (SR) is a mechanism in which nodes forward packets using SR forwarding tables and segment IDs Like MPLS, SR enables very fast and simple forwarding engines in the data plane of nodes. SR is not dependent on a particular Open Systems Interconnection (OSI) model data link layer technology to forward packets.
SR nodes (i.e., nodes employing SR) make packet forwarding decisions based on segment IDs as opposed to LDP distributed labels, and as a result SR nodes need not employ LDP in their control planes. In one embodiment, segment IDs are substantially shorter than labels. The range for segment IDs may be distinct from the range for labels. Unless otherwise indicated, the SR nodes lack LDP in their control plane.
Packets can enter an SR enabled network (i.e., a network of nodes that are SR enabled) via an ingress edge SR node, travel hop-by-hop along a segment path (SP) that includes one or more core SR nodes, and exit the network via an egress edge SR node.
Like labels, segment IDs are short (relative to the length of an IP address or a FEC), fixed-length identifiers. In one embodiment, segment IDs are shorter than labels. Segment IDs may correspond to topological segments of a network, services provided by network nodes, etc. Topological segments represent one hop or multi hop paths to SR nodes. Topological segments act as sub-paths that can be combined to form an SP. Stacks of segment IDs can represent SPs, and SPs can be associated with FECs as will be more fully described below.
There are several types of segment IDs including nodal-segment IDs, adjacency-segment IDs, etc. Nodal-segment IDs are assigned to SR nodes so that no two SR nodes belonging to a network domain are assigned the same nodal-segment ID. Nodal-segment IDs can be mapped to unique node identifiers such as node loopback IP addresses (hereinafter node loopbacks). In one embodiment, all assigned nodal-segment IDs are selected from a predefined ID range (e.g., [32, 5000]). A nodal-segment ID corresponds to a one-hop or a multi-hop, shortest path (SPT) to an SR node assigned the nodal-segment ID as will be more fully described below.
An adjacency-segment ID represents a direct link between adjacent SR nodes in a network. Links can be uniquely identified. For purposes of explanation only, this disclosure will identify a link using the loopbacks of nodes between which the link is positioned. To illustrate, for a link between two nodes identified by node loopback X and node loopback Y, the link will be identified herein as link XY. Because loopbacks are unique, link IDs are unique. Link IDs should not be confused with adjacency-segment IDs; adjacency-segment IDs may not be unique within a network. This disclosure will presume that only one link exists between nodes in a network, it being understood the present disclosure should not be limited thereto.
Each SR node can assign a distinct adjacency-segment ID for each of the node's links. Adjacency-segment IDs are locally significant; separate SR nodes may assign the same adjacency-segment ID, but the adjacency-segment ID represents distinct links. In one embodiment, adjacency-segment IDs are selected from a predefined range that is outside the predefined range for nodal-segment IDs.
SR nodes can advertise routing information including nodal-segment IDs bound to loopbacks, adjacency-segment IDs mapped to link IDs, etc., using protocols such as IGP and/or BGP with SR extension. Nodes may use the routing information they receive in order to create topology maps of the network. The maps can be used to create or update forwarding tables. To illustrate, a node can use the map it creates to identify next hop egress interfaces for shortest paths (SPTs) to respective node loopbacks. The identified SPT or next hop egress interfaces for the loopbacks are then mapped to respective nodal-segment IDs in the forwarding table. SR nodes can also map their adjacency-segment IDs to egress interfaces for respective links in SR forwarding tables. Because adjacency-segment IDs are locally significant, however, adjacency-segment IDs should only be mapped in SR forwarding tables of the nodes that advertise the adjacency-segment IDs. In other words, an SR node that advertises an adjacency-segment ID should be the only node in the network area that has a SR forwarding table that maps the adjacency-segment ID to an egress interface.
As noted above, SR enables segment paths (SPs) through a network. SPs can be associated with FECs. Packets associated with the same FEC normally traverse the same SP towards their destination. Nodes in SPs make forwarding decisions based on segment IDs, not based on the contents (e.g., destination IP addresses) of packets. As such, packet forwarding in SPs is not dependent on a particular Layer 2 technology.
SR edge nodes and/or other devices (e.g., a path computation node) can use advertised routing information (nodal-segment IDs bound to loopbacks, adjacency-segment IDs mapped to link IDs, etc.) and topological maps to create ordered lists of segment IDs (i.e., segment ID stacks). Segment ID stacks correspond to respective SPs. Individual segment IDs in a stack may correspond to respective segments or sub paths of a corresponding SP.
When an ingress edge SR node receives a packet, the node or a path computation element in data communication with the node, can select an SP for the packet based on information contained in the packet. In one embodiment, a FEC may be determined for the packet using the packet's destination address. Like MPLS, this FEC may take form in an identifier (e.g., loopback) of the egress edge node that is closest to the destination IP address of the received packet. The FEC is then used to select a segment ID stack mapped thereto. The ingress edge node can attach the selected segment ID stack to the packet via a header. The packet with attached stack is forwarded along and traverses the segments of the SP in an order that corresponds to the list order of the segment IDs in the stack. A forwarding engine operating in the data plane of each SR node can use the top segment ID within the stack to lookup the egress interface for next hop. As the packet and attached segment ID stack are forwarded along the SP in a hop-by-hop fashion, segment IDs can be popped off the top of the stack. In another embodiment, the attached stack of segment IDs remains unchanged as the packet is forwarded along the SP. In this embodiment, a pointer to an active segment ID in the stack can be advanced as the packet is forwarded along the SP. In contrast to MPLS, however, segment IDs are not swapped as the packet and attached segment ID stack are forwarded along the SP.
To illustrate general concepts of SR, FIG. 2 shows an example SR enabled provider network that is in data communication with nodes AE1 and AE2. Network 202 consists of SR nodes 204-222. Nodes 204-210 are assigned unique nodal-segment IDs 64-67, respectively, nodes 212-218 are assigned unique nodal-segment IDs 68-71, respectively, and node 222 is assigned unique nodal-segment ID of 72. Each of the SR nodes 204-222 have interfaces that are identified as shown. For example, node 204 has three interfaces designated 1-3, respectively. Each of the nodes 204-222 is assigned a unique loopback. Loopbacks A-D are assigned to nodes 204-210, respectively, loopbacks M-P are assigned to nodes 212-218 respectively, and loopback Z is assigned to node 222. These loopbacks are unique in the network and can be used for several purposes such as calculating the topology of network 202, which in turn can be used to create SPs and/or to identify SPTs and thus next hop egress interfaces for SR forwarding tables. Nodes 204-222 can also assign locally significant adjacency-segment IDs. For example, node 208 can assign adjacency-segment IDs 9001-9003 to links CB, CD, and CO, respectively.
Each of SR nodes 204-222 can advertise routing information to the other nodes in network 202 using IGP with SR extension. For example, node 208 can generate and send one or more advertisements that include adjacency-segment IDs 9001-9003 bound to link IDs CB, CD, and CO, respectively, and nodal-segment ID 66 bound to loopback C. One of ordinary skill understands that advertisements may contain additional information. Using the advertisements they receive, the control planes of nodes 204-222 can generate respective SR forwarding tables for use in the data planes. For example, node 208 can generate example SR forwarding table 240 that maps adjacency-segment IDs 9001-9003 to node interface IDs 1-3, respectively, and nodal-segment IDs such as 64, 65, 67, 70, and 72, to node 208 interfaces 1, 1, 2, 3, and 2, respectively, which are the SPT next hop egress interfaces determined by node 208 for loopbacks A, B, D, O, and Z respectively. It is noted that in the embodiment shown, only SR forwarding table 240 maps adjacency-segment IDs 9001-9003 to interfaces; SR forwarding tables in the other nodes of network 202 should not map adjacency-segment IDs 9001-9003.
In addition to creating SR forwarding tables, SR nodes or a path computation node (not shown) can create segment ID stacks for respective SPs. For example, ingress edge node 204 creates example segment ID stack 224 for an SP between ingress edge node 204 and egress edge node 222. Example segment stack 224 can be created for a particular FEC (e.g., FEC Z). Example stack 224 includes three segment IDs: nodal-segment IDs 66 and 72 advertised by nodes 208 and 222, respectively, and adjacency-segment ID 9003 advertised by node 208. Stack 224 corresponds to an SP in which packets flow in order through nodes 204, 206, 208, 216, 218, and 222.
In response to receiving a packet that is destined for a device that can be reached via AE2, which in turn can be reached via node 222, SR node 204 can select a segment ID stack based on information contained in the packet. For example, node 204 can select FEC Z (i.e., the loopback for node 222) for a received packet P based on the destination IP address in packet P and/or other information. FEC Z is mapped to example stack 224 in a table not shown. Node 204 attaches stack 224 to packet P. Example segment stack 224 lists segment IDs that correspond to one hop and multi hop segments that packets traverse to reach egress edge node 222. The one hop and multi hop segments collectively form the SP corresponding to stack 224. Once the segment stack 224 is attached to packet P, ingress SR enable node 204 may access a SR forwarding table (not shown) using the top segment ID (e.g., segment ID=66) to read egress interface identifier 2, which is the next hop egress interface for the SPT to the SR node assigned nodal-segment ID 66.
With continuing reference to FIG. 2, FIG. 3 illustrates example process of packet forwarding using segment IDs according to one embodiment. More particularly, FIG. 3 illustrates an example method performed by an SR node, including an edge node, in a network like that shown in FIG. 2. In response to receiving a packet with an attached segment ID stack, or in response to attaching a segment ID stack to a packet, the SR node determines in step 304 whether the top segment ID of the stack matches the nodal-segment ID assigned to the SR node. If there is a match, the process proceeds to step 306 where the SR node pops the top segment ID, which may expose an underlying segment ID as the new top segment ID. If there is no new top segment ID (i.e., the segment popped in 306 was the last segment ID of the stack) the packet P has arrived at the egress edge node, and the process ends. If a new top segment ID is exposed, or if there is no match of segment IDs in step 304, the SR node accesses its SR forwarding table in step 314 to read the egress interface that is mapped to the top segment ID. In step 316 the SR node determines whether the top segment ID is an adjacency-segment ID. This determination can be implemented by simply comparing the top segment ID with the designated range of adjacency-segment IDs that are available for assignment within the network. If the top segment ID is found to be within the designated range, the top segment ID is an adjacency-segment ID and it is popped. In step 322 the SR node forwards packet P and attached stack to the next node via the egress interface identified in step 314.
With continuing reference to FIGS. 3, FIG. 2 shows packet P and attached stack 224 as it is forwarded by nodes. As shown, nodes 204 and 206 forward packet P and stack 224 without popping a segment ID. However, node 208 pops nodal-segment ID 66 and adjacency-segment ID 9003 in accordance with steps 306 and 320, respectively, before the packet P and stack 224 are forwarded to node 216 in accordance with step 322. Nodes 216 and 218 forward packet P and stack 224 without popping segment IDs. SR egress edge node 222 recognizes itself as the last hop of the SP. Eventually, node 222 may employ traditional IP routing and forward packet P to access node AE2 based on routing table lookup using the destination IP address within packet P.
3. Hybrid Networks
FIGS. 1 and 2 illustrate example provider networks that contain LDP nodes and SR nodes, respectively. Some providers may employ hybrid networks or networks that contain both LDP and SR nodes. Differences in the packet forwarding mechanisms described above with respect to SR and LDP nodes, however, may preclude interoperability between SR and LDP nodes when directly coupled to each other.
A hybrid network can successfully implement packet transport to its destination if, in one embodiment, the hybrid network is subdivided into intermediate system-intermediate system (IS-IS) or open shortest path (OSPF) areas or levels with SR/LDP nodes (i.e., nodes that implement both SR and LDP) at borders or boundaries therebetween. As will be more fully described below the SR/LDP nodes can act as level 1/level 2 border nodes that facilitate the exchange packets and routing information between SR and LDP areas. FIGS. 4-6 illustrate respective portions of example provider networks that employs SR, LDP, and SR/LDP nodes.
3.1 LDP Into SR
FIG. 4 illustrates a portion of an example hybrid network 400 that employs LDP, SR, and SR/LDP nodes. Nodes 402 and 404 are LDP enabled, nodes 410 and 412 are SR enabled, and border node 406 is SR/LDP enabled. Network 400 is subdivided into SR and LDP areas with nodes 402-406 contained in an LDP area and nodes 406-412 contained in an SR area. Additional areas are contemplated. SR/LDP node 406 may be contained in both the LSP and SR areas.
Nodes in network 400 have unique loopbacks. For example, nodes 402-412 are assigned loopbacks A-E, respectively. Each of the SR and SR/LDP nodes is assigned a nodal-segment ID that is unique within its SR area. Specifically, nodes 406-412 are assigned nodal-segment IDs 66-70, respectively.
All nodes in an area may employ IGP to advertise their routing information including their loopbacks to other nodes in the area. SR and SR/LDP nodes such as nodes 406-412 extend their IGP advertisements by binding their loopbacks to their respective nodal-segment IDs. Advertisements originating from the SR and SR/LDP nodes may also contain additional information indicating they are SR enabled. SR/LDP nodes are also capable of propagating loopback addresses of edge nodes of one area into an adjacent area using, for example, internal IGP. In this regard, an SR/LDP node can advertise that an edge node in one area can be reached via the SR/LDP node. For example SR/LDP node 406 can advertise to the LDP nodes that loopback E of node 412 is reachable via loopback C of SR/LDP node 406. This enables reachability of SR edge nodes via SR/LDP nodes. In one embodiment, SR/LDP nodes do not propagate nodal-segment IDs bound to SR edge node loopbacks into the LDP area.
The nodes in an area can use the routing information they receive, including loopbacks of edge nodes in other areas, to create paths and/or forwarding tables. For example nodes in the LDP area can create or update topology maps of their area, which in turn can be used to create or update LSPs and LDP forwarding tables. Since SR/LDP node 406 advertises loopback E can be reached via loopback C, the LDP nodes 402 and 404 use loopback C to calculate the SPT to loopback E. Entries in the LDP tables can map labels for FEC E using the SPT egress to loopback C. Similarly, nodes in the SR area within can create or update topology maps of their area, which in turn can be used to create or update SPs and SR forwarding tables. SR/LDP nodes can create a forwarding table for each area in which they are contained. These tables can be logically linked to enable SR/LDP nodes such as node 406 to function as a merge point between LSPs and SPs as will be more fully described below. It is noted that while SR/LDP and LDP nodes in the LDP area exchange labels with each other via LDP peering sessions, SR/LDP and SR nodes in the SR area do not exchange LDP labels with each other. SR/LDP nodes can track neighbor nodes that are or are not LDP enabled. For example, SR/LDP node 406 will recognize that neighbor node 410 is not LDP enabled when node 410 fails to respond to an LDP peering request from SR/LDP node 406.
SR/LDP nodes, like node 406, are capable of exchanging packets between areas. SR/LDP nodes can transfer a packet from an LSP in the LDP area to an SP in the SR area. To illustrate, ingress edge LDP node 402 may receive a packet P destined for a device that is reachable via node AE2. In response to receiving the packet, ingress edge LDP node 402 or a path computation node may determine a FEC for packet P based upon information such as the packet's destination IP address. For example, node 402 may select loopback E as the FEC for an LSP the packet P should take to edge node 412. The selected FEC is mapped to label L1 by a table stored in memory of node 402. In accordance with the MPLS forwarding mechanism described above, ingress node 402 forwards the packet P and label L1 to the next hop (i.e., LDP node 404) of the LSP via the egress interface mapped to label L1. The next hop node 404 accesses its LDP forwarding table to read label L2, which is mapped to label L1. Node 404 swaps L1 with L2, and forwards the packet P to SR/LDP node 406.
SR/LDP node 406 is the last hop in the LSP. SR/LDP node 406 can forward the packet P on a corresponding SP. Using its logically linked LDP and SR forwarding tables, SR/LDP node 406 maps label L2 to loopback E, and loopback E to nodal-segment ID 70. It is noted that SR/LDP node 406 can map loopback E to nodal-segment ID 70 as a result of the advertisement it received previously from node 412 that included loopback E bound to segment ID 70. In an alternative embodiment, the SR/LDP can determine the nodal-segment ID by first determining the FEC (i.e., loopback E) based upon information such as the destination IP address contained with the packet P. Regardless of the fashion in which SR/LDP node 406 determines the nodal-segment ID 70, SR/LDP node 406 essentially swaps the incoming label L2 with the nodal-segment ID 70 and forwards packet P with attached segment ID 70 in accordance with the process shown in FIG. 3 or a modification thereof.
3.2 SR Into LDP
SR/LDP nodes can transfer a packet from an LSP to an SP as described above. SR/LDP nodes can also transfer a packet from an SP to an LSP. FIG. 5 illustrates another example hybrid network 500 that includes nodes 502-512 coupled between nodes AE1 and AE2. Network 500 is subdivided into SR and LDP areas with nodes 502-506 contained in an SR area, and nodes 506-512 contained in an LDP area. SR/LDP node 506 is contained in both the LSP and SR areas. SR/LDP node 506 implements features of SR/LDP node 406 described above that enable transport of a packet from LDP enabled edge node 512 to SR enabled edge node 502. As will be more fully described below, SR node 506 also enables packet transport from SR edge node 502 to LDP edge node 512.
All nodes are assigned a unique loopback. For example, nodes 502-512 are assigned loopbacks F-J, respectively. Each of the SR and SR/LDP nodes is assigned a nodal-segment ID that is unique within network 500. Specifically, nodes 502-506 are assigned nodal-segment IDs 72-76, respectively.
Like the areas in FIG. 4, each area in the subdivided network 500 can be configured to implement its own IGP. In one embodiment, SR and SR/LDP nodes such as nodes 502-506 extend their IGP advertisements to include loopbacks bound to respective nodal-segment IDs. Advertisements originating from the SR and SR/LDP nodes may also contain additional information indicating they are SR enabled.
SR/LDP nodes can propagate the loopbacks of SR edge nodes into the LDP using, for example, internal IGP. For example SR/LDP node 506 can propagate the loopback J of node 512 into the SR area in addition to propagating the loopback F of node 502 into the LDP area. In this regard, SR/LDP node 506 can advertise to nodes 502 and 504 that loopback J assigned to LDP edge node 512 can be reached via loopback H assigned to SR/LDP node 506. Before an SR/LDP node propagates the loopback of an LDP edge node into the SR area, the SR/LDP node may bind a unique nodal-segment ID to the LDP edge node loopback. In the illustrated example, the loopback J is propagated into the SR area with a nodal-segment ID 80. This enables reachability of LDP edge nodes via SR/LDP nodes and SPs as will be more fully described below. An SR-TLV flag can be disabled to prevent PHP from occurring.
Area nodes use the routing information they receive, including loopbacks of edge nodes in other areas, to create paths and/or forwarding tables. Nodes in the LDP area can create or update topology maps of their area, which in turn can be used to create or update LSPs and LDP forwarding tables. Similarly, nodes in the SR area within can create or update topology maps of their area, which in turn can be used to create or update SPs and SR forwarding tables. Since SR/LDP node 506 advertises that loopback J/nodal-segment 80 can be reached via loopback H, the SR nodes 502 and 504 use loopback H to calculate the SPT to loopback J/nodal-segment ID 80. Entries in the SR tables map nodal-segment ID 80 to the SPT egress towards loopback H. SR/LDP nodes can create a forwarding table for each area in which they are contained. These tables can be logically linked or merged to enable SR/LDP nodes such as node 506 to function as a merge point between LSPs and SPs as will be more fully described below.
SR/LDP nodes can transfer a packet from an SP in the SR area to an LSP in the LDP area. To illustrate, ingress edge SR node 502 may receive a packet P destined for a device that is reachable via node AE2. In response to receiving the packet, ingress edge SR node 502 or a path computation node may determine a FEC for packet P based upon information such as the packet's destination IP address. For example, node 502 may select loopback J as the FEC for a path the packet P should take to edge node 512, which is connected to AE2. The selected FEC can be mapped to nodal-segment ID 80 by node 502. In accordance with the procedure described in FIG. 3, ingress node 502 forwards the packet P and nodal-segment ID 80 to the next hop (i.e., SR node 504) of the SP towards node 512 via the egress interface mapped to nodal-segment ID 80. The next hop node 504 accesses its SR forwarding table to read the egress interface to the next hop SR node 506.
When SR/LDP node 506 receives the packet P with attached nodal-segment ID 80, its forwarding table does not provide an egress interface for the next SR hop. Rather, the forwarding table entry mapped to the nodal-segment ID may indicate the packet should be forwarded on an LSP associated with FEC J. As a result, SR/LDP node 506 can forward the packet P on a corresponding LSP. To that end, the forwarding table may map loopback J to a label L3, which was received from LDP node 510 during a prior LDP peering session. SR/LDP node 506 removes nodal-segment ID 80 and attaches the label L3 to packet P. SR/LDP node 506 can then forward packet P with attached label L3 from the egress interface mapped to FEC J in the node's forwarding table. The next hop node 510 forwards the packet P to egress node 512 after swapping or popping label L3 in accordance with the MPLS forwarding mechanism described above.
3.3 LDP over SR
FIGS. 4 and 5 illustrate SR/LDP nodes that enable packet transfer from an SR area into LDP area, and LDP area into SR area, respectively. FIG. 6 illustrates a hybrid network 600 that enables packet transfer from an LDP area to another LDP area via an SR area.
FIG. 6 illustrates a portion of an example hybrid network 600 that employs LDP, SR, and SR/LDP nodes. LDP nodes 602, 604, 612, and 614 are LDP enabled, node 608 is SR enabled, and nodes 606 and 610 are SR/LDP enabled. Network 600 is divided into an SR and two LDP areas. Specifically, nodes 602-606 are contained in a first LDP area 620, nodes 610-614 are contained in a second LDP area 622, and nodes 606-610 are contained in an SR area 624. In this embodiment, SR/LDP nodes 606 and 610 act as border nodes. Operational aspects of SR/LDP nodes 406 and/or 506 can be employed by SR/LDP nodes 606 and 610 to enable packet transfer via the SR area.
Nodes in network 600 have unique loopbacks. For example, nodes 606-614 are assigned loopbacks K-Q, respectively. Each of the SR and SR/LDP nodes is assigned a nodal segment ID that is unique within its SR area. Specifically, nodes 606-610 are assigned nodal segments 82-86, respectively.
Nodes in each of the areas 620-624 may employ IGP to advertise their routing information including their loopbacks to other nodes in the area. SR and SR/LDP nodes 606-610 extend their advertisements by binding their loopbacks to their respective nodal-segment IDs. SR/LDP nodes 606 and 610 can propagate the loopbacks of edge nodes from one area into another in much the same manner in which SR/LDP nodes 406 and 506 propagate loopbacks of edge nodes into adjacent areas. To illustrate, SR/LDP node 610 can advertise to nodes in SR area 624, including nodes 608 and 606, that loopback Q, which is assigned to LDP edge node 614, can be reached via loopback O assigned to SR/LDP node 610. SR/LDP node 610 may bind a unique nodal-segment ID 90 to loopback Q in the advertisement that is propagated into SR area 624. This enables reachability of LDP edge node 614 via SR/LDP node 610 and an SP in area 624 as will be more fully described below. After SR/LDP node 610 advertises loopback Q to SR/LDP node 606, SR/LDP node 606 can in turn advertise to LDP nodes in area 620 that loopback Q is reachable via loopback M of SR/LDP node 606. This enables reachability of LDP edge node 614 via SR/LDP node 606 and an LSP in area 622. In one embodiment, SR/LDP node 606 does not propagate nodal-segment ID 90 bound to LDP edge node 614 by SR/LDP node 610.
The nodes in each area use the routing information they receive, including loopbacks of edge nodes in other areas, to create paths and/or forwarding tables. Nodes within the first LDP area 620 can create or update topology maps in this area, which in turn can be used to create LSPs and LDP forwarding tables. Since SR/LDP node 606 advertises loopback Q can be reached via loopback M, the LDP nodes 602 and 604 use loopback M to calculate the SPT to loopback Q. Nodes in the second LDP area 622 may likewise create or update topology maps in this area, which in turn can be used to create or update LSPs and forwarding tables. Lastly, nodes 606-610 in the SR area 624 can create or later update topology maps in this area, which in turn can be used to create or update SPs and SR forwarding tables. Since SR/LDP node 610 advertises that loopback Q/nodal-segment 90 can be reached via loopback O, nodes 606 and 608 use loopback O to calculate the SPT to loopback Q/nodal-segment ID 00. The SR/LDP nodes 606 and 610 can create a forwarding table for each area in which they are contained. The tables within an SR/LDP node can be logically linked to enable the SR/LDP nodes to function as merge points between LSPs and SPs.
SR/LDP node 606 can transfer a packet, which is received via an LSP in LDP area 620, to an SP in SR area 624, and SR/LDP 610 can transfer a packet, which is received via the SP in area 624, to an LSP in LDP are 622. In this fashion, network 600 can implement packet transfer over an SP. To illustrate, ingress LDP node 602 may receive a packet P destined for a device reachable via node AE2. In response to receiving the packet, ingress LDP node or a path computation node may determine a FEC for the packet P based upon information such as the packet's destination IP address. To illustrate, node 602 may select loopback Q as the FEC for packet P. The selected FEC corresponds to an LSP and is mapped in memory of node 602 to label L1. In accordance with the MPLS procedure described above, ingress node 602 forwards the packet P and label L1 to the next hop (LDP node 604) of the LSP toward node 614 via the egress interface mapped to label L1. The next hop 602 swaps L1 with L2, and forwards the packet P to SR/LDP node 606, which is the next and last hop on the LSP through LDP area 620.
SR/LDP node 606, using the logical combination of its LDP and SR forwarding tables it has in memory, maps labeled L2 to loopback O, loopback O to nodal segment 90. SR/LDP node 606 essentially swaps the incoming label L2 that is attached to the packet P with the nodal-segment ID 90. Thereafter, SR/LDP node 606 forwards the packet P with attached nodal-segment ID 90 in accordance with the process shown in FIG. 3 or a modification thereof. SR node 608 receives packet P with attached nodal-segment ID 90, and forwards the packet along with the segment ID 90 to SR/LDP node 610 in accordance with FIG. 3.
SR/LDP node 610 is the last hop in the SP-SR area 624. When SR/LDP node 610 receives packet P with attached nodal-segment ID 90, its forwarding table does not provide an egress interface for the next SR hop. Rather, the forwarding table entry mapped to the nodal-segment ID may indicate the packet should be forwarded on LSP in LDP area 622, which LSP is associated with a FEC identified by loopback Q. As a result, SR/LDP node 610 can forward the packet P toward its ultimate destination. To this end, SR/LDP node 610 may map loopback Q to a label L3, which was received from node 612 during a prior LDP peering session. SR/LDP node 610 removes nodal-segment ID 90 and attaches label L3 to packet P. SR/LDP node 610 can then forward packet P with attached label L3 via the egress interface mapped to FEC Q in the node's forwarding table. The next hop node 612 forwards the packet P to egress node 614 after swapping or popping label L3 in accordance the MPLS forwarding mechanism described above.
FIG. 7 is a block diagram illustrating certain additional and/or alternative components of nodes that can be employed in the networks shown in FIG. 1, 2, 4, 5, or 6. In this depiction, node 700 includes a number of line cards (line cards 702(1)-(N)) that are communicatively coupled to a forwarding engine or packet forwarder 710 and a processor 720 via a data bus 730 and a result bus 740. Line cards 702(1)-(N) include a number of port processors 750(1,1)-(N,N) which are controlled by port processor controllers 760(1)-(N). It will also be noted that forwarding engine 710 and processor 720 are not only coupled to one another via data bus 730 and result bus 740, but are also communicatively coupled to one another by a communications link 770.
The processors 750 and 760 of each line card 702 may be mounted on a single printed circuit board. When a packet or packet and header are received, the packet or packet and header may be identified and analyzed by router 700 in the following manner. Upon receipt, a packet (or some or all of its control information) or packet and header is sent from the one of port processors 750(1,1)-(N,N) at which the packet or packet and header was received to one or more of those devices coupled to data bus 730 (e.g., others of port processors 650(1,1)-(N,N), forwarding engine 710 and/or processor 720). Handling of the packet or packet and header can be determined, for example, by forwarding engine 710. For example, forwarding engine 710 may determine that the packet or packet and header should be forwarded to one or more of port processors 750(1,1)-(N,N). This can be accomplished by indicating to corresponding one(s) of port processor controllers 760(1)-(N) that the copy of the packet or packet and header held in the given one(s) of port processors 750(1,1)-(N,N) should be forwarded to the appropriate one of port processors 750(1,1)-(N,N). In addition, or alternatively, once a packet or packet and header has been identified for processing, forwarding engine 710, processor 720 or the like can be used to process the packet or packet and header in some manner or add packet security information, in order to secure the packet. On a node sourcing such a packet or packet and header, this processing can include, for example, encryption of some or all of the packet's or packet and header's information, the addition of a digital signature or some other information or processing capable of securing the packet or packet and header. On a node receiving such a processed packet or packet and header, the corresponding process is performed to recover or validate the packet's or packet and header's information that has been thusly protected.
Although the present invention has been described in connection with several embodiments, the invention is not intended to be limited to the specific forms set forth herein. On the contrary, it is intended to cover such alternatives, modifications, and equivalents as can be reasonably included within the scope of the invention as defined by the appended claims.

## Claims

1. A method comprising:
receiving a packet at a hybrid node, wherein
a first label distribution protocol (LDP) label is attached to the packet,
the first LDP label is advertised using LDP, and
the hybrid node employs LDP and segment routing (SR) in a control plane implemented by the hybrid node;

detaching the first LDP label from the packet;
attaching a segment identifier (ID) to the packet after the first LDP label is detached, wherein
the segment ID is advertised using a protocol other than LDP; and

forwarding the packet with the attached segment ID on a path towards a segment routing (SR) enabled node, wherein
the path comprises a first SR enabled node,
the first SR enabled node employs SR and lacks LDP in a control plane implemented by the first SR enabled node, and
the SR enabled node is uniquely assigned the segment ID.

2. The method of claim 1 wherein the packet with the attached first LDP label is received from a LDP enabled node that is not SR enabled.

3. The method of claim 1 wherein the first SR enabled node is not LDP enabled.

4. The method of claim 1 further comprising:
the first SR enabled node receiving the packet with the attached segment ID; and
the first SR enabled node forwarding the packet with the attached segment ID towards the SR enabled node, wherein the path comprises a second SR enabled node.

5. The method of claim 1 further comprising:
the SR enabled node receiving the packet with the attached segment ID;
the SR enabled node detaching the segment ID from the packet;
the SR enabled node attaching a second LDP label to the packet after detaching the segment ID; and
the SR enabled node forwarding the packet with the attached second LDP label to a second LDP enabled node.

6. The method of claim 5 further comprising:
the SR enabled node mapping the segment ID to the second LDP label.

7. The method of claim 1 further comprising:
mapping the first LDP label to the segment ID.

8. The method of claim 1 further comprising:
receiving a second packet with a second LDP label attached thereto;
detaching the second LDP label from the second packet;
attaching a second segment ID to the second packet after the second LDP label is detached; and
forwarding the second packet with the attached second segment ID on a second path towards an additional SR enabled node, wherein the second path comprises a second SR enabled node, and wherein the additional SR enabled node is uniquely assigned the second segment ID.

9. The method of claim 8 further comprising:
the second SR enabled node receiving the second packet with the attached second segment ID; and
the second SR enabled node forwarding the second packet with the attached second segment ID towards the additional SR enabled node, wherein the second path comprises a third SR enabled node, and the second SR enabled node is not LDP enabled.

10. An apparatus comprising:
a hybrid node, wherein the hybrid node is configured to:
receive packets with an attached first label distribution protocol (LDP) label, wherein
the first LDP label is advertised using LDP, and
the hybrid node employs LDP and segment routing (SR) in a control plane implemented by the hybrid node;

detach the first LDP label from the packets;
attach a segment identifier (ID) to the packets after the first LDP label is detached, wherein
the segment ID is advertised using a protocol other than LDP; and

forward the packets with the attached segment ID towards a SR enabled node on a path, wherein
the path comprises a first SR enabled node,
the first SR enabled node employs SR and lacks LDP in a control plane implemented by the first SR enabled node, and
the SR enabled node is uniquely assigned the segment ID.

11. The apparatus of claim 10, wherein
the hybrid node is coupled to a first LDP enabled node, and
the first LDP enabled node is configured to:
attach the first LDP label to the packets; and
forward the packets with the attached first LDP label to hybrid node, wherein
the first LDP node is not SR enabled.

12. The apparatus of claim 11, wherein
the first SR enabled node is configured to:
receive the packets with the attached segment ID; and
forward the packets with the attached segment ID to a second SR enabled node on the path.

13. The apparatus of claim 12, wherein
the SR enabled node is configured to:
receive the packets with the attached segment ID;
detach the segment ID from the received packets;
attach a second LDP label to the packets after the segment ID is detached; and
forward the packets with the attached second LDP label to a second LDP enabled node.

14. The apparatus of claim 13 wherein the SR enabled node is configured to map the segment ID to the second LDP label.

15. The apparatus of claim 10 wherein the hybrid node is configured to map the first LDP label to the segment ID.

16. A memory storing executable instructions, wherein a node is configured to implement a method in response to executing the instructions, the method comprising:
receiving a packet at a hybrid node, wherein
a first label distribution protocol (LDP) label is attached to the packet,
the first LDP label is advertised using LDP, and
the hybrid node employs LDP and segment routing (SR) in a control plane implemented by the hybrid node;

detaching the first LDP label from the packet;
attaching a segment identifier (ID) to the packet after the first LDP label is detached, wherein
the segment ID is advertised using a protocol other than LDP; and

forwarding the packet with the attached segment ID on a path towards a segment routing (SR) enabled node, wherein
the path comprises a first SR enabled node,
the first SR enabled node employs SR and lacks LDP in a control plane implemented by the first SR enabled node, and
the SR enabled node is uniquely assigned the segment ID.

17. The memory of claim 16 wherein the first SR enabled node is not LDP enabled, and wherein the packet with the attached first LDP label is received from a LDP enabled node that is not SR enabled.

18. The memory of claim 17 wherein the method further comprises:
mapping the first LDP label to the segment ID.

19. The memory of claim 17 wherein the method further comprises:
receiving a second packet with second LDP label attached thereto;
detaching the second LDP label from the second packet;
attaching a second segment ID to the second packet after the second LDP label is detached; and
forwarding the second packet with the attached second segment ID on a second shortest path towards an additional node, wherein the second shortest path comprises a second SR enabled node, and wherein the additional node is uniquely assigned the second segment ID.