Patent Publication Number: US-9432245-B1

Title: Distributed load balancer node architecture

Description:
BACKGROUND 
     Conventional load balancers are typically single, dedicated boxes that include multiple network interface controllers (NICs), for example eight NICs, with some of the NICs handling inbound traffic from/outbound traffic to clients and the other NICs handling outbound traffic from/inbound traffic to the host devices (e.g., servers such as web servers) that are being load balanced. Bandwidth or throughput on these conventional load balancers is typically in the range of 40 Gigabits per second (Gbps) on the client side and 40 Gbps on the server side. As the scale and scope of network-based applications and network-based services such as cloud computing services have increased, data centers may house hundreds or even thousands of host devices (e.g., web servers) that need to be load balanced. Conventional load balancers may not scale well in such environments. 
     Further, conventional load balancers typically use techniques such as max connections (or max conns), round robin, and/or least connections (least conns) applied to data collected from the host devices to select which host device will handle a connection. In addition, conventional load balancers typically serve as proxies to the host devices that they front and thus terminate connections (e.g., Transmission Control Protocol (TCP) connections) from the clients and send the client traffic to the host devices on TCP connections established between the host devices and the load balancer. Thus, a host device and a client do not communicate over a direct TCP connection when using these conventional load balancers. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of an example distributed load balancing system, according to at least some embodiments. 
         FIG. 2  is a high-level flowchart of a load balancing method that may be implemented by the distributed load balancer system of  FIG. 1 , according to at least some embodiments. 
         FIG. 3  shows an example load balancer node that includes ingress, egress, and flow tracker components, according to at least some embodiments. 
         FIG. 4  illustrates routing and packet flow in the distributed load balancer, according to at least some embodiments. 
         FIG. 5  illustrates advertising ingress nodes to the edge router, according to at least some embodiments. 
         FIG. 6  is a flowchart of a multipath routing method, according to at least some embodiments. 
         FIG. 7  graphically illustrates asymmetric packet flow, according to at least some embodiments. 
         FIG. 8  illustrates packet flow in the distributed load balancing system, according to at least some embodiments. 
         FIGS. 9A and 9B  provide a flowchart of packet flow when establishing connections in the distributed load balancing system, according to at least some embodiments. 
         FIGS. 10A through 10G  illustrate packet flow in the distributed load balancing system, according to at least some embodiments. 
         FIGS. 11A through 11D  illustrate the handling of events that effect membership in the load balancer node consistent hash ring, according to at least some embodiments. 
         FIG. 12  is a high-level flowchart of a health check method that may be performed by each load balancer node according to a health check interval, according to at least some embodiments. 
         FIG. 13  illustrates a method for health checking a load balancer node from another load balancer node, according to at least some embodiments. 
         FIG. 14  graphically illustrates a load balancer node health checking one or more other load balancer nodes, according to at least some embodiments. 
         FIG. 15  illustrates the load balancer nodes health checking the server nodes, according to at least some embodiments. 
         FIG. 16  graphically illustrates a view of health of another node that may be maintained by a load balancer node  110 , according to at least some embodiments. 
         FIG. 17  illustrates health information that may be maintained by each load balancer node, according to at least some embodiments. 
         FIGS. 18A and 18B  illustrate handling a load balancer node failure, according to at least some embodiments. 
         FIGS. 19A and 19B  graphically illustrate a connection publishing technique, according to at least some embodiments. 
         FIG. 20  is a high-level flowchart of a connection publishing method that may be performed by each load balancer module, according to at least some embodiments. 
         FIG. 21  is a flowchart of a method for distributing the active connection information received in a connection publishing packet to target load balancer nodes, according to at least some embodiments. 
         FIG. 22  illustrates an alternative method for distributing the active connection information received in a connection publishing packet to target load balancer nodes, according to at least some embodiments. 
         FIG. 23  illustrates example software stack architecture for a load balancer node according to at least some embodiments. 
         FIG. 24  illustrates aspects of core packet processing technology that may be used in embodiments. 
         FIG. 25  illustrates an example multicore packet processor for processing data flows on the load balancer nodes, according to at least some embodiments. 
         FIG. 26  illustrates another example multicore packet processor for processing data flows on the load balancer nodes, according to at least some embodiments. 
         FIG. 27  illustrates processing of incoming packets by a load balancer node process, according to at least some embodiments. 
         FIG. 28  illustrates processing of outgoing packets by a load balancer node process, according to at least some embodiments. 
         FIG. 29  illustrates a load balancing system that includes a distributed load balancer in a production environment, according to at least some embodiments. 
         FIG. 30  illustrates a distributed load balancer test system that incorporates a message bus mechanism that enables multiple distributed load balancing system components to be configured and executed in or as a single process, according to at least some embodiments. 
         FIGS. 31 and 32  illustrate message bus packet adapters and packet pipelines, according to at least some embodiments. 
         FIG. 33A  illustrates an example provider network environment, according to at least some embodiments. 
         FIG. 33B  illustrates a distributed load balancer implementation in an example provider network environment as shown in  FIG. 33A , according to at least some embodiments. 
         FIG. 34A  illustrates an example physical rack implementation of the distributed load balancer and server nodes according to at least some embodiments. 
         FIG. 34B  illustrates another example physical rack implementation of the distributed load balancer and server nodes according to at least some embodiments. 
         FIG. 35  illustrates an example networking environment in which one, two or more distributed load balancers are implemented in a network, according to at least some embodiments. 
         FIG. 36  is a block diagram illustrating an example computer system that may be used in some embodiments. 
     
    
    
     While embodiments are described herein by way of example for several embodiments and illustrative drawings, those skilled in the art will recognize that embodiments are not limited to the embodiments or drawings described. It should be understood, that the drawings and detailed description thereto are not intended to limit embodiments to the particular form disclosed, but on the contrary, the intention is to cover all modifications, equivalents and alternatives falling within the spirit and scope as defined by the appended claims. The headings used herein are for organizational purposes only and are not meant to be used to limit the scope of the description or the claims. As used throughout this application, the word “may” is used in a permissive sense (i.e., meaning having the potential to), rather than the mandatory sense (i.e., meaning must). Similarly, the words “include”, “including”, and “includes” mean including, but not limited to. 
     DETAILED DESCRIPTION 
     Various embodiments of methods and systems for distributed load balancing in network environments are described. Embodiments of a distributed load balancing method and system are described that may be implemented according to embodiments of a distributed load balancer in various network environments. Embodiments of the distributed load balancer may, for example, be used to facilitate and maintain packet flows, for example Transmission Control Protocol (TCP) technology packet flows, between clients on an external network such as the Internet and destinations, typically servers (e.g., web servers, application servers, data servers, etc.) on a local network, such as a provider network  1900  as illustrated in  FIGS. 33A and 33B . While embodiments are primarily described herein in relation to processing TCP packet flows, note that embodiments may be applied to other data communications protocols than TCP, and to other applications than processing packet flows. 
     The distributed load balancer may act to facilitate and maintain TCP packet flows between particular clients and selected servers (e.g., web servers). However, the distributed load balancer does not terminate the TCP flows from the clients and does not act as a proxy to the servers as is done in conventional load balancers. Instead, the load balancer nodes of the distributed load balancer route TCP packets received from the clients to target servers, and the servers use their TCP stacks to manage the TCP connections to the clients. In other words, the servers terminate the TCP packet flows from the clients. 
     In addition, instead of the load balancer node(s) making decisions as to which server will service a connection request based on a load balancing technique or algorithm applied to information collected from the servers as is done in conventional load balancer technology, the load balancer nodes may randomly select a server to receive a new connection request, and a component of the distributed load balancer that resides on the server node makes the decision locally as to whether the selected server will accept or reject the new connection request based on one or more metrics of the current status of the respective server. Thus, the decisions as to which servers are to accept connection requests is moved from the load balancer node(s) to the server nodes that will be handling the connections. In other words, the decision is moved closer to where and when the connection request will be serviced. 
     To facilitate and maintain the packet flows between the clients and the servers, embodiments of the distributed load balancer may employ various techniques or technologies including but not limited to multipath routing technology, consistent hashing technology, distributed hash table (DHT) technology, Border Gateway Protocol (BGP) technology, membership tracking, health checking, connection publishing, and packet encapsulation and decapsulation. These as well as other aspects of the distributed load balancing system are described below in relation to the Figures. 
     Distributed Load Balancing System 
       FIG. 1  is a block diagram of an example distributed load balancing system, according to at least some embodiments. Embodiments of the distributed load balancer may be implemented in a network  100 , for example a provider network  1900  of a service provider as illustrated in  FIGS. 33A and 33B . As a high-level overview of client packet handling in the distributed load balancer system, one or more clients  160  of the network  100  may connect to a border router  102  of the network  100 , for example via an external network  150  such as the Internet. The border router  102  may route incoming packets (e.g., TCP packets) from clients  160  to an edge router  104  component of the distributed load balancer that routes the incoming packets to the load balancer (LB) nodes  110  in a load balancer node layer of the distributed load balancer system. In at least some embodiments, the edge router  104  may make the routing decisions according to a per-flow hashed multipath routing technique, for example an equal-cost multipath (ECMP) hashing technique. The load balancer nodes  110  in turn encapsulate the packets (e.g., according to User Datagram Protocol (UDP)) and route the encapsulated packets to the local load balancer modules  132  on the server nodes  130  via a network fabric  120  (e.g., an L3 network) on network  100 . The fabric  120  may include one or more networking devices or components including but not limited to switches, routers, and cables. On the server nodes  130 , the local load balancer modules  132  decapsulate the packets and send the client TCP packets to the TCP stacks of the servers  134 . The servers  134  on the server nodes  130  then use their TCP stacks to manage the connections to the clients  160 . 
       FIG. 2  is a high-level flowchart of a load balancing method that may be implemented by the distributed load balancer system of  FIG. 1 , according to at least some embodiments. Embodiments of the distributed load balancer system may not solve the hard problem of assigning load among multiple destinations (e.g., web servers) as is done in conventional load balancers. For example, conventional load balancers typically use techniques or algorithms such as max connections, round robin, and/or least connections techniques to select which server should handle a connection. However, these techniques have drawbacks, and in particular are difficult to perform successfully in a distributed system where the data used to make load balancing decisions is often almost immediately stale. In at least some embodiments of the distributed load balancer system, instead of attempting to select a server node  130  to satisfy a connection request using one or more of the load balancing techniques as is done in conventional load balancers, a load balancer node  110  in the load balancer node layer may randomly determine a server node  130  to receive a request for a client connection. If that server node  130  considers itself overloaded, the server node  130  may send the connection request back to the load balancer node  110  thus informing the load balancer node  110  that the server node  130  cannot currently handle the connection. The load balancer node layer may then randomly determine another server node  130  to receive the connection request, or alternatively may return an error message to the requesting client  160  to inform the client  160  that the connection cannot currently be established. 
     As indicated at  10  of  FIG. 2 , the load balancer node layer of the distributed load balancer system receives a request for a communication session (e.g., a TCP connection) from a source. The source may, for example, be a client  160  on an external network  150  to the network  100  that implements the distributed load balancer system. In at least some embodiments, the request may be received from the client  160  at a border router  102  of the network  100 , and routed to an edge router  104  that routes the incoming packets to the load balancer (LB) nodes  110  in a load balancer node layer, for example using a per-flow equal-cost multipath (ECMP) hashing technique to pseudorandomly select a load balancer node  110  to which a particular connection request from a client  160  is to be routed. 
     As indicated at  20 , the load balancer node layer randomly selects a destination node and forwards the connection request to the selected destination node. The destination node may, for example, be one of a plurality of server nodes  130  fronted by the load balancer. In at least some embodiments, a load balancer node  110  in the load balancer layer may randomly select a server node  130  to receive a connection request from among all known server nodes  130 . However, other methods than purely random selection from among all known server nodes  130  may be used in some embodiments to select server nodes  130  to receive the connection requests. For example, in some embodiments, information about the server nodes  130  may be used by the load balancer nodes  110  to weight the random selection of server nodes  130 . As an example, if the load balancer nodes  110  know that different server nodes  130  are different types of devices or are configured with different CPUs and thus have different capabilities or capacities, the information may be used to bias the random selection towards (or away from) particular type(s) or configuration(s) of server node  130 . 
     As indicated at  30 , the destination node determines if it can accept the communications session. In at least some embodiments, a local load balancer (LB) module  132  on the server node  130  determines if the respective server  134  on the server node  130  can accept the new connection based on one or more metrics of the current status of the respective server  134 . 
     At  40 , if the connection request is accepted, then as indicated at  50  the destination node informs the load balancer node layer that the destination node can handle the connection. As indicated at  60 , a communications session is then established between the source (e.g., a client  160 ) and the destination node (e.g., a server  134  on a server node  130 ) via the load balancer node layer. In at least some embodiments, the server  134  on the server node  130  uses a TCP stack to manage the connection to the client  160 . 
     At  40 , if the connection request is not accepted, then as indicated at  70  the destination node notifies the load balancer node layer, and the method may return to element  20 . The load balancer node layer may then randomly select another destination node at  20 , or alternatively may inform the requesting client  160  that the connection cannot currently be established. Note that the client  160  may, but does not necessarily, resubmit the connection request to begin the method again at element  10 . 
     Referring again to  FIG. 1 , at least some embodiments of the distributed load balancer system may use commodity hardware to route client traffic received at an edge router  104  on network  100  to server nodes  130  on the network  100 . At least some embodiments of the distributed load balancer may include a load balancer node layer that includes multiple load balancer nodes  110 . In at least some embodiments, each load balancer node  110  may serve in one or more of multiple roles in the load balancer node layer. These roles of the load balancer nodes  110  may include the roles of an ingress node, and egress node, and a flow tracker node (as a primary flow tracker or a secondary flow tracker for a given packet flow). In at least some embodiments, each load balancer node  110  may be implemented in the load balancer node layer as or on a separate computing device, such as a commodity rack-mounted computing device. In at least some embodiments, each load balancer node  110  may serve in each of the three roles of ingress node, egress node, and flow tracker node (as a primary or secondary flow tracker for a packet flow), with the load balancer node  110  generally serving in only one (but possibly in two or three) of the roles for particular packet flows. Note, however, that in at least some embodiments, a load balancer node  110  is not allowed to serve as both the primary flow tracker and the secondary flow tracker for a particular packet flow. Alternatively, in some embodiments, each load balancer node  110  may serve in only one of the three roles. In this embodiment, separate sets of computing devices may be implemented in the load balancer node layer specifically as ingress nodes, egress nodes, and flow tracker nodes. 
     In at least some embodiments, consistent hashing and consistent hash ring technology may be applied to determine the primary and secondary flow trackers for the packet flows. Each packet flow from a client may be uniquely identified, for example by a 4-tuple consisting of: the client IP address, client port, server (public) IP address, and server port. This identifier may be abbreviated as CP or CcPp indicating the client and public endpoint pair. Packets associated with any given TCP flow (or CP pair) can appear on any load balancer node  110  operating as an ingress server  112  due to the hashed multipath (e.g., ECMP) flow distribution from the edge router  104 . Consistent hashing is used so that when a packet arrives at a load balancer node  110  serving as an ingress node, the ingress node can determine which load balancer node  110  is responsible for maintaining the state for the packet flow (i.e., the primary flow tracker node). The CP pair may be hashed by the ingress node into a consistent hash ring to determine which load balancer node  110  is responsible for maintaining state information for the packet flow. The node  110  determined according to the consistent hash of the CP pair for the packet flow in the consistent hash ring is the node  110  that serves as the primary flow tracker for the packet flow. In at least some embodiments, the successor node in the consistent hash ring serves as the secondary flow tracker for the packet flow. 
       FIG. 3  shows an example load balancer (LB) node  110  that includes components that implement all three roles (ingress, egress, and flow tracker), according to at least some embodiments. In this example, an ingress server  112  component performs the ingress role of receiving inbound TCP packets from client(s) and sending the TCP packets as encapsulated packets to the server(s). An egress server  114  component performs the egress role of receiving outbound encapsulated packets from the server(s) and sending the decapsulated TCP packets on to the client(s). A flow tracker  116  component performs as a primary or secondary flow tracker for one or more packet flows that are established between the client(s)  160  and the server(s)  134 . The ingress server  112  may also communicate with the flow tracker  116  on load balancer node  110 , or with the flow tracker  116  on another load balancer node  110  to initiate a TCP connection between a client and one of the servers  134  in response to a connection request received from the respective client  160 , or to obtain mapping information for the packet flow. 
     Load Balancer Nodes 
     Referring again to  FIG. 1 , in at least some embodiments, the load balancer nodes  110  in the load balancer node layer receive client traffic (packets, e.g. TCP packets) from one or more routers  104  on the network and encapsulate the packets according to a protocol (e.g., the User Datagram Protocol (UDP)) used by the distributed load balancer system on the fabric  120 . The load balancer node layer then forwards the encapsulated packets to the destination server nodes  130  over fabric  120 . Each server node  130  includes a local module  132  that is a component of the load balancer system. The module  132  may be referred to herein as a load balancer module or simply LB module, and may be implemented in software, hardware, or a combination thereof on the server node  130 . At each server node  130 , the respective load balancer module  132  decapsulates the packets and sends the TCP packets to a local TCP stack for normal TCP processing. In at least some embodiments, the load balancer node layer may maintain state information for every client-server TCP flow; however, the load balancer nodes  110  in the load balancer node layer may not interpret anything about the TCP flow. Each flow is managed between the server  134  on the respective server node  130  and the client  160 . The distributed load balancer system insures that the TCP packets arrive at the correct destination server  134 . The load balancer module  132  at each server node  130  makes the decision as to whether the respective server  134  will accept or reject a new connection in response to a client connection request received from a load balancer node  110 . 
     In at least some embodiments, the distributed load balancing system may use consistent hashing technology to, for example, determine which load balancer node(s)  110  should remember which server node  130  is responsible for a particular TCP packet flow. Using consistent hashing technology, the load balancer nodes  110  in the load balancer node layer may be viewed as a consistent hash ring, and the load balancer nodes  110  may keep track of membership in the ring and determine particular members in the ring that are responsible for particular packet flows according to a consistent hashing function. In at least some embodiments, there are two load balancer nodes  110  that are responsible for tracking each packet flow between the clients  160  and the servers  134 ; these nodes  110  may be referred to as the primary flow tracker (PFT) node and the secondary flow tracker (SFT) node. In at least some embodiments, the primary flow tracker is the first load balancer node  110  on the consistent hash ring for the flow, and the secondary flow tracker is the next or subsequent load balancer node  110  on the consistent hash ring distinct from the primary flow tracker node. In this arrangement, if the primary flow tracker node fails, then the secondary flow tracker node may become the new primary flow tracker, and another load balancer node  110  (e.g., the next node  110  on the consistent hash ring) may assume the role of the secondary flow tracker. Note that, in at least some embodiments, a load balancer node  110  is not allowed to serve as both the primary flow tracker and the secondary flow tracker for a given packet flow. This and other membership changes in the consistent hash ring are discussed later in this document. In at least some embodiments, configuration information for the load balancer implementation (e.g., authoritative list(s) of the load balancer nodes  110  and server nodes  130  that are currently in the implementation) may be maintained by a configuration service  122  component of the distributed load balancing system, which may for example be implemented on one or more server devices coupled to the load balancer nodes  110  via the fabric  120 . 
     In at least some embodiments, in addition to serving as primary and secondary flow tracker nodes, the load balancer nodes  110  may also perform in one of two other roles for a given flow: the role of an ingress node and the role of an egress node. An ingress node for a packet flow is the load balancer node  110  that receives the respective packet flow from the edge router  104  and forwards the packet flow (as encapsulated packets) to a selected server  134  on a server node  130  via fabric  120 . An ingress node is the only load balancer node  110  that moves actual client data (TCP data packets) to the respective destination server node  130 . The ingress node maintains a mapping of the TCP flow to a respective load balancer module  132  on the destination server node  130  so that the ingress node knows which load balancer module  132  to forward the client traffic to. An egress node is a load balancer node  110  that is responsible for forwarding the response traffic for a packet flow received from the server node  130  via fabric  120  to the respective client  160  via the border network. The load balancer module  132  encapsulates response packets obtained from the server  134  according to a load balancer protocol (e.g., UDP) and sends the encapsulated response packets to the respective egress node for the flow via fabric  120 . The egress nodes are stateless and simply decapsulate the packets and send the response packets (e.g., TCP packets) onto the border network to a border router  102  for delivery to the respective client  160  via the external network  150 . 
     As previously mentioned, in at least some embodiments, each load balancer node  110  may perform the roles of an ingress node, an egress node, and/or a flow tracker node (as either a primary or a secondary flow tracker) for different packet flows. A single load balancer node  110  in the load balancer node layer may perform in any one of the roles depending on what packet flow the node is processing. For example, in at least some embodiments, a load balancer node  110  may perform as an ingress node for one packet flow, as a primary or secondary flow tracker for another packet flow, and as an egress node for yet another packet flow. In addition, in at least some embodiments a load balancer node  110  may perform multiple roles for the same packet flow, for example as the ingress node and as the primary (or secondary) flow tracker node for a given packet flow. However, in at least some embodiments, for redundancy and recovery purposes, a load balancer node  110  is not allowed to serve as both the primary and secondary flow tracker node for the same packet flow. 
     The above describes embodiments where each load balancer node  110  may serve in any of the three roles of ingress server, egress server, and flow tracker. However, in some embodiments, different groups of computing devices may be assigned to the different roles in the load balancing system. For example, in some embodiments, there may be distinct sets of ingress nodes, egress nodes and flow tracker nodes each implemented on a separate computing device. As another example, in some embodiments, one set of computing devices may serve as both ingress nodes and flow tracker nodes, while another set of computing devices may serve only as egress nodes. 
     Load Balancer Modules 
     As previously mentioned, each server node  130  includes a local load balancer module  132  that is a component of the load balancer system. The module  132  may be implemented in software, hardware, or a combination thereof on the server node  130 . In at least some embodiments, the load balancer module  132  on a server node  130  may perform three main roles: encapsulating outgoing and decapsulating incoming packets, making local load balancing decisions for the server  134  on the node  130 , and connection publishing. These three roles are briefly described below, and described in more detail later in this document. 
     At least some embodiments of the distributed load balancing system do not terminate TCP connections and do not spoof packets; the source and destination IP addresses of all packets sent through the load balancer node layer are the actual IP addresses of the endpoints (i.e., the clients  160  and servers  134 ) involved in the packet flows. Instead of spoofing, these embodiments encapsulate all packets sent between the load balancer nodes  110  and the server nodes  130  on fabric  120 , for example as UDP packets. Since the inbound packets in a packet flow arriving at a server node  130  from a load balancer node  110  acting as the ingress node for the flow are encapsulated by the load balancer node  110 , the packets need to be decapsulated and redirected to a localhost TCP flow for the server  134  on the node  130 . The load balancer module  132  on node  130  performs this decapsulation. Similarly, outgoing packets for a packet flow from the server  134  are encapsulated by the load balancer module  132  and sent via fabric  120  to a load balancer node  110  acting as the egress node for the packet flow. 
     In at least some embodiments, the load balancer modules  132  on the server nodes  130  also make local decisions related to load balancing for the servers  134  on the respective server nodes  130 . Specifically, the load balancer module  132  on a node  130  decides whether the respective server  134  will accept another TCP flow in response to receiving a request for a new TCP connection. As previously noted, the load balancer nodes  110  encapsulate all packets sent to the load balancer module  132 , so the load balancer module  132  actually does not receive a TCP synchronize (SYN) packet from the client  160 ; instead, the load balancer module  132  receives a connection request message according to the encapsulation protocol (e.g., UDP) from a flow tracker  116  which the load balancer module  132  can either accept or reject. If the load balancer module  132  accepts the connection request message, the load balancer module  132  creates a SYN packet destined for the localhost. When the localhost accepts the connection, this becomes the actual TCP stack handling the respective client connection. 
     In at least some embodiments, to make the decision as to whether a connection request message should be accepted, the load balancer module  132  looks at one or more metrics regarding current resource consumption on the server node  130 , and if there are sufficient resources available to handle the new connection, the load balancer module  132  accepts the connection. In at least some embodiments, resource metrics that may be considered by the load balancer module  132  may include one or more of, but are not limited to, CPU utilization, recent bandwidth consumption, and number of established connections. Other metrics may be considered instead of or in addition to these metrics in some embodiments. For example, in some embodiments, the load balancer module may consider server latency (i.e., the amount of time requests are spending in the server connection backlog) as a metric, and may reject the connection request if server latency is above a threshold. Using these and/or other metrics, the load balancer module  132  can decide for the respective server  134  whether the server  134  is to accept or reject new packet flows. In at least some embodiments, a resource utilization rate (e.g., N % utilization) may be determined from the metric(s) individually or in combination and compared to a threshold (e.g., 90% utilization). If the determined resource utilization rate is at or above the threshold, or if adding the connection would move the rate to above the threshold, then the connection request may be rejected. 
     In at least some embodiments, the load balancer modules  132  may implement a probabilistic method for determining if connection request messages are to be rejected. Instead of rejecting all connection requests if resource utilization is at or above a threshold as described above, in this method may reject connection requests at different probabilities at two or more different levels of utilization. For example, if resource utilization is 80%, a load balancer module  132  may reject connection requests at 20% probability; if resource utilization is 90%, the load balancer module  132  may reject connection requests at 25% probability; if resource utilization is 95%, the load balancer module  132  may reject connection requests at 50% probability; and at 98% or above, the load balancer module  132  may reject all connection requests. 
     In at least some embodiments, each connection request message may include an indication of how many times the connection request message has been rejected by load balancer modules  132 . If a connection request message received by a load balancer module  130  indicates that it has been rejected over a threshold number of times, the load balancer module  130  may accept the connection even though the performance metrics of the server node  130  indicate that the connection request should be rejected. 
     In some cases, it is possible that all of the load balancer modules  132  that a connection request message is sent to may reject the connection request. In at least some embodiments, to prevent a connection request message from being bounced from load balancer module  132  to load balancer module  132  for an indefinite period, each connection request message may be given a time to live. If this time to live expires, the flow tracker node may terminate the request and notify the respective client  160  that the request cannot currently be serviced. 
     In at least some embodiments, the load balancer modules  132  on the server nodes  130  also perform connection publishing to the load balancer nodes  110 . In at least some embodiments, to perform connection publishing, periodically or aperiodically (e.g., once a second) each load balancer module  132  looks at the routing table (e.g., a netstat routing table) on the server node  130  and publishes a list of active connections (TCP flows) back to the load balancer nodes  110 . The load balancer nodes  110  that need to be informed about the existence of a given packet flow are the load balancer nodes  110  that are serving as the ingress node and as the primary and secondary flow trackers for the respective packet flow. In some embodiments, the load balancer module  132  may use a consistent hashing technique to filter the list of load balancer nodes  110  that need to be informed about the active TCP flows on the server node  130 . For example, the load balancer module  132  may determine which load balancer nodes  110  are serving as the primary and secondary flow trackers for a given packet flow according to the consistent hash ring. In some embodiments, the load balancer module  132  tracks which load balancer node  110  last sent a data packet to the load balancer module  132  for each packet flow, and uses this information to determine which load balancer nodes  110  are serving as ingress nodes for the packet flows, since only ingress nodes forward client data to the load balancer module  132 . In some embodiments, the load balancer module  132  then formulates messages for each of the load balancer nodes  110  that it has determined need to be informed about the packet flows and sends the messages to the load balancer nodes  110  to inform the nodes  110  that the respective server node  130  is still maintaining the connection(s) to the client(s)  160 . This connection publishing to the load balancer nodes  110  by the load balancer modules  132  may be viewed as extending a lease at the load balancer nodes  110 . If a load balancer node  110  has not received a connection publishing message indicating a particular packet flow within a period of time (e.g., ten seconds), then the load balancer node  110  is free to forget about the respective packet flow. 
     Multipath Routing to Load Balancer Nodes 
       FIG. 4  illustrates aspects of routing and packet flow in the distributed load balancer, according to at least some embodiments. In at least some embodiments, each ingress node (ingress nodes are shown in  FIG. 4  as ingress servers  112 ) advertises its ability to route one or more public endpoints (e.g., IP address and port) to the edge router  104  for the distributed load balancer, for example via the border gateway protocol (BGP). In at least some embodiments, rather than each ingress node advertising itself to the edge router  104  via a BGP session, one or more other ingress nodes, for example two neighbor nodes, may establish BGP sessions with the edge router  104  to advertise the ingress node, as shown in  FIG. 5 . 
     Conventional load balancers can typically only serve a single public endpoint. In contrast, embodiments of the distributed load balancer enable multiple load balancer nodes  110  to service a single public endpoint. Depending on router capabilities, this enables configurations in which a single public IP address routed to all the ingress servers  112  may handle the entire bandwidth (e.g., 160 Gbps) through the edge router(s)  104 . In at least some embodiments, to achieve this, the edge router(s)  104  may utilize a layer 4 per-flow hashed multipath routing technique, for example an equal-cost multipath (ECMP) routing technique, to distribute traffic across multiple ingress servers  112  each advertising the same public IP address. Distributing incoming packets to all of the ingress servers  112  using layer-4 source and destination ports for the flows as part of the edge router(s)  104  flow hash may generally keep the packets for each connection routed to the same load balancer node  110  serving as the ingress server  112  to avoid out-of-order packets. Note, however, that the edge router(s)  104  may use other techniques to distribute traffic across the ingress servers  112  in some embodiments. 
       FIG. 4  also shows that two or more distributed load balancers may be implemented on a network  100 . The two or more distributed load balancers may each act as an independent load balancer that fronts a plurality of servers  130  and that each advertises a different public IP address, or alternatively as shown in  FIG. 4  two or more distributed load balancers may each advertise the same IP address, and a hashing technique (e.g., a layer 4 per-flow hashed multipath routing technique) may be used at the border router(s)  102  to partition the packet flows out to the edge routers  104 , which in turn distribute the packet flows to their respective ingress servers  112 . 
       FIG. 5  illustrates using Border Gateway Protocol (BGP) to advertise ingress nodes to the edge router, according to at least some embodiments. In this example, there are four load balancer nodes serving as ingress nodes  110 A through  110 D in the load balancer implementation. Edge router  104  routes incoming packets from clients (not shown) to the load balancer nodes  110 . In at least some embodiments, the edge router  104  may make the routing decisions according to a layer 4 per-flow hashed multipath routing technique, for example an equal-cost multipath (ECMP) routing technique. 
     In at least some embodiments, edge router  104  learns about the ingress nodes  110  that are currently available in the load balancer implementation to receive client traffic via Border Gateway Protocol (BGP) technology advertising sessions initiated by the ingress nodes  110 . Each ingress node  110  could use BGP to advertise itself to the edge router  104 . However, BGP typically takes a relatively long time to converge (three seconds or more). Using this technique where each ingress node  110  advertises itself via BGP, if an ingress node  110  goes down, it may take considerable time in networking terms (three seconds or more) for the BGP session on the edge router  104  to time out and thus for the edge router  104  to learn about the failure close down and reroute the current TCP flows to the ingress node  110 . 
     To avoid the convergence problem with BGP and to recover more quickly upon node  110  failure, in at least some embodiments, instead of an ingress node  110  advertising itself to the edge router  104  via a BGP session, at least one other ingress node  110  in the load balancer implementation takes responsibility for advertising the ingress node  110  to the edge router  104  via BGP. For example, in some embodiments as shown in  FIG. 5 , the left and right neighbor ingress nodes  110  of a given ingress node  110 , for example the left and right neighbors in an ordered listing of the nodes  110 , for example a consistent hash ring formed by the nodes  110 , may advertise the given ingress node  110  to the edge router  104 . For example, in  FIG. 5 , ingress node  110 A advertises ingress nodes  110 B and  110 D, ingress node  110 B advertises ingress nodes  110 A and  110 C, ingress node  110 C advertises ingress nodes  110 B and  110 D, and ingress node  110 D advertises ingress nodes  110 C and  110 A. The ingress nodes  110  check and gossip each other&#39;s health as described later in this document. Using the health check method as described, unhealthy nodes can be detected and the information can be propagated among the nodes  110  in less than a second, for example in 100 milliseconds (ms). Upon determining that an ingress node  110  is not healthy, the ingress nodes  110  that advertise the unhealthy node may immediately stop advertising the unhealthy node  110 . In at least some embodiments, the ingress nodes  110  end the BGP sessions with the edge router  104  by sending a TCP Close or similar message for the BGP session to the edge router  104 . Thus, rather than having to wait for a BGP session established by a failed node  110  to time out to detect the node  110  failure, the edge router  104  may discover the failed node  110  when the other ingress nodes  110  that advertise on behalf of the failed node  110  terminate the BGP sessions with the edge router  104  that advertise the node  110  upon detecting that the node  110  is unhealthy. The handling of load balancer node failures is further discussed in relation to  FIGS. 18A and 18B  later in this document. 
       FIG. 6  is a flowchart of a multipath routing method, according to at least some embodiments of the distributed load balancing system. As indicated at  900 , the ingress nodes  110  in a load balancer implementation advertise their neighbor nodes  110  to the edge router  104 . In at least some embodiments, the ingress nodes  110  may determine their neighbor nodes  110  according to an ordered listing of the nodes  110  such as a consistent hash ring. In at least some embodiments, the ingress nodes  110  advertise their neighbor node(s)  110  to the edge router  104  using BGP sessions, with one BGP session established to the edge router  104  for each advertised node  110 . 
     As indicated at  902 , the edge router  104  distributes traffic received from clients  160  to the active (advertised) ingress nodes  110  according to a per-flow hashed multipath routing technique, for example an equal-cost multipath (ECMP) routing technique. In at least some embodiments, the edge router  104  exposes a public IP address to the clients  160 ; the ingress nodes  110  all advertise the same public IP address to the edge router  104 . The edge router uses layer-4 source and destination ports as part of the edge router&#39;s  104  flow hash to distribute the incoming packets among the ingress nodes  110 . This generally keeps the packets for each connection routed to the same ingress node  110 . 
     As indicated at  902 , the ingress nodes forwards the data flows to target server nodes  130 . In at least some embodiments, the ingress nodes  110  interact with primary and secondary flow tracker nodes for the data flows to map the data flows to the target server nodes  130 . Thus, each ingress node  110  may maintain mappings of active data flows through the node  110  that may be used to appropriately forward the received packets to the target server nodes  130 . 
     Elements  906  through  910  relate to detecting and recovering from ingress node  110  failures. As indicated at  906 , the ingress nodes  110  may detect that an ingress node  110  is down, for example according to a health checking technique as described herein. Upon detecting that the node  110  is down, its neighbor nodes  110  stop advertising the node  110  to the edge router  104 . In at least some embodiments, this involves sending a TCP Close to the edge router  104  for the respective BGP session. 
     As indicated at  908 , the edge router  104 , upon detecting that the ingress node  110  is down via the closing of the BGP sessions, redistributes incoming traffic from the clients  160  to the remaining ingress nodes  110  according to the per-flow hashed multipath routing technique. Thus, at least some data flows may be routed to different ingress nodes  110 . 
     As indicated at  910 , the ingress nodes  110  may recover mappings as necessary and forward the data flows to the appropriate target server nodes. Methods for recovering from node  110  failures on ingress nodes  110  are discussed elsewhere in this document. As one example, an ingress node  110 , upon receiving a packet for which it does not have a current mapping, may use a consistent hash function to determine a flow tracker node for the data flow according to a consistent hash ring and recover the mapping from the flow tracker node. 
     Asymmetric Packet Flow 
     In at least some embodiments, to efficiently utilize ingress node bandwidth and CPU usage when the ratio of outbound traffic to inbound data is greater than 1, the distributed load balancing system forwards outbound packets from the server nodes  130  to multiple egress nodes as shown in  FIG. 7 . In at least some embodiments, for each connection, the load balancer module  132  on the respective server node  130  hashes the client endpoint/public endpoint tuple and uses a consistent hash algorithm to select a load balancer node  110  to serve as the egress server  114  for the respective outbound packet flow. However, in some embodiments other methods and/or data may be used to select the egress servers  114  for connections. The selected egress server  114  may typically be, but is not necessarily, a different load balancer node  110  than the load balancer node  110  that serves as the ingress server  112  for the connection. In at least some embodiments, unless there is a failure of that load balancer node  110 /egress server  114 , all of the outbound packets for the particular connection will be forwarded to the same egress server  114  in order to avoid out-of-order packets. 
     In at least some embodiments, the method and data used for selecting an egress server  114  by the server nodes  130  may be different than the method and data used for selecting an ingress server  112  performed by the edge router(s)  104 . Using the different methods and data may generally result in a different load balancer node  110  being selected as the egress node for a given connections than the load balancer node  110  selected as the ingress node for the connection, and may also result in multiple load balancer nodes  110  being selected as egress nodes to handle the outgoing traffic for connections that pass through a single load balancer node  110  serving as an ingress node. 
       FIG. 7  graphically illustrates asymmetric packet flow, according to at least some embodiments. At least one connection has been established from clients  160  on external network  150  through ingress server  112  to each of server nodes  130 A,  130 B,  130 C, and  130 D. In at least some embodiments, to select egress nodes for the connections, for each connection, the load balancer module  132  on the respective server node  130  hashes the client endpoint/public endpoint tuple and uses a consistent hash algorithm to select a load balancer node  110  to serve as the egress server  114  for the respective outbound packet flow. For example, server node  130 A has selected egress server  114 A for a connection, and server node  130 B has selected egress server  114 A for one connection and egress server  114 B for another connection. However, in some embodiments other methods and/or data may be used to select the egress nodes for connections. 
     Recovering from Load Balancer Node Failures without Dropping Client Connections 
     While it is possible for the load balancer nodes  110  to use consistent hashing to determine which server node  130  should receive client traffic, due to the long lifespan of some connections this approach may not maintain existing flows in cases where a new server node  130  joins the consistent hash membership and there is a subsequent ingress load balancer node  110  failure. In this scenario, a load balancer node  110  that takes over a flow from the failed node  110  may not be able to determine the original mapping selected, as the consistent hash ring for the servers  130  would have different membership. Thus, in at least some embodiments, distributed hash table (DHT) technology may be used by the load balancer nodes  110  to select server nodes  130  for connections and to route packets to the selected server nodes  130 . Once a server node  130  has been selected according to the DHT to receive a particular connection, and assuming that the server node  130  stays healthy and that the load balancer module  132  on the server node  130  continues to extend the lease by periodically transmitting the status of that active connection to the DHT (e.g., via connection publishing), the DHT will retain the mapping until the connection completes. An ingress node  110  failure impacts the distribution of packets from the edge router  104  to the remaining load balancer nodes  110 , resulting in the load balancer nodes  110  receiving traffic from a different set of client connections. However, since the DHT tracks all active connections, the load balancer nodes  110  can query the DHT to obtain leases for any active mappings. As a result, all the load balancer nodes  110  will pass traffic to the correct server nodes  130 , thus preventing failure of active client connections even in the event of an ingress load balancer node  110  failure. 
     Packet Flow in the Distributed Load Balancing System 
       FIG. 8  illustrates packet flow in the distributed load balancing system, according to at least some embodiments. Note that the solid lines with arrows in  FIG. 8  represent TCP packets, while the dotted lines with arrows represent UDP packets. In  FIG. 8 , an ingress server  112  receives TCP packets from one or more clients  160  via the edge router  104 . Upon receipt of a TCP packet, ingress server  112  determines if it has a mapping for the TCP packet flow to a server node  130 . If the ingress server  112  does have a mapping for the TCP packet flow, then the server  112  encapsulates the TCP packet (for example according to UDP) and sends the encapsulated packet to the target server node  130 . If the ingress server  112  does not have a mapping for the TCP packet flow, then the ingress server  112  may send a UDP message including information about the TCP packet flow extracted from the TCP packet to the primary flow tracker  116 A to establish a connection to a server node  130  and/or obtain a mapping for the TCP packet flow.  FIGS. 9A and 9B  and  FIGS. 10A through 10G  illustrate methods for establishing a connection between a client  160  and a server node  130 . The load balancer module  132  on a server node  130  randomly selects load balancer node(s)  110  to serve as the egress server(s)  114  for TCP connection(s) on the server node  130  and sends UDP-encapsulated TCP response packets to the client(s)  160  via the egress server(s)  114 . 
       FIGS. 9A and 9B  provide a flowchart of packet flow when establishing connections in the distributed load balancing system, according to at least some embodiments. As indicated at  200  of  FIG. 9A , an ingress server  112  receives a TCP packet from a client  160  via the edge router  104 . At  202 , if the ingress server  112  has a mapping for the TCP flow to a server node  130 , then the ingress server  112  encapsulates and sends the TCP packet to the respective server node  130  as indicated at  204 . Note that the ingress server  112  may be continuously receiving and processing packets for one, two or more TCP flows from one, two, or more clients  160 . 
     At  202 , if the ingress server  112  does not have a mapping for the TCP flow, the packet may be a TCP synchronize (SYN) packet from a client  160 . As indicated at  206 , upon receipt of a SYN packet, the ingress server  112  extracts data from the SYN packet and forwards the data to the primary flow tracker  116 A, for example in a UDP message. In at least some embodiments, the ingress server  112  can determine the primary flow tracker  116 A and/or secondary flow tracker  116 B for the TCP flow according to a consistent hash function. At  208 , the primary flow tracker  116 A stores the data, for example in a hash table, generates an initial TCP sequence number for the server node  130  side of the TCP connection, and forwards the data and the TCP sequence number to the secondary flow tracker  116 B. At  210 , the secondary flow tracker  116 B may also store the data, and fabricates and sends a SYN/ACK packet to the client  160 , the SYN/ACK packet containing at least the TCP sequence number. 
     As indicated at  212 , the ingress server  112  receives a TCP acknowledgement (ACK) packet from the client  160  via the edge router  104 . The ingress server  112  does not at this time have a mapping for the TCP flow to a server  130  node, so at  214  the ingress server  112  sends a message including data extracted from the ACK packet to the primary flow tracker  116 A. As indicated at  216 , upon receiving the message, the primary flow tracker  116 A confirms the TCP flow according to the stored data, and confirms that the acknowledged sequence number (+1) from the ACK packet matches the value sent in the SYN/ACK. The primary flow tracker  116 A then selects a server node  130  to receive the TCP flow, and sends a message containing the data, TCP sequence number, and IP address of the local load balancer module  132  on the selected server node  130  to the secondary flow tracker  116 B. As indicated at  218 , the secondary flow tracker  116 B also confirms the data and TCP sequence number, fabricates a SYN message, and sends the fabricated SYN message to the local load balancer module  132  on the selected server node  130 . The method continues at element  220  of  FIG. 9B . 
     As indicated at  220  of  FIG. 9B , in response to the fabricated SYN message, the load balancer module  132  may examine one or more metrics of the server node  130  to determine if the server node  130  can accept the connection. At  222 , if the load balancer module  132  determines that the server node  130  cannot currently accept the connection, then at  224  the load balancer module  132  messages the secondary flow tracker  116 B. The secondary flow tracker  116 B may delete the information for the flow that it previously stored. At  226 , the secondary flow tracker  116 B messages the primary flow tracker  116 A. The primary flow tracker  116 A may then select a new target server node  130  and message the secondary flow tracker  116 B as indicated at  216  of  FIG. 9A . 
     At  222 , if the load balancer module  132  determines that the server node  130  can accept the connection, then as indicated at  228  of  FIG. 9B  the local load balancer module  132  constructs a TCP SYN packet from the fabricated SYN and sends the TCP SYN packet to the server  134  on the server node  130 . The source IP address of the TCP SYN packet is populated with client  160 &#39;s actual IP address so that server  134  will believe that it has received a direct TCP connection to the client  160 . The load balancer module  132  stores relevant details about the TCP flow, for example in a local hash table. As indicated at  230 , the server  134  responds with a SYN/ACK packet that the load balancer module  132  intercepts. As indicated at  232 , the load balancer module  132  then sends a message including connection information to the secondary flow tracker  116 B to indicate that the connection has been accepted. Upon receipt of this message, at  234  the secondary flow tracker  116 B records the mapping to the server  134 , and sends a similar message to the primary flow tracker  116 A, which also records the mapping information. As indicated at  236 , the primary flow tracker  116 A then forwards a mapping message to the ingress server  112 . Ingress server  112  now has a mapping for the TCP flow from the client  160  to the server  130 . 
     At  238 , ingress server  112  encapsulates and forwards any buffered data packets for the data flow to the local load balancer module  132  on the server node  130 . Additional incoming packets for the data flow from the client  160  received by the ingress server  112  are encapsulated and forwarded directly to the load balancer module  132 , which decapsulates the packets and sends the data packets on to the server  134 . 
     At  240 , the load balancer module  132  randomly selects an egress server  114  for the data flow. Subsequent outbound TCP packets from the server  134  are intercepted by the load balancer module  132 , encapsulated according to UDP, and forwarded to the arbitrarily selected egress server  114 . The egress server  114  decapsulates the outgoing packets and sends the TCP packets to the client  160 . 
     As noted above, at  202 , if the ingress server  112  does not have a mapping for the TCP flow of a received packet, the packet may be a TCP synchronize (SYN) packet from a client  160 . However, the packet may not be TCP SYN packet. For example, if load balancer node  110  membership changes due to addition or failure of a load balancer node  110 , the edge router  104  may start routing packets for one or more TCP flows to the ingress server  112  that the ingress server  112  does not have mappings for. In at least some embodiments, upon receiving such a packet for which the ingress server  112  does not have a mapping, the ingress server  112  may use the consistent hash function to determine the primary flow tracker  116 A and/or secondary flow tracker  116 B for the TCP flow according to the consistent hash ring and message either the primary flow tracker  116 A or the secondary flow tracker  116 B to request the mapping. Upon receiving the mapping for the TCP flow from a flow tracker  116 , the ingress server  112  can store the mapping and begin encapsulating and forwarding the TCP packet(s) for the TCP flow to the correct destination server node  130 . 
     Load Balancer Node Details 
     In at least some embodiments, the load balancer nodes  110  each have three roles:
         Ingress—Receiving all incoming packets from a client  160  in a client connection, routing the packets to a server node  130  if the mapping is known, or messaging a flow tracker if the mapping is not known. The outgoing packets from an ingress node are encapsulated (e.g., according to UDP) by the ingress node.   Flow tracking—Keeping track of connection states (e.g. which server node  130 /server  134  has been assigned to service each client connection). Flow trackers also participate in establishing connections between clients  160  and servers  134 .   Egress—Decapsulating and forwarding outbound packets received from a server  134  to a client  160 .       

     In at least some embodiments, in the ingress role, a load balancer node  110  is responsible for forwarding packets to servers  134  when a client→server mapping is known, or forwarding a request to a flow tracker when the mapping is unknown. In at least some embodiments, a load balancer node  110  serving as an ingress node for a particular client connection/data flow may also serve as either the primary flow tracker or the secondary flow tracker for the client connection, but not both. 
     In at least some embodiments, in the flow tracker role, a load balancer node  110  is responsible for maintaining the state of connections that are still being established, as well as maintaining the client→server mapping for established connections. Two flow trackers are involved with each individual client connection, referred to as the primary flow tracker and the secondary flow tracker. In at least some embodiments, the flow trackers associated with client connections may be determined using a consistent hash algorithm. The flow trackers also perform load-balancing functionality, including but not limited to pseudorandomly selecting a server node  130  for each new client connection. Note that the local load balancer module  132  on a selected server node  130  may reject a connection request if it determines that the server  134  cannot handle the connection. If this happens, then the flow trackers may select another server node  130  and send the connection request to the other server node  130 . In at least some embodiments, the primary flow tracker role and the secondary flow tracker role for a given connection are performed by different load balancer nodes  110 . 
     In at least some embodiments, in the egress role, a load balancer node  110  is stateless and decapsulates incoming packets received from server nodes  130 , performs some validation, and forwards the outbound TCP packets to respective clients  160 . In at least some embodiments, a local load balancer module  132  on a server node  130  may arbitrarily select a load balancer node  110  for a given connection. 
     Load Balancer Node Consistent Hash Ring Topology 
     In at least some embodiments, the load balancer nodes  110  form a ring topology based on the consistent hashing of input keyspace (client endpoint, public endpoint). The input keyspace may be partitioned among the available flow tracker nodes, and every flow tracker node may be responsible for answering queries corresponding to its keyspace. In at least some embodiments, data may be replicated to the primary and secondary flow tracker nodes based on the successor in the consistent hash ring (e.g., the secondary flow tracker node is the successor node, or next node in the consistent hash ring, to the primary flow tracker node). If a flow tracker node goes down for some reason, the next load balancer node in the consistent hash ring acquires the keyspace of the failed node. When a new flow tracker node joins, the node registers its endpoint (e.g., with a configuration service  122  as shown in  FIG. 1 ) so that other load balancer nodes may learn about the configuration change in the load balancer implementation and thus in the consistent hash ring. The handling of additions and failures of flow trackers in the consistent hash ring is discussed in more detail in reference to  FIGS. 11A through 11D . 
     Ingress Node &lt;-&gt; Flow Tracker Node Communications 
     In at least some embodiments, the load balancer nodes  110  serving as ingress nodes may learn about the load balancer nodes  110  serving as flow tracker nodes from configuration service  122 . The ingress nodes may monitor the configuration service  122  for membership changes in the load balancer implementation and thus in the consistent hash ring. When an ingress node receives a packet from a client  160  that the ingress node does not have a mapping for, the ingress node may use a consistent hash function to determine which flow tracker node should service the packet. In at least some embodiments, the input to the hash function is the (client endpoint, public endpoint) pair from the packet. In at least some embodiments, the ingress nodes and flow tracker nodes communicate using UDP messages. 
     When a primary flow tracker node receives a message from an ingress node for a new packet flow, the primary flow tracker node randomly determines a TCP sequence number and forwards another message to the secondary flow tracker node. The secondary flow tracker node generates a TCP SYN/ACK message for the client. Both flow trackers remember the client connection endpoint pair and the TCP sequence number, and retain this information until memory pressure or expiration causes the state to be purged. 
     When the primary flow tracker node receives a message from an Ingress node that a TCP ACK packet has been received, the primary flow tracker node verifies that the acknowledged TCP sequence number matches the stored value that was sent in the SYN/ACK packet, selects a server node  130  to service the request, and forwards a message to the secondary flow tracker node. The secondary flow tracker node sends a message to the load balancer module  132  on the selected server node  130  to initiate an actual TCP connection with the TCP stack on the server node  130 , and then waits for an acknowledgement response from the server node  130 . 
     When the secondary flow tracker node receives a connection acknowledgement from the load balancer module  132  on the server node  130 , a reverse message flow through the primary flow tracker to the ingress node is triggered that stores information about the associated server node  130  in both nodes. From this point forward, additional TCP packets received at the ingress node are forwarded directly to the load balancer module  132  on the server node  130 . 
     Load Balancer Module &lt;-&gt; Load Balancer Node Communications 
     In at least some embodiments, every load balancer module  132  registers its endpoint with configuration service  122  and monitors configuration service  122  continuously for membership changes in the load balancer node layer. The following describes functions of the load balancer module  132 , according to at least some embodiments:
         Connection publishing—periodically (e.g., once a second) or aperiodically publish the set of active connections (client endpoint, public endpoint) on the respective server node  130  to both the primary and the secondary flow tracker nodes responsible for those connections, as well as to the ingress nodes that last sent packets to the load balancer module  132  for those connections. The connection publishing function renews the lease for the connection states at the responsible load balancer nodes  110 .   Monitor membership changes in the load balancer layer. If the membership changes, the load balancer modules  132  may use this change information to immediately send active connections to the load balancer nodes that are now responsible for the connections.
 
Packet Flow in the Distributed Load Balancing System—Details
       

     The distributed load balancing system may include multiple load balancer nodes  110 . In at least some embodiments, each load balancer node  110  in the distributed load balancing system may serve in the roles of a flow tracker node, an egress node, and an ingress node for client  160  connections to the servers  134 . The distributed load balancing system may also include a load balancer module  132  on each server node  130 . 
       FIGS. 10A through 10G  illustrate packet flow in the distributed load balancing system, according to at least some embodiments. In  FIGS. 10A through 10G , packets exchanged between load balancer nodes  110  and packets exchanged between load balancer nodes  110  and server nodes  130  are either UDP messages or UDP-encapsulated client TCP packets. In at least some embodiments, client TCP packets only exist on network  100  in decapsulated form on the north side of the load balancer nodes  110  in transit to and from the border router  102  (see  FIG. 1 ). Note that the solid lines with arrows in  FIGS. 10A-10G  represent TCP packets, while the dotted lines with arrows represent UDP packets. 
     In at least some embodiments, the distributed load balancing system may attempt to preserve established connections in the event of a single load balancer node  110  failure. In at least some embodiments, this may be achieved by replicating connection details in a primary flow tracker node and a secondary flow tracker node so that, if either of these nodes fails, a connection&#39;s client→server mapping may be restored by the remaining flow tracker node. In at least some embodiments, some packet loss may occur in the event of a node failure; however, client/server TCP packet retransmissions may recover the lost packets. 
     Each TCP connection from a client may be referred to as a TCP flow, and is uniquely identified by a 4-tuple consisting of: the client IP address, client port, server (public) IP address, and server port. This identifier may be abbreviated as CP or CcPp indicating the client and public endpoint pair. Packets associated with any given TCP flow (or CP pair) can appear on any load balancer node  110  operating as an ingress server  112  due to the hashed equal-cost multipath (ECMP) flow distribution from the upstream edge router  104 . However, packets for a TCP flow may generally continue to arrive at the same load balancer node  110  unless there is a link or load balancer node  110  failure that causes the TCP flows to be redirected. The load balancer node  110  that receives packets for a TCP flow from the upstream router  104  is referred to as the ingress node for the TCP flow. 
     In at least some embodiments, consistent hashing is used so that when packets arrive at a load balancer node  110  serving as an ingress node for the TCP flow, the ingress node can determine which load balancer node  110  contains the state for the TCP flow (i.e., the flow tracker node). The CP pair may be hashed by the ingress node into a consistent hash ring to determine which load balancer node  110  is responsible for maintaining state regarding the TCP flow. This node serves as the primary flow tracker for the TCP flow. The successor node in the consistent hash ring serves as the secondary flow tracker for the TCP flow. 
     In at least some embodiments, all load balancer nodes  110  may serve as ingress nodes, primary flow tracker nodes, and secondary flow tracker nodes. Depending on the consistent hash result for a TCP flow, a load balancer node  110  serving as the ingress node for the TCP flow may also serve as the primary or the secondary flow tracker node for the TCP flow. However, in at least some embodiments, different physical load balancer nodes  110  perform the primary and secondary flow tracker roles for the TCP flow. 
     Establishing Connections 
     Referring to  FIG. 10A , new connections from a client  160  may be triggered by a client TCP synchronize (SYN) packet. The load balancer nodes  110  do not actually establish a connection with a server node  130  upon receipt of the SYN packet, nor do they immediately select a server node  130  to receive the connection. Instead, the load balancer nodes  110  store relevant data from the client&#39;s SYN packet, and generate a SYN/ACK packet on behalf of the yet-to-be-chosen server node  130 . Referring to  FIG. 10C , once the client  160  responds with the first ACK packet in the TCP three-way handshake, the load balancer nodes  110  select a server node  130 , generate an equivalent SYN packet for that server node  130 , and attempt to establish an actual TCP connection with the server node  130 . 
     Referring again to  FIG. 10A , upon receipt of a client SYN packet at the load balancer node  110  serving as the ingress server  112  for the TCP flow, the ingress server  112  extracts the data fields from the SYN packet and forwards the data to the primary flow tracker  116 A for the TCP flow. The primary flow tracker  116 A stores the data, for example in a hash table, generates an initial TCP sequence number (for the server side of the TCP connection), and forwards the same data to the secondary flow tracker  116 B. The secondary flow tracker  116 B fabricates a SYN/ACK packet for the client  160  containing that server TCP sequence number. 
     In  FIG. 10A , the ingress server  112 , primary flow tracker  116 A, and secondary flow tracker  116 B roles are each performed by different load balancer nodes  110 . However, in some cases, the load balancer node  110  serving as the ingress server  112  for a TCP flow may be the same node  110  that serves as the primary flow tracker  116 A or the secondary flow tracker  116 B for the TCP flow (but not both). The reason that the ingress server  112  for a packet flow may be on the same node  110  as a flow tracker  116  for the flow is that the edge router  104  pseudorandomly selects the ingress server  112  for the flow according to a per-flow hashed multipath routing technique (e.g., an ECMP routing technique), while the flow trackers  116  for the packet flow are determined on a consistent hash ring according to a consistent hash function applied to the packet flow&#39;s address information. If the ingress server  112  for a packet flow is on the same node  110  as a flow tracker  116  for the packet flow, the data from the SYN packet may only be forwarded from the node  110  that implements the ingress server  112  to the other flow tracker  116  node  110 . For example, in  FIG. 10B , the primary flow tracker  116 A is on the same load balancer node  110 A as the ingress server  112  for the TCP flow, while the secondary flow tracker  116 B is on a different load balancer node  110 B, and thus the data from the SYN packet is forwarded from node  110 A (by flow tracker  116 A) to the secondary flow tracker  116 B on load balancer node  110 B 
     Referring to  FIG. 10C , when non-SYN packets arrive at an ingress server  112 , the ingress server  112  either knows or does not know which server node  130  to forward the packets to. The first non-SYN packet to arrive at an ingress server  112  for a TCP flow should be the first TCP acknowledgement (ACK) packet in the TCP three-way handshake (or possibly a subsequent data packet), where the TCP acknowledgement number field matches the server sequence number (+1) that was sent in the SYN/ACK packet in  FIG. 10A . When the ingress server  112  receives a non-SYN packet for which it has no server mapping, it forwards a message to the primary flow tracker  116 A for the TCP flow, the message including information from the ACK packet such as a sequence number, or alternatively containing the ACK packet itself. In at least some cases, the primary flow tracker  116 A remembers the stored data for the TCP flow and confirms that the acknowledged sequence number (+1) matches the value that was sent to the client  160  in the SYN/ACK packet. The primary flow tracker then selects a server node  130  for the TCP flow and forwards another message containing the previously stored data for the TCP flow, the server sequence number, and an IP address for the load balancer module  132  on the selected server node  130  to the secondary flow tracker  116 B. The secondary flow tracker  116 B confirms the server sequence number, records the information, and sends a fabricated SYN message to the load balancer module  132  on the selected server node  130 . The TCP flow&#39;s CP endpoint pair is now mapped to the load balancer module  132 /server node  130 . The load balancer module  132  on the server node  130  is responsible for creating a legitimate TCP SYN packet for the server  134  on the server node  130  when it receives the fabricated SYN message from the secondary flow tracker  116 B. In creating the SYN packet, the source IP address is populated with the client  160 &#39;s actual IP address so that the server  134  will believe that it has received a direct TCP connection request from the client  160 . The load balancer module  132  stores the relevant details about the TCP flow, for example in a local hash table, and sends the TCP SYN packet to the server  134  (e.g., injects the SYN packet into the Linux kernel of the server  134 ). 
     In  FIG. 10C , the ingress server  112 , primary flow tracker  116 A, and secondary flow tracker  116 B roles are each performed by different load balancer nodes  110 . However, in some cases, the load balancer node  110  serving as the ingress server  112  for a TCP flow will be the same node  110  that serves as the primary flow tracker  116 A or the secondary flow tracker  116 B for the TCP flow (but not both). For example, in  FIG. 10D , the secondary flow tracker  116 B is on the same load balancer node  110 A as the ingress server  112  for the TCP flow, while the primary flow tracker  116 A is on a different load balancer node  110 B. 
     Referring to  FIG. 10E , the server  134  (e.g., the Linux kernel) responds with a SYN/ACK packet that the load balancer module  132  also intercepts. The SYN/ACK packet may contain a different TCP sequence number than was originally delivered to the client  160  in the generated SYN/ACK from the secondary flow tracker  116 B (see  FIG. 10A ). The load balancer module  132  is responsible for applying the sequence number delta to incoming and outgoing packets. The SYN/ACK packet from the server  134  also triggers a message (e.g., a UDP message) from the load balancer module  132  back to the secondary flow tracker  116 B to indicate that the connection to the selected server node  130 /load balancer module  132 /server  134  has succeeded. Upon receipt of this message, the secondary flow tracker  116 A may record the client and public endpoint pair (CP) mapping between the client  160  and the server  134  as committed, and send a similar message to the primary flow tracker  116 A which will also record the CP mapping. The primary flow tracker  116 A may then forward a CP mapping message to the ingress server  112 , which causes the ingress server  112  to forward any buffered data packets for the connection to the local load balancer module  132  on the server node  130  as encapsulated data packets. 
     Referring to  FIG. 10F , the CP mapping for the connection is known to the ingress server, so incoming TCP packets received by the ingress server  112  for the connection may be encapsulated (e.g., according to UDP) and forwarded directly to the to the local load balancer module  132  on the server node  130  as encapsulated data packets. The load balancer module  132  decapsulates the data packets and sends the TCP packets to the server  134  on the server node  130 , for example by injecting the TCP packets on to a TCP stack of the kernel. Outbound packets from the server  134  are intercepted by the load balancer module  132  on the server node  130 , encapsulated (e.g., according to UDP), and forwarded to an arbitrary load balancer node  110  that the load balancer module  132  randomly selects as the egress server  114  for this connection. The egress server  114  decapsulates the packets and sends the decapsulated packets to the client  116 . The egress function of the selected load balancer node  110  is stateless, so a different load balancer node  110  can be selected as the egress server  114  for the connection in the event of failure of the load balancer node  110  serving as the egress server. However, generally the same load balancer node  110  is used as the egress server  114  for the duration of the connection to reduce or eliminate re-ordering of the outbound packets. 
     Referring to  FIG. 10G , in at least some embodiments, if the load balancer module  132 A on a server node  130 A that is selected by the primary flow tracker  116 A (see  FIG. 10C ) determines that it is overloaded, it has the option to reject the fabricated SYN message received from the secondary flow tracker  116 B (see  FIG. 10C ). In at least some embodiments, the fabricated SYN message includes a time to live (TTL) value or counter that allows for a maximum number of rejections. In at least some embodiments, if this TTL value reaches zero, the load balancer module  132 A may either accept the connection or drop the connection to shed load. If the load balancer module  132 A decides to reject the connection, it decrements the TTL value and sends a reject message to the secondary flow tracker  116 B. The secondary flow tracker  116 B resets the CP mapping and sends a release message to the primary flow tracker  116 A to do the same. The primary flow tracker  116 A chooses a new load balancer module  132 B on another server node  130 B and sends a new target message back to the secondary flow tracker  116 B, which sends a new fabricated SYN message to the newly chosen load balancer module  132 B. Note that packet drops may result in this sequence failing to complete; however, a retransmission from the client  160  may trigger the load balancer module selection process again at the primary flow tracker  116 A, which may, but does not necessarily, choose the same load balancer module  132  for the connection if it has not learned about the previous rejection of the fabricated SYN packet. 
     In at least some embodiments, the TTL counter may be used to prevent continuously sending connection requests to server nodes  130 , which may occur for example if all the server nodes  130  are busy. In at least some embodiments, each time a load balancer module  132  rejects a connection request on behalf of a respective server node  130 , the load balancer module  132  decrements the TTL counter. The flow tracker nodes  116  may monitor the TTL counter and, as long as the TTL counter is not zero (or is above some specified threshold), may select another server node  130  and try again. If the TTL counter reaches zero (or reaches the specified threshold), the connection request is dropped and no further attempts are made by the flow tracker nodes  116  to send a connection request to a selected one of the server nodes  130  for that connection. In at least some embodiments, an error message may be sent to the respective client  160 . 
     In at least some embodiments, the distributed load balancer system supports multiple public IP addresses. As such, it is possible that a client  160  may initiate two TCP connections from the same client port number to two different public IP addresses. These TCP connections are distinct from the client  160 &#39;s point of view, but internally the distributed load balancer may map the connections to the same server node  130 , which would result in a collision. In at least some embodiments, to detect and handle possible collisions, the load balancer module  132 , upon receiving the fabricated SYN packet from the secondary flow tracker  116 B as shown in  FIGS. 10C and 10D , may compare the address information to its active connections and, if this connection would cause a collision, reject the connection request as shown in  FIG. 10G . 
     Handling Load Balancer Node Failures and Additions 
     In many conventional load balancers, some or all existing connections are lost in the event of a load balancer failure. In at least some embodiments, in the event of failure of a single load balancer node  110 , the distributed load balancing system may maintain at least some of the established connections so that the clients and servers can continue to exchange packets via the connections until the connections complete normally. In addition, the distributed load balancing system may continue to service connections that were in the process of being established at the time of failure. 
     In at least some embodiments of the distributed load balancing system, a failure recovery protocol may be implemented that may recover existing client connections in the event of a single load balancer node  110  failure. Multiple load balancer node  110  failures, however, may result in lost client connections. In at least some embodiments, TCP retransmissions between a client  160  and a server  134  may be used as a means of recovery following a load balancer node  110  failure. 
     In addition to potential load balancer node  110  failures, new load balancer nodes  110  may be added to the distributed load balancer system. These new nodes  110  may be added to the load balancer layer and thus to the consistent hash ring, and load balancer node  110  roles regarding existing client connections may be adjusted according to the change, as necessary. 
     Handling Flow Tracker Node Failures and Additions 
     In at least some embodiments, as each connection is established (see, e.g.,  FIGS. 10A through 10G ), the connection state information is passed through two load balancer nodes  110 , referred to as the primary and secondary flow trackers, which may be determined using a consistent hash algorithm that, for example, uses the (client IP:port, public IP:port) tuple as hash function input. In the event of a single load balancer node  110  failure, at least one of the surviving load balancer nodes  110  may continue to be mapped via the consistent hash function and may contain the necessary state information for a connection to direct packets to the selected server node  130  for a connection. In addition, in the case of an addition of a load balancer node  110  to the consistent hash ring, state information for connections may be refreshed to the appropriate flow trackers. 
       FIGS. 11A through 11D  illustrate the handling of events that effect membership in the load balancer node consistent hash ring, according to at least some embodiments. These events may include, but are not limited to, adding a new primary flow tracker node, adding a new secondary flow tracker node, failure of a primary flow tracker node, and failure of a secondary flow tracker node. 
       FIG. 11A  illustrates handling the addition of a new primary flow tracker node to the consistent hash ring. The top row of  FIG. 11A  shows flow tracker  116 A as the primary flow tracker for one or more client connections and flow tracker node  116 B as the secondary flow tracker for the same connection(s). In the bottom row of  FIG. 11A , a new flow tracker node  116 C has been added, and becomes the primary flow tracker for the client connection(s). Flow tracker node  116 A, formerly the primary flow tracker, becomes the secondary flow tracker, while flow tracker node  116 B, formerly the secondary flow tracker, becomes a next flow tracker in the consistent hash ring. State information for the client connection(s) that was maintained by flow trackers  116 A and  116 B may be provided to the new primary flow tracker  116 C. In addition, flow tracker  116 B may “forget” its formerly tracked connections in the role of secondary flow tracker. 
       FIG. 11B  illustrates handling the addition of a new secondary flow tracker node to the consistent hash ring. The top row of  FIG. 11B  shows flow tracker  116 A as the primary flow tracker for one or more client connections and flow tracker node  116 B as the secondary flow tracker for the same connection(s). In the bottom row of  FIG. 11B , a new flow tracker node  116 C has been added, and becomes the secondary flow tracker for the client connection(s). Flow tracker node  116 A remains as the primary flow tracker for the connection(s), while flow tracker node  116 B, formerly the secondary flow tracker, becomes a next flow tracker in the consistent hash ring. State information for the client connection(s) that was maintained by flow trackers  116 A and  116 B may be provided to the new secondary flow tracker  116 C. In addition, flow tracker  116 B may “forget” its formerly tracked connections in the role of secondary flow tracker. 
       FIG. 11C  illustrates handling the failure of a primary flow tracker node in the consistent hash ring. The top row of  FIG. 11C  shows flow tracker  116 A as the primary flow tracker for one or more client connections, flow tracker node  116 B as the secondary flow tracker for the same connection(s), and flow tracker node  116 C as a next flow tracker in the consistent hash ring. In the bottom row of  FIG. 11C , primary flow tracker node  116 A has failed. Flow tracker node  116 B becomes the primary flow tracker for the connection(s), while flow tracker node  116 C becomes the secondary flow tracker for the connection(s). State information for the client connection(s) is maintained by flow tracker  116 B and may be provided to the new secondary flow tracker  116 C. 
       FIG. 11D  illustrates handling the failure of a secondary flow tracker node in the consistent hash ring. The top row of  FIG. 11D  shows flow tracker  116 A as the primary flow tracker for one or more client connections, flow tracker node  116 B as the secondary flow tracker for the same connection(s), and flow tracker node  116 C as a next flow tracker in the consistent hash ring. In the bottom row of  FIG. 11D , secondary flow tracker node  116 B has failed. Flow tracker node  116 A remains as the primary flow tracker for the connection(s), while flow tracker node  116 C becomes the secondary flow tracker for the connection(s). State information for the client connection(s) is maintained by flow tracker  116 B and may be provided to the new secondary flow tracker  116 C. 
     In at least some embodiments, the load balancer modules  132  on the server nodes  130  perform connection publishing to the load balancer nodes  110 . In at least some embodiments, the connection publishing periodically (e.g., once a second) or aperiodically pushes current connection state information from the server nodes  130  to the load balancer nodes  110  serving as flow tracker nodes and ingress nodes, which acts to refresh or restore the connection mappings to both the primary and secondary flow tracker nodes for the connections. In at least some embodiments, a load balancer module  132  may detect a flow tracker membership change, for example as illustrated in  FIGS. 11A through 11D . In response, the load balancer module  132  may perform a connection publication to populate the state information for the connections in the primary and secondary flow tracker nodes, which may have changed for the connections when the membership changed. Note that connection publishing may allow at least some established connections to be recovered in the event of multiple load balancer node failures. 
     Failure-Related Message Flow 
     In at least some embodiments, the protocol between the primary and secondary flow tracker nodes may include a correction or synchronization functionality. For example, referring to  FIG. 11A , when a new primary flow tracker node  116 C joins the consistent hash ring, the new node  116 C may lay claim to the consistent hash keyspace for some number of connections (˜1/N) and begin receiving traffic related to these connections from the edge router  104 . However, the new primary flow tracker node  116 C does not have any state stored for the connections, so it may operate on each packet as if it were the first packet received from the client  160 . The primary flow tracker is responsible for generating server TCP sequence numbers in response to SYN packets (see, e.g.,  FIG. 10A ) and for selecting server nodes  130  in response to the first ACK packet from a client  160  (see, e.g.,  FIG. 1 ), and these generated values may disagree with values chosen by the previous primary flow tracker (flow tracker node  116 A in  FIG. 11A ). However, in at least some embodiments the consistent hash algorithm assigns the previous primary flow tracker (flow tracker node  116 A in  FIG. 11A ) into the secondary flow tracker role, and this flow tracker still retains the previously stored state for the connections. Thus, in at least some embodiments, when the secondary flow tracker (flow tracker node  116 A in  FIG. 11A ) detects a discrepancy in information received from the primary flow tracker  116 C, it can send update messages back to the primary flow tracker  116 C to bring the two load balancer nodes  110  serving as flow trackers for the connections into synchronization. Similar methods may be used to synchronize the flow trackers after other changes in the consistent hash ring membership. 
     Load Balancer Module Details 
     In at least some embodiments, the load balancer module  132  is a component of the distributed load balancer system that resides on each of the server nodes  130 . Roles of the load balancer node  132  include, but are not limited to, decapsulating packets received from the load balancer nodes  110  and sending the decapsulated packets to the server  134  on the server node  130 , and encapsulating outgoing packets from the server  134  and sending the encapsulated packets to a load balancer node  110 . 
     In at least some embodiments, incoming packets to the load balancer modules  132  on the server nodes  130  from the load balancer nodes  110  serving as ingress servers  112  are stateless protocol (e.g., UDP) packets that encapsulate the actual client data packets. Each encapsulated client data packet has the original clientIP:port of a respective client  160  as the source address and the server  134  publicIP:port as the destination address. The load balancer modules  132  strip the encapsulation from the client data packets and send the packets to the respective servers  134  on the server nodes  130 , for example by redirecting the packets to a localhost TCP flow. 
     In at least some embodiments, outgoing packets from the servers  134  to the load balancer nodes  110  serving as egress servers  114  are stateless protocol (e.g., UDP) packets that encapsulate the outgoing IP packets. The load balancer modules  132  encapsulate the outgoing IP packets and send the encapsulated packets to the egress servers  114  via the fabric  120 . Each encapsulated outgoing IP packet has the server  134  publicIP:port as the source address and the clientIP:port of a respective client  160  as the destination address. 
     Load Balancer Module Functionality 
     In at least some embodiments, functions of the load balancer module  132  on a server node  130  may include one or more of, but are not limited to:
         Terminating UDP tunnels from the load balancer node(s)  110 , e.g. from the ingress server  112  handling a connection to a client  160 . This includes stripping UDP encapsulation from incoming client data packets received from the ingress servers  112 .   Selecting an egress server  114  to receive outgoing traffic for a connection.   Intercepting outgoing IP packets on a connection to the respective server  134 , encapsulating the outgoing IP packets for the connection, and sending the encapsulated packets to the egress server  114 .   Mangling the sequence number in incoming and outgoing packets so that the sequence number aligns with the sequence number generated by the flow tracker nodes  116  when the flow tracker nodes  116  sent a SYN/ACK to the client  160 .   Making the decision on whether to accept or reject a connection for the respective server  134 , for example based on one or more metrics indicating the respective server  134 &#39;s current load.   Detecting and rejecting connections from the same clientIP:port address to the respective server  134  if there is an active connection for that clientIP:port address to avoid collisions.   Connection tracking and connection publishing.
 
Load Balancer Module Configuration Information
       

     In at least some embodiments, each load balancer module  132  may acquire and locally store one or more of, but is not limited to, the following sets of information for its configuration: a set of load balancer node  110  endpoints; a set of valid public IP addresses which it is to serve; and the port number(s) on which the respective server  134  accepts incoming connections. In a least some embodiments, this information may be acquired from or updated by accessing or querying a configuration service  122  component of the distributed load balancer system, as illustrated in  FIG. 1 . Other methods of acquiring the information may be used in some embodiments. 
     Load Balancer Module Packet Handling 
     The following describes load balancer module  132  operations for inbound traffic and outbound traffic according to at least some embodiments. In at least some embodiments, when an inbound data packet is received by the load balancer module  132 , the data packet is decapsulated from the UDP packet, and the destination address in the decapsulated TCP packet is first validated against a set of configured valid public IP addresses. If there is no match, the packet is dropped or ignored. In at least some embodiments, the load balancer module  132  may adjust the sequence number in the TCP header by a constant delta so that the sequence number matches the randomly chosen sequence number generated by the flow tracker nodes  116  that sent the SYN/ACK packet to the client  160 . The load balancer module  132  records the mapping from the [Client:Public] endpoint to the [Client:Server] endpoint as an internal state. 
     In at least some embodiments, for outbound TCP packets from the server  134 , the load balancer module  132  first checks its internal state to determine if the packet is for an active connection that the load balancer module is managing. If it is not, the load balancer module  132  just passes the packet through. If it is, the load balancer module  132  encapsulates the outgoing TCP packet, for example according to UDP, and forwards the encapsulated packet to a load balancer node  110  that was selected as the egress server  114  for this connection. In at least some embodiments, the load balancer module  134  may adjust the TCP sequence number in the outgoing TCP packet by a constant delta so that it aligns with the sequence number generated by the flow tracker nodes  116  that sent the SYN/ACK packet to the client  160 . 
     Connection Tracking 
     In at least some embodiments, the load balancer module  132  on each server node  130  manages a hash table containing connection details for every active client connection to the respective server  134 . In at least some embodiments, the key for the hash table is the (clientIp:port, publicIp:port) tuple. In at least some embodiments, the connection state for each client connection may include one or more of, but is not limited to:
         The client IP:Port   The public IP:Port   The initial server TCP sequence number provided by the flow tracker  116  nodes.   The server TCP sequence number delta.   The original primary flow tracker IP address.   The original secondary flow tracker IP address.   The IP address of the last detected ingress server  112 .   An expiration time for this entry   Least Recently Used (LRU)/Collision indices.       

     In at least some embodiments, each load balancer module  132  periodically generates connection publishing messages to the primary and secondary flow tracker nodes for all active client connections. In at least some embodiments, the content of /proc/net/tcp is scanned and intersected with the active connections in the load balancer module&#39;s hash table so that they will continue to be published to the flow tracker nodes until the Linux kernel stops tracking the connection. Connection publishing will be discussed in more detail later in this document. 
     Sequence Number Mangling 
     As previously described, in at least some embodiments the load balancer nodes  110  generate SYN/ACK packets in response to client  160  SYN packets on behalf of the server  134 . Only after the client  160  sends an ACK packet (the TCP three-way handshake) does a load balancer module  110  send any data to a load balancer module  132  on a server node  130 . When the load balancer module  132  is first instructed to establish a client connection, the load balancer module  132  locally fabricates a SYN packet to begin a TCP connection with the server  134  on the server node  130 , and intercepts the server  134 &#39;s corresponding SYN/ACK packet. Typically, the server  134  (e.g., the Linux kernel on the server node  130 ) selects an entirely different TCP sequence number than the one the client received in the SYN/ACK packet from the load balancer nodes  110 . Thus, in at least some embodiments, the load balancer module  132  may correct for the sequence numbers in all the packets in the TCP connection between the client  160  and the server  134 . In at least some embodiments, the load balancer module  132  computes the difference between the sequence number generated by the load balancer nodes  110  and the sequence number generated by the server  134  and stores the difference as a delta value in the hash table entry for the TCP connection. When incoming data packets arrive from the client  160  on the connection, the TCP header will contain acknowledgement numbers that will not align with the sequence number used by the server  134 , so the load balancer module  132  subtracts the delta value (e.g., using two&#39;s complement) from the sequence number value in the TCP header. The load balancer module also adds the delta value to the sequence number in outbound packets from the server  134  to the client  130  on the connection. 
     Health Checking in the Distributed Load Balancer System 
     In at least some embodiments of the distributed load balancer system, each load balancer node  110  requires a consistent view of the healthy members in the load balancer implementation (i.e., of the healthy load balancer nodes  110  and server nodes  130 ) for at least the following reasons:
         Load balancing—The load balancer nodes  110  need to detect server node  130  failures and converge on a set of healthy server nodes  130  that can accept client traffic.   Distributed state management—The load balancer is a distributed system with the state shared/replicated across multiple load balancer nodes  110  (e.g., according to a consistent hashing mechanism). In order to properly handle client traffic, each load balancer node  110  needs to have an eventually consistent view of the healthy member nodes  110  in the load balancer implementation.       

     To accomplish this, at least some embodiments of the distributed load balancer system may implement embodiments of a health check protocol that monitors nodes in the load balancer implementation and detects unhealthy nodes as soon as possible. The health check protocol may propagate health information among the nodes in the load balancer implementation, and may provide methods that enable the nodes to converge on a set of healthy nodes. In addition, the health check protocol may provide mechanisms for reporting healthy/unhealthy nodes and state changes in the load balancer implementation. 
     In at least some embodiments, the health check protocol may be based on one or more of, but not limited to, the following assumptions:
         All nodes in the load balancer implementation are known. (I.e., the health check protocol may not perform discovery).   All node failures are fail-stop.   All messages between nodes are stateless protocol (e.g., UDP) messages, and the messages may be dropped, delayed, duplicated or corrupted. There are no guarantees on message delivery.       

     In at least some embodiments, a node in a load balancer implementation (e.g., a load balancer node  110  or server node  130 ) may be considered healthy under the following conditions:
         All of the node&#39;s internal components are in ready state (ready to handle client traffic).   The node&#39;s incoming/outgoing network links are healthy (for at least the network interface controllers (NICs) on which client traffic flows).       

       FIG. 12  is a high-level flowchart of a health check method that may be performed by each load balancer node according to a health check interval, according to at least some embodiments. As indicated at  1000 , at each load balancer interval, for example every 100 milliseconds, each load balancer (LB) node  110  may health check at least one other LB node  110  and at least one server node  130 . As indicated at  1002 , the load balancer node  110  may update its locally stored health information according to the health checks. As indicated at  1004 , the load balancer node  110  may then randomly select at least one other load balancer node  110  and send its health information to the selected load balancer node(s)  110 . In at least some embodiments, the node  110  may also send a list of healthy load balancer nodes  110  to one or more server nodes  130 , for example to the same server node(s)  130  that are health checked by the node  110 . The elements of  FIG. 12  are explained in more detail in the following discussion. 
     In at least some embodiments of the health check protocol, a load balancer node  110  does not assert its own health to the other load balancer nodes  110 . Instead, one or more other load balancer nodes  110  may health-check the node  110 . For example, in at least some embodiments, each load balancer node  110  may periodically or aperiodically randomly select one or more other nodes  110  to health-check. As another example, in at least some embodiments, one or more other load balancer nodes  110 , for example the two nearest neighbors of a given load balancer node  110  on an ordered list of nodes  110  such as a consistent hash ring, may each periodically or aperiodically check the health of the given node  110 . In at least some embodiments, health checking a node  110  may include using health pings sent to the NICs  1114  on the node  110  as illustrated in  FIG. 23 . In at least some embodiments, if a first node  110  determines that a second node  110  is healthy via a health check, the first node  110  may update (e.g., increment) the heartbeat counter for the second node  110  stored in local health information for the load balancer nodes  110 . The first node  110  periodically or aperiodically sends its local health information to one or more other load balancer nodes  110  in the load balancer implementation, which may update their own local health information accordingly (e.g., by incrementing the heartbeat counter for the second node) and send their updated local health information to one or more other nodes  110 . The heartbeat information for the second node  110  may thus be propagated to the other nodes  110  in the load balancer implementation. As long as the second node  110  is healthy, all of the other nodes  110  that are reachable from the second node  110  should thus see the second node  110 &#39;s heartbeat counter getting incremented on a consistent basis, e.g. once a second or once every ten seconds. If the second node  110  is detected to be unhealthy by the node(s)  110  that check its health, no heartbeat for the node  110  is sent by the health checking nodes  110  and, after some time threshold, the other nodes  110  in the load balancer implementation  110  consider the node  110  in question to be unhealthy, or down. 
     In at least some embodiments, a load balancer node  110  may check one or more aspects of its own internal state and, if the node  110  detects that it is unhealthy for some reason, the node  110  may stop responding to the health pings from other nodes  110  that check its health. Thus, the nodes  110  checking the unhealthy node  110 &#39;s health may consider the node  110  as unhealthy, and may not propagate heartbeat increments on behalf of the node  110 . 
     Health Check Protocol Details 
     In at least some embodiments, the health check protocol may leverage a heartbeat counter technique and gossip protocol technology. The health check protocol may be considered to have two main parts—health checking and gossip/failure detection. 
     Health checking—Every load balancer node  110  in the load balancer implementation may periodically or aperiodically health check one or more other nodes  110  in the implementation. Methods by which the one or more other nodes are determined are discussed later. A core idea of health checking is that if a node  110  health checks another node  110  and determines that the other node  110  is healthy, the checking node  110  asserts that the other node  110  is healthy by incrementing and propagating a heartbeat counter for the other node  110 . In other words, the nodes  110  do not assert their own health to the other nodes; instead, one or more other nodes  110  check and assert the health of each node  110  in the load balancer implementation. 
     Gossip/failure detection—In at least some embodiments, the health check protocol may leverage a gossip protocol to propagate load balancer node  110  health information among the member load balancer nodes  110  in the load balancer implementation. The gossip protocol converges rapidly, and provides eventual consistency guarantees that are sufficient for the purposes of the distributed load balancing system. In at least some embodiments, using the gossip protocol, each load balancer node  110  maintains a heartbeat counter for each other node  110  in the load balancer implementation, for example in a heartbeat list. Each load balancer node  110  periodically or aperiodically performs a health check of at least one other load balancer node  110  as described above, and increments the heartbeat counter for a node  110  upon determining via the health check that the checked node  110  is healthy. In at least some embodiments, each load balancer node  110  periodically or aperiodically randomly selects at least one other node  110  in the load balancer implementation to which it sends its current heartbeat list. Upon receipt of a heartbeat list from another node  110 , a load balancer node  110  merges the heartbeat information in the received list with its own heartbeat list by determining the maximum heartbeat counter for each node  110  in the two lists (the received lists and its own list) and using the determined maximum heartbeat counter in its own heartbeat list. In turn, this heartbeat list gets sent to another randomly selected node  110 , which updates its own heartbeat list accordingly, and so on. Using this technique, heartbeat information for each healthy node  110  is eventually (e.g., in a few seconds) propagated to all of the other load balancer nodes  110  in the load balancer implementation. As long as the heartbeat counter keeps increasing for a given load balancer node  110 , it is considered to be healthy by the other nodes  110 . If a load balancer node  110 &#39;s heartbeat counter does not get incremented for a specified period by the health checking and gossiping method, then other load balancer nodes  110  may converge on the load balancer node  110  being considered unhealthy. 
     Health Checking Load Balancer Nodes 
     The following describes a method for health checking a load balancer node  110  that may be performed by another load balancer node  110 , according to at least some embodiments. With reference to  FIG. 23 , in at least some embodiments, a load balancer node  110  may be considered healthy if one or more of the following conditions are determined for the node  110 :
         The processor threads (e.g., core packet processing code  1108  threads) of the node  110  are in the ready state (internal).   The node  110  knows the edge router  104 &#39;s IP address and/or MAC address (internal).   All of the threads and/or protocol handlers of the node  110  are in the ready state (internal).   The incoming and outgoing links from the north side (edge router  104 /border network) and from the south side (servers  130 /production network) are active (external).   The node  110  can receive and dispatch packets via the network interface controllers (NICs) used in the load balancer implementation. For example, in an example load balancer node  110  embodiment as shown in  FIG. 23 , the node  110  should successfully receive and dispatch packets via the north-facing NIC  1114 A and the south-facing NIC  1114 B.       

     If one or more of these health conditions does not hold for a given node  110 , the node  110  may be considered not healthy. Note that, in some embodiments, a node  110  is only considered healthy if all of the above conditions hold for the node  110 . 
     In at least some embodiments, in addition to the above health conditions, a third NIC, shown in  FIG. 23  as NIC  1114 C, on each load balancer node  110  that may, for example, be used for control plane communications may also be checked by a health-checking node  110  by sending packets to and receiving packets from the NIC and, if the check of the third NIC fails, the node  110  being checked may be considered unhealthy. 
       FIG. 13  illustrates an example method for health checking a load balancer node from another load balancer node, according to at least some embodiments. In this example, load balancer node  110 A is health checking load balancer node  110 B. Each node  110 A and  110 B has a north-facing NIC (NIC  1114 A in  FIG. 23 ) and a south-facing NIC (NIC  1114 B in  FIG. 23 ). At  1 , node  110 A sends a packet (e.g., a ping packet) from its north-facing NIC to the north-facing NIC of node  110 B via edge router  104 . Node  110 B receives the packet on its north-facing NIC, and at  2  sends a response from its north-facing NIC to the north-facing NIC of node  110 A via fabric  120 , provided the conditions given in the list above are satisfied. After receiving the response on its north-facing NIC, at  3 , node  110 A sends a packet (e.g., a ping packet) from its south-facing NIC to the south-facing NIC of node  110 B via fabric  120 . Node  110 B receives the packet on its south-facing NIC, and at  4  sends a response from its south-facing NIC to the south-facing NIC of node  110 A via edge router  104 , provided the conditions given in the list above are satisfied. Upon receiving the response on its south-facing NIC, node  110 A considers node  110 B to be healthy and increments node  110 B&#39;s local heartbeat counter, which may then be propagated to other nodes  110  according to a gossip protocol as previously described. 
     As an alternative to the above, in some embodiments, load balancer node  110 B may respond to the first ping message, received at its north-facing NIC, via its south-facing NIC to the south-facing NIC of node  110 A, and respond to the second ping message, received at its south-facing NIC, via its north-facing NIC to the north-facing NIC of node  110 A. 
     In addition, in some embodiments, node  110 A may also health check a third NIC of node  110 B that is used for control plane communications (shown as NIC  1114 C in  FIG. 23 ) by pinging node  110 B&#39;s third NIC from its own third NIC and receiving a response to the ping message on its third NIC from node  110 B&#39;s third NIC if node  110 B is healthy. The ping message and response may pass through one or more control plane device(s)  170 , for example a network switch. 
     The above-described health check mechanism exercises all of the incoming and outgoing links and data paths of node  110 B in all directions (north, south, and through the control plane) as well as all of node  110 B&#39;s NICs, and also verifies the internal health of node  110 B as the ping packets traverse the internal queues and dispatching of node  110 B as would a client packet. 
     Assigning Health Checking Responsibilities to Load Balancer Nodes 
     In at least some embodiments, every load balancer node  110  in a load balancer implementation has access to a list (e.g., a sorted list) of all of the other load balancer nodes  110  in the load balancer implementation, for example via a configuration function and/or via a configuration service  122  component as shown in  FIG. 1 . In at least some embodiments, each load balancer node  110  may randomly select one or more other nodes  110  on the list to health check at each health check interval, incrementing their heartbeat counter if determined healthy. Note that the list includes all load balancer nodes  110  in the load balancer implementation whether currently considered healthy or unhealthy via the health check mechanism, and currently unhealthy nodes  110  may be randomly selected from the list and health checked as well as healthy nodes  110 . Thus, a currently unhealthy node  110  may be determined to be healthy by one or more nodes  110  that health check the node  110 , its heartbeat counter may be incremented and propagated to the other nodes  110 , and the unhealthy node  110  may thus return to healthy status. 
     Alternatively, in some embodiments, each load balancer node  110  may assume responsibility for health checking one or more other nodes  110  in the list and incrementing their heartbeat counter if determined healthy. For example, in some embodiments, each node  110  may assume responsibility for two other nodes, for example its “left” (or previous) and “right” (or next) nearest neighbor nodes  110  in the list. Note that the list may be considered circular and a node  110  at the “end” of the list may assume responsibility for health checking a node  110  at the “beginning” of the list, and vice versa. In some embodiments, the two other nodes  110  may be otherwise selected, for example as the two nearest neighbors next on the list. In some embodiments, each node  110  may assume responsibility for health checking more than two other nodes  110  on the list, for example three or four other nodes  110 . In at least some embodiments, if a neighbor node  110  that is being checked by a node  110  is determined to be unhealthy, then the node  110  may assume responsibility for health checking at least one node on the list that the unhealthy neighbor node  110  was responsible for checking. In at least some embodiments, in addition to health checking its neighbor nodes  110  (e.g., a “left” and “right” neighbor node), each load balancer node  110  may also periodically or aperiodically randomly select a node  110  in the ring and perform a health check of that randomly selected node  110  and, if healthy, increment and propagate the random node  110 &#39;s heartbeat. In at least some embodiments, all other nodes  110  in the ordered list are considered for the random selection and health check regardless of whether the other node  110  was previously considered healthy or not. 
     In at least some embodiments, each node  110  performs the health check of one or more randomly selected nodes  110 , or alternatively of its neighbor nodes  110  and a randomly selected node, at a regular interval, which may be referred to as the health check interval. For example, in some embodiments, the heartbeat interval may be 100 milliseconds, although shorter or longer intervals may be used. In addition, in at least some embodiments, each node  110  sends or “gossips” its current heartbeat list to at least one other randomly selected node  110  at a regular interval, which may be referred to as a gossip interval. In some embodiments, the health check interval and the gossip interval may be the same, although they are not necessarily the same. 
       FIG. 14  graphically illustrates a load balancer node health checking one or more other load balancer nodes, according to at least some embodiments. In this example, there are eight load balancer nodes  110 A- 110 H in the load balancer implementation. The dotted circle represents an ordered list of all nodes  110  in the implementation. In some embodiments, each node  110  may randomly select one or more other nodes  110  on the list to health check at each interval. As an alternative, in some embodiments, each load balancer node  110  may assume responsibility for checking one or more particular nodes  110  on the ordered list, for example node  110 A may take responsibility for health-checking its two nearest neighbor nodes  110 B and  110 H according to the ordered list as shown in  FIG. 14 . In addition, the load balancer node may also randomly select another node  110  from the ordered list at each health check interval. As shown in this example, node  110 A has also randomly selected node  110 F to health check. At the gossip interval, node  110 A randomly selects some other healthy node  110 , for example node  110 D, and sends its current heartbeat list to the selected other node  110 , for example in a UDP message. A node  110 , upon receiving a heartbeat list from another node  110 , may update its own heartbeat list accordingly and propagate the heartbeat list to one or more randomly selected nodes  110  at the next gossip interval. 
     Health Checking the Server Nodes 
     In addition to health checking the load balancer nodes  110  as described above, embodiments of the health check protocol may perform health checking of the server nodes  130  including the load balancer modules  132  and servers  134  on those nodes  130 . In at least some embodiments, a server node  130  may be considered healthy if one or both of the following conditions are determined for the node  130 :
         The load balancer module  132  is healthy.   The server node  130  responds successfully to health pings (e.g., L7 health pings).       

       FIG. 15  illustrates the load balancer nodes health checking the server nodes, according to at least some embodiments. In at least some embodiments, every load balancer node  110  in a load balancer implementation has access to a list of all of the other load balancer nodes  110  in the load balancer implementation, as well as a list of all server nodes  130  in the load balancer implementation. The list(s) may be obtained and updated, for example via a configuration function and/or via a configuration service  122  component as shown in  FIG. 1 . In at least some embodiments, the server nodes  130  may be consistent hashed against the healthy load balancer nodes  110  to form a consistent hash ring as illustrated in  FIG. 15 . In at least some embodiments, each server node  130  in the ring is health checked by two healthy load balancer nodes  110  in the ring. For example, in  FIG. 15 , server node  130 A is health checked by load balancer nodes  110 A and  110 C. These two nodes  110  may be referred to as the first (node  110 A) and second (node  110 B) health checking nodes  110  for the server node  130  in the consistent hash ring. Note that a given healthy load balancer node  110  may health check more than one server node  130 . For example, in  FIG. 15 , load balancer node  110 A also health checks server nodes  130 B and  130 C. In addition, a given node balancer node  110  may be a first health checking node  110  for one or more server nodes  130  and a second health checking node  110  for one or more other server nodes  130 . For example, in  FIG. 15 , load balancer node  110 A is the first health checker node for server nodes  130 A and  130 B and the second health checker node for server nodes  130 C and  130 D. 
     In at least some embodiments, if a load balancer node  110  fails, the membership in the consistent hash ring changes, and one or more others of the load balancer nodes  110  that are still healthy and thus still on the consistent hash ring may assume responsibility for health checking the server nodes  130  previously health checked by the failed node  110 . 
     In at least some embodiments, each healthy node  110  performs the health check of its assigned server nodes  130  at a regular interval, which may be referred to as a server check interval. In at least some embodiments, the server check interval may be greater than or equal to the gossip interval previously mentioned. 
     In at least some embodiments, to perform a health check of a server node  130 , a healthy load balancer node  110  (e.g., node  110 A in  FIG. 15 ) initiates a health ping message (e.g., a L7 HTTP health ping message) to a server node  130  (e.g., server node  130 A in  FIG. 15 ). If healthy, the server node  130  sends a ping response back to the load balancer node  110 . In at least some embodiments, the ping message is received and processed by the load balancer module  132  on the server node  130 , so the health check ping, if successful, establishes that the module  132  on the server node  130  is healthy. Upon receiving the response to the ping, the load balancer node  110  considers the server node  130  as healthy, and increments a heartbeat counter for the server node  130 . 
     In at least some embodiments, the heartbeat counters for all server nodes  130  health checked by a given healthy load balancer node  110  may be propagated to the other load balancer nodes  110 , for example according to the gossip technique previously described for the load balancer node  110  heartbeat counters in which each node  110  sends its heartbeat list to at least one other randomly selected node  110  at a regular interval (the gossip interval), and the receiving node  110  updates its own heartbeat list according to the maximum values in the two lists. 
     Failure Detection and Gossip 
     In at least some embodiments, the information obtained through the load balancer node  110  health checks and the server node  130  health checks described above may need to be propagated to all the nodes  110  in the load balancer implementation so that all load balancer nodes  110  can maintain consistent view of the load balancer implementation. As described above, in at least some embodiments, the load balancer nodes  110  may communicate with each other according to a gossip protocol to exchange and propagate this health information and to detect load balancer node  110  and server node  130  failures. 
     In at least some embodiments, at a regular interval (referred to as the gossip interval), each load balancer node  110  randomly selects another load balancer node  110  and sends the other node  110  its view of healthy load balancer nodes  110  and server nodes  130  along with the heartbeat counters for the load balancer nodes  110  and server nodes  130 . As long as a load balancer node or server node  130  is healthy, the node will pass its health checks and its heartbeat counter will keep increasing. If the heartbeat counter for a node does not change for a specified interval (which may be referred to as a fail time interval), then the node is suspected to have failed by the load balancer nodes  110 . Once a node is suspected to have failed, the load balancer nodes  110  may wait for a specified interval (which may be referred to as the unhealthy time interval) before determining that the node is unhealthy. This unhealthy time interval allows the load balancer nodes  110  to wait until all the load balancer nodes  110  learn that the node has failed. 
       FIG. 16  graphically illustrates a state for, or view of, health of another node (either a load balancer node  110  or server node  130 ) that may be maintained by a load balancer node  110 , according to at least some embodiments. Assume the load balancer node  110  starts with a view of the node in question as being healthy, as indicated at  300 . This indicates that the heartbeat counter for the node has been incrementing. However, if the node&#39;s heartbeat counter does not increase for a specified interval (the fail time interval) as indicated at  302 , then the load balancer node  110  suspects that the node has failed, as indicated at  304 . If the node&#39;s heartbeat counter does not increase for a specified interval (the unhealthy time interval) as indicated at  306 , then the load balancer node  110  considers the node unhealthy, as indicated at  308 . However, if the heartbeat counter for the node increments before the unhealthy time interval expires as indicated at  310 , the load balancer node  110  again considers the node as healthy  300 . Similarly, receiving a heartbeat increment for an unhealthy node as indicated at  312  can cause the node to be considered as healthy  300 . 
     Determining that a node is unhealthy may involve different actions by the load balancer node(s)  110  depending on whether the unhealthy node is a load balancer node  110  or a server node  130 , and also depending on the load balancer node&#39;s  110  relationship with the unhealthy node, as described elsewhere herein. 
     Load Balancer Node Data 
     In at least some embodiments, each load balancer node  110  may maintain data about the state of the load balancer implementation. In at least some embodiments, this data may be maintained in one or more data structures on each load balancer node  110  including but not limited to a healthy load balancer node list, a suspect load balancer node list, and a heartbeat list.  FIG. 17  illustrates an example load balancer node  110  that maintains a healthy load balancer node list  320 , a suspect load balancer node list  322 , an unhealthy load balancer node list  324 , and a load balancer node heartbeat list  326 . 
     In at least some embodiments, each load balancer node  110  may maintain a healthy load balancer node list  320 , which is a list of healthy load balancer nodes  110  that may, for example, be used to determine which nodes  110  are healthy and are thus participating in the gossip protocol. Only the nodes  110  on the list  320  are involved in the propagation of load balancer information via the gossip protocol, only the nodes  110  on the list  320  are considered to be in the consistent hash ring, and only the nodes  110  on this list health-check server nodes  130 . A node  110  may randomly select another node  110  from this list  320  to which its heartbeat information is sent. In addition, heartbeat counters are exchanged only for the nodes  110  that are currently in the healthy load balancer node list  320 . In at least some embodiments, a load balancer node N can be added to the healthy load balancer node list  320  of another load balancer node  110  if node N passes a health check by the load balancer node  110  or if the load balancer node  110  receives a gossip message about node N from some other load balancer node  110  on the list  320 . 
     In at least some embodiments, each load balancer node  110  may maintain a suspect load balancer node list  322 , which is a list of load balancer nodes whose heartbeat counter (see heartbeat list  326 ) has not increased for a specified interval (referred to as the fail time interval). If a load balancer node E is in the suspect load balancer node list  322  of a load balancer node  110 , then the load balancer node  110  will not gossip about node E. If some other load balancer node  110  on the healthy list  320  gossips to the load balancer node  110  about node E with a higher heartbeat counter than the counter for node E in the node  110 &#39;s heartbeat list  326 , then node E will be moved from the suspect list  322  to the healthy list  320 . If the node E stays on the load balancer node  110 &#39;s suspect list  322  for a specified interval (referred to as the unhealthy time interval), node E is considered unhealthy by the load balancer node  110  and is moved onto an unhealthy node list  324 . A node  110  on the unhealthy node list  324  (in this example, node G) may be moved to the healthy node list  320  of a load balancer node  110  upon node G passing a health check by the node  110  or upon receiving an updated heartbeat counter for the node G from another node  110 . 
     In at least some embodiments, each load balancer node  110  may maintain a heartbeat list  326  for all known load balancer nodes  110 . For each node,  110 , this list  326  may include a heartbeat counter and a timestamp that indicates when the heartbeat counter last changed. 
     In at least some embodiments, each load balancer node  110  may also maintain a heartbeat list for all known server nodes, not shown in  FIG. 17 . This list may be similar to the load balancer node heartbeat list  326 . In some embodiments, the two lists may be combined. In at least some embodiments, the heartbeat information for the server nodes  130  may be propagated among the load balancer nodes  110 , for example according to a gossip protocol, along with or in addition to the heartbeat information for the load balancer nodes  110 . 
     While  FIG. 17  shows four separate lists, it is to be noted that two or more of the lists may be combined into a single list. For example, in some embodiments, a single list of all nodes  110  may be maintained on each load balancer node  110 , and bit flags or other data structures may be used to indicate whether each node is currently healthy, suspect, or unhealthy. 
     Server Node Data 
     In at least some embodiments, the server nodes  130  and local load balancer modules  132  on the nodes  130  do not participate in the gossip protocol with the load balancer nodes  110 . The load balancer nodes  110  gossip the heartbeat information about the other load balancer nodes  110  obtained by the load balancer node health check method and the heartbeat information about the server nodes  130  obtained by the server node health check method only among themselves (specifically, each load balancer node  110  gossips only to nodes currently on its healthy load balancer node list  320 ). 
     However, each server node  130 /load balancer module  132  may need information about healthy load balancer nodes  110  in the load balancer implementation so that the server node  130  can determine load balancer nodes  110  (specifically, egress nodes) to which the server node  130  can forward outgoing client traffic and determine which load balancer nodes to which connection publishing information is to be sent. In at least some embodiments, to provide this information to the server nodes  130 , the load balancer nodes  110  may periodically or aperiodically update the server nodes  130  with information identifying the currently healthy load balancer nodes  110  (e.g., healthy load balancer node list  320  in  FIG. 17 ). In at least some embodiments, the load balancer nodes  110  that are responsible for health checking a given server node  130  (see  FIG. 15 ) are responsible for providing the information identifying the currently healthy load balancer nodes to the server  130 . For example, referring to  FIG. 15 , load balancer node  110 A may send its healthy load balancer node list  320  to server nodes  130 A,  130 B,  130 C, and  130 D, load balancer node  110 B may send its healthy load balancer node list  320  to server nodes  130 C,  130 D, and  130 E, and so on. 
     Handling Load Balancer Node Failures 
       FIGS. 18A and 18B  illustrate handling a load balancer node failure, according to at least some embodiments.  FIG. 18A  shows an example load balancer implementation. There are four load balancer nodes  110 A through  110 D currently in the load balancer implementation. Edge router  104  routes incoming packets from clients (not shown) to the load balancer nodes  110 . In at least some embodiments, the edge router  104  may make the routing decisions according to a layer 4 per-flow hashed multipath routing technique, for example an equal-cost multipath (ECMP) routing technique. In at least some embodiments, edge router  104  learns about the load balancer nodes  110  that are currently available in the load balancer implementation to receive client traffic via load balancer node  110  advertisements, for example advertisements via Border Gateway Protocol (BGP) technology sessions initiated by the load balancer nodes  110 . However, in at least some embodiments, instead of a load balancer node  110  advertising itself to the edge router  104  via a BGP session, at least one other node  110  in the load balancer implementation takes responsibility for advertising the node  110  to the edge router  104  via BGP. For example, in some embodiments as shown in  FIG. 18A , the left and right neighbor nodes  110  of a given node  110  advertise the given node  110  to the edge router  104 . For example, load balancer node  110 A advertises nodes  110 B and  110 D, load balancer node  110 B advertises nodes  110 A and  110 C, and load balancer node  110 C advertises nodes  110 B and  110 D. 
     As shown in the example of  FIG. 18A , each load balancer node  110  also periodically health checks one or more other load balancer nodes  110 , for example one or more randomly selected nodes  110 , one or more neighbor nodes  110  as determined by an ordered list of load balancer nodes, or one or more neighbor nodes and one or more randomly selected nodes. In addition, each load balancer node  110  may periodically health check at least one server node  130  and also may send its list of healthy load balancer nodes  110  to the server node(s) it health checks. Health information for the load balancer nodes  110  and the server nodes  130  may be propagated among the nodes  110 , for example according to a gossip protocol. 
       FIG. 18B  illustrates handling the failure of a single load balancer node  110  in the example load balancer implementation of  FIG. 18A . In this example, load balancer node  110 B has failed for some reason. For example, nodes  110 A and  110 C may health check node  110 B, and both may detect that node  110 B is failing its health checks. Thus, nodes  110 A and  110 C do not increment the heartbeat counter for node  110 B. The heartbeat information from both nodes  110 A and  110 B is propagated to the other healthy load balancer nodes  110  (in this example, the only other load balancer node is node  110 D) according to the gossip protocol. As soon as all of the healthy load balancer nodes  110  (in this example, nodes  110 A,  110 C, and  110 D) converge on node  110 B&#39;s failure, one or more of, but not limited to, the following events may occur. Note that these events do not necessarily occur in this order.
         Nodes  110 A and  110 C stop advertising node  110 B to the edge router  104 . In at least some embodiments, this involves ending the BGP session that the node  110  established with the edge router  104  to advertise node  110 B. Note that each node  110  establishes a separate BGP session with the edge router  104  for each other node  110  that it advertises, so ending the BGP session for node  110 B does not affect other nodes  110  that are advertised. In at least some embodiments, a node  110  ends a BGP session with the edge router  104  by sending a TCP Close or similar message for the BGP session to the edge router  104 .   In response to detecting that node  110 B is no longer being advertised by any of the nodes, edge router  104  stops routing client data packets to node  110 B. The edge router  104  also adjusts the multipath (e.g., ECMP) hashing to redistribute packet flows from the clients to the remaining healthy load balancer nodes  110 , specifically to the ingress servers  112  on the nodes  110 . For any packet flow routed to an ingress server  112  for which the ingress server  112  does not have a client→server mapping, the mapping may be obtained from a flow tracker node for the client→server connection, or alternatively a new client→server connection may be established according to the technique as illustrated in  FIGS. 10A through 10G .   Nodes  110 A and  110 C may each opens a BGP session to edge router  104  to advertise each other. Note that, since both nodes  110 A and  110 C are advertised to edge router  104  by load balancer node  110 D as well as node  110 B, the fact that node  110 B may stop advertising nodes  110 A and  110 B to the edge router  104  when it fails does not cause edge router  104  to stop routing packets to these two nodes  110 .   In at least some embodiments, nodes  110 A and  110 C may take responsibility for health checking each other, since they are now neighbor nodes  110 . Note that node  110 B, even though considered unhealthy, may still be randomly health checked by one or more of the other nodes  110 .   One or more of the remaining healthy load balancer nodes  110  may assume responsibility for flow tracking connections formerly flow tracked by node  110 B. For example, node  110 C and/or node  110 D may take over as primary or secondary flow trackers as illustrated in  FIGS. 11C and 11D  for one or more connections for which node  110 B was a primary or secondary flow tracker.   One or more of the remaining healthy load balancer nodes  110  may assume responsibility for health checking the server nodes  130  previously health checked by node  110 B. The server nodes  130  are updated with the healthy load balancer node list (now not including node  110 B) by the remaining load balancer nodes  110 . For example, in  FIG. 18B , load balancer node  110 A begins health checking and updating server node  130 C, and load balancer node  110 C begins health checking and updating server node  130 B.   On the edge router  104 , the BGP sessions from the failed node  110 B eventually time out. Alternatively, the edge router  104  may terminate the BGP sessions upon recognizing that node  110 B has failed.       

     It is possible that two load balancer nodes  110  can fail at or at close to the same time. If the two failed load balancer nodes are not adjacent to each other, then the failures are independent and may be handled as separate single node  110  failures according to the method illustrated in  FIG. 18B . However, if the two failed nodes are adjacent to each other (e.g., nodes  110 B and  110 C in  FIG. 18A , then as soon as all of the healthy load balancer nodes  110  (in this example, nodes  110 A and  110 D) detect and converge on the failure, one or more of, but not limited to, the following events may occur. Note that these events do not necessarily occur in this order.
         Node  110 A ends the BGP session to edge router  104  for node  110 B.   Node  110 D ends the BGP session to edge router  104  for node  110 C.   Nodes  110 A and  110 D start BGP session with edge router  104  to advertise each other.   Nodes  110 A and  110 D may begin health checking each other. Note that nodes  110 A and  110 D may also continue to health check the failed nodes  110 .   The remaining healthy nodes  110  update the server nodes  130  with the healthy load balancer node lists.   Traffic may continue to flow from the edge router  104  to node  110 B and/or node  110 C since these two nodes  110  may continue to advertise each other to edge router  104 . However, these BGP sessions will eventually timeout, and the edge router  104  will redistribute the flows to the remaining advertised nodes  110  accordingly.   Nodes  110 B and  110 C may close their BGP sessions with edge router  104  on which they advertise nodes  110 A and  110 D, respectfully, if nodes  110 B and  110 C think they are still healthy.
 
Connection Publishing
       

     Referring again to  FIG. 1 , in at least some embodiments, the load balancer nodes  110  in a load balancer implementation maintain state information for client TCP connections to servers  130 . This state information allows the load balancer nodes  110  to route incoming client traffic from the edge router  104  to the server nodes  130  responsible for the TCP connections. The load balancer modules  132  on the server nodes  130  maintain lists of active TCP connections to their respective servers  134 . Connection publishing is a mechanism via which the load balancer modules  132  on the server nodes  130  may publish their lists of active client TCP connections to the load balancer nodes  110 . In at least some embodiments, the connection publishing packets are formed and published to the load balancer nodes  110  by the load modules  132  at a regular interval, which may be referred to as the connection publishing interval. 
     In at least some embodiments, the connection state information maintained by the load balancer nodes  110  may be viewed as a form of cache, and maintaining the state information for a particular connection may be viewed as maintaining a lease on the load balancer node  110  for that connection. Unless the cache entries are renewed, the load balancer nodes  110  may not be able to route client data flows to the server nodes  130  that are handling the data flows. The connection publishing mechanism periodically renews the caches, and thus the leases, on the load balancer nodes  110  with current connection state information from the server nodes  130  to thus keep the TCP packets flowing from the clients  160  to the appropriate server nodes  130 . When a client  160  ends a TCP connection to a server  134 , the load balancer module  132  on the server node  130  associated with that connection will drop the connection from its list of active connections and thus will no longer publish the TCP connection through the connection publishing mechanism. Thus, the connection state information for that connection (the cache entry or entries) on the load balancer nodes  110  associated with that connection (specifically, the ingress server  112  and the primary and secondary flow trackers  116  for the connection) is no longer renewed, and the connection is dropped by the load balancer nodes  110 . In at least some embodiments, the cache entry or entries for the connection may remain in the cache on a load balancer node  110  until the memory is required for some other active connection. 
     Thus, the connection publishing mechanism periodically or aperiodically extends the connection leases on the ingress servers  112  and the primary and secondary flow trackers  116  to keep the client traffic flowing. In addition, the connection publishing mechanism may help recover from at least some load balancer node  110  failures. When one or more load balancer nodes  110  holding state information for a client connection fails, the active connection information provided to the remaining load balancer nodes  110  by connection publishing may in some cases be used to recover the connection. 
     Using the connection publishing mechanism, the server nodes  130  are the authoritative sources for the states of the connections between the servers  134  and the clients  160 . In addition, closing of connections to the servers  134  is passively handled by the load balancer modules  132  on the server nodes  130  and the load balancer nodes  110 . Handshaking is not required between the server nodes  130  and the load balancer nodes  110 . In other words, the load balancer modules  132  do not have to send messages to the load balancer nodes  110  to actively inform the nodes that particular connections have been closed. When a server  134  closes a connection, the server  134  clears its internal state for the connection. The load balancer module  132  uses the server  134 &#39;s internal state to populate the connection publishing packet. Since the connection is no longer in the server  134 &#39;s internal state, the connection does not get published to the load balancer nodes  110 . The lease for the connection on the load balancer node  110  thus expires, and the load balancer nodes  110  passively forget about the connection. Memory in a load balancer node  110 &#39;s cache that was used for the connection can then be used for other connections as necessary. 
     In some embodiments, the leases for connections maintained by the load balancer nodes  110  may involve time-stamping entries for the connections in the cache. When a connection&#39;s lease is renewed by a connection publishing packet, the timestamp may be updated. If a connection&#39;s lease is not renewed because the connection is no longer being published by the load balancer module  132  on the server node  130 , then the timestamp is no longer updated. In at least some embodiments, a lazy garbage collection method may be used in which the entry for the connection may remain in the cache until the memory is needed. For example, in at least some embodiments, timestamps on cache entries may be compared to a lease renewal time threshold; if the timestamp for a cache entry is older than the threshold, then the entry is stale and may be reused. However, in some embodiments, stale entries may be actively garbage collected. 
     Connection Publishing Recipients 
     In at least some embodiments, for each client TCP connection, there are three load balancer nodes  110  that maintain a connection state—the node  110  serving as the ingress server  112 , the node  110  serving as the primary flow tracker  116  and the node serving as the secondary flow tracker  116 . For a given TCP flow, the primary and secondary flow trackers  116  can be determined, for example by a load balancer node  110 , by applying a consistent hash function to the TCP flow to find the primary flow tracker  116  node and its successor node in the consistent hash ring. The load balancer node  110  serving as the ingress server  112  for a TCP flow is the node  110  that receives traffic for that flow from the edge router  104  based on the edge router  104 &#39;s internal multipath (e.g., ECMP) hash function. If there is a node  110  failure or addition, the load balancer node  110  serving as the ingress server  112  may change for many of the active TCP flows; and the load balancer nodes  110  serving as flow trackers for at least some active TCP flows may change (see, e.g.,  FIGS. 11A through 11D ). For every TCP flow to the server  132  on a server node  130 , the load balancer module  132  on that server node  130  maintains state information indicating which of the load balancer nodes  110  is the ingress server  112  for that TCP flow, since it receives traffic from that load balancer node  110 . However, in at least some embodiments, the load balancer module  132  may not know and may not be able to determine which load balancer nodes  110  are serving as the primary and secondary flow trackers for a TCP flow, since the load balancer module  132  may not know the consistent hash function that is used. In other words, in at least some embodiments, the load balancer modules  132  do not do consistent hashing. 
     Publishing the Active Connection Information 
       FIGS. 19A and 19B  graphically illustrate a connection publishing technique, according to at least some embodiments.  FIG. 19A  illustrates load balancer (LB) modules publishing active connection information to the load balancer nodes. In at least some embodiments, each load balancer module  132  collects information for each active TCP flow on the server node  130  and forms a connection publishing packet. The information for a given TCP flow includes information identifying the load balancer node  110  serving as the ingress server  112  for the flow. When a connection publishing packet is ready (e.g., when the connection publishing interval has been reached), the load balancer module  132  randomly selects a load balancer node  110 , for example from the list of healthy load balancer nodes  110  that are periodically sent to the server nodes  130  from the load balancer nodes  110  that health-check the server nodes  130  as previously described. The load balancer module  132  then sends the connection publishing packet to the selected node  110 . For example, in  FIG. 19A , load balancer module  132 A has sent one connection publishing packet to load balancer node  110 A, and later sends another connection publishing packet to load balancer node  110 B. 
       FIG. 20  is a high-level flowchart of a connection publishing method that may be performed by each load balancer module  132 , according to at least some embodiments. As indicated at  500 , the load balancer (LB) module  132  creates a connection publishing entry for every active TCP flow on the respective server node  130 . In at least some embodiments, the load balancer module  132  retrieves the set of active TCP connections that the server  134  on the server node  130  handles, for example from /proc/net/tcp on the server node  130 . For every active TCP connection, the load balancer module  132  looks up (e.g., in a locally maintained table of active connections) the load balancer node  110  that is serving as the ingress server  112  for the TCP flow and creates a connection publishing entry that indicates the TCP tuple for the connection (e.g., a 4-tuple consisting of: the client IP address, client port, server (public) IP address, and server port) and the ingress server  112  for the connection. Note that each load balancer module  132  maintains information for each active TCP connection indicating the last load balancer node  110  from which a packet was received for the connection, and this information may be used by the load balancer module  132  to identify the ingress node  110  for each active connection. 
     As indicated at  502 , the load balancer module  132  randomly selects a load balancer node  110  to which the connection publishing packet (containing one or more connection publishing entries, with one entry for each active TCP connection) is to be sent. In at least some embodiments, the load balancer module  110  may be randomly selected when the load balancer module  132  determines that the connection publishing packet is ready to be sent. In at least some embodiments, this determination is made according to a connection publishing interval. As non-limiting examples, the connection publishing interval may be 100 milliseconds (ms), or one second. In at least some embodiments, the load balancer module  110  is selected from a list of healthy load balancer nodes  110  that has been previously received from one of the load balancer nodes  110 . As indicated at  504 , the load balancer module then publishes the connection publishing packet to the selected load balancer node  110 . In at least some embodiments, the connection publishing packet is a stateless packet, for example a UDP packet. In some embodiments, the connection publishing packet may be compressed prior to sending the packets to the target load balancer node  110 . In at least some embodiment, the connection publishing information may be sent to the target load balancer node  110  in two or more packets. 
     As indicated by the arrow returning from element  504  to element  500 , the load balancer module  132  may continuously build connection publishing packets, select random nodes  110 , and send the packets to the selected nodes. As noted above, this may be performed according to a connection publishing interval so that the load balancer nodes  110  are relatively regularly refreshed with current active connection information to maintain the connection leases on the load balancer nodes  110 . 
     In at least some embodiments, since the connection publishing packets are randomly distributed to the load balancer nodes  110  by the load balancer modules, the load balancer nodes  110  that receive the connection publishing packets are responsible for distributing the active connection information in the connection publishing packets to the correct ingress/primary/secondary nodes  110  for the connections.  FIG. 19B  and  FIGS. 21 and 22  illustrate methods for distributing the active connection information that may be used in at least some embodiments. 
       FIG. 19B  illustrates distributing the active connection information among the load balancer nodes  110 , according to at least some embodiments. When a load balancer node  110  receives a connection publishing packet from a load balancer module  132 , the load balancer node  110  may analyze the information for each TCP flow indicated therein to determine the ingress node and the primary and secondary flow tracker nodes for that flow. If the load balancer node  110  is serving in one of those roles for a flow, the load balancer node  110  consumes the information for the flow (e.g., by updating its cache of state information). In at least some embodiments, the load balancer node  110  may also put the information for the flow in packet(s) to be sent to the one or more other nodes  110  that are serving in the other roles for the flow. For the remaining flows indicated by the connection publishing packet, the load balancer node  110  splits the active connection information into two or more smaller packets and sends each packet to one or more other load balancer nodes  110 . For example, in at least some embodiments, a packet containing active connection information for one or more flows may be sent to the load balancer nodes  110  that are serving as the ingress server  112 , primary flow tracker  116 A, and secondary flow tracker  116 B for the flow(s). 
       FIG. 21  is a flowchart of a method for distributing the active connection information received in a connection publishing packet to target load balancer nodes  110 , according to at least some embodiments. As indicated at  520 , a load balancer node  110  receives a connection publishing packet from a load balancer module  132 . The load balancer module  132  generated the packet and selected the load balancer node  110  to receive the packet, for example as described above in reference to  FIGS. 19A and 20 . The connection publishing packet may include information identifying the server node  130  from which the packet was received (e.g., an IP address of the load balancer module  132  on the server node  130 ) and a list of entries identifying active TCP connections (e.g., a 4-tuple consisting of: the client IP address, client port, server (public) IP address, and server port for each connection). 
     In elements  522 - 530  of  FIG. 21 , the load balancer module  110  iteratively processes the active TCP connection information indicated in the received connection publishing packet. As indicated at  522 , the load balancer node  110  analyzes the entry for a next TCP flow in the packet to determine the ingress node  110  and the primary and secondary flow tracker nodes  110  for the respective TCP flow. In at least some embodiments, the load balancer node  110  gets the identity of the ingress node  110  from the connection publishing entry. In at least some embodiments, the primary and secondary flow tracker nodes  110  for the TCP flow may be determined according to the consistent hash function. At  524 , if the load balancer node  110  is serving in one of the roles for the TCP flow being examined, then at  526  the load balancer node  110  consumes the information for the flow, for example by updating its cache of state information. As indicated at  528 , the load balancer node  110  may add the connection publishing entry for the TCP flow to a packet being constructed that is to be sent to another load balancer node  110 . At  530 , if there are more connection publishing entries for flows in the connection publishing packet, then the method returns to  522  to process the next entry. Otherwise, the load balancer node sends the newly constructed packet(s) each containing a subset of the connection publishing entries from the original connection publishing packet to target load balancer nodes  110  for the packets, as indicated at  532 . In at least some embodiments, the packets sent to the target load balancer nodes  110  are stateless packet, for example UDP packets. In some embodiments, the packets may be compressed prior to sending the packets to the target load balancer nodes  110 . 
     Thus, in at least some embodiments, in elements  522 - 528  of  FIG. 21 , the flow tracker node  110  constructs one or more packets (e.g., UDP packets) each to be sent to a particular one of the other nodes  110  according to the information determined at  522  from the connection publishing entries in the received connection publishing packet. In at least some embodiments, a packet sent to another node  110  contains entries for TCP flows for which the target node  110  is serving as the ingress node  110 , primary flow tracker node  110 , or secondary flow tracker node  110 . Note that in some embodiments a given load balancer node  110  may serve as both the ingress and primary flow tracker node for a TCP flow, or as both the ingress and secondary flow tracker node for a TCP flow. 
       FIG. 22  illustrates an alternative method for distributing the active connection information received in a connection publishing packet to target load balancer nodes  110 , according to at least some embodiments. As indicated at  550 , a load balancer node  110  receives a connection publishing packet from a load balancer module  132 . In this method, as indicated at  552 , a process on the load balancer module  110  analyzes the connection publishing entries in the packet and splits the received packet into one or more smaller packets accordingly. The load balancer module  110  does not locally consume the flow information during this process. Once the connection publishing packet has been split into one or more packets, the packets are then processed as indicated at  554 - 560 . At  554 , if the target node  110  for the packet is this load balancer node  110 , then the load balancer node  110  locally consumes the packet as indicated at  556 . Otherwise, the packet is sent to the target load balancer node  110 . At  560 , if there are more packets to be processed, then the method returns to  554 . Otherwise, the method is done. 
     Thus, the load balancer node  110  that receives a connection publishing packet from a load balancer module  132  may split the connection publishing packet into two or more smaller packets that are specific to particular ones of the other load balancer nodes  110  and distribute the packets accordingly, while internally consuming flow information for any TCP flows currently being handled by the load balancer node  110 . In the meantime, other load balancer nodes  110  may also be receiving connection publishing packets from the load balancer modules  132 , splitting the connection publishing entries in multiple smaller packets, and sending the smaller packets to target nodes  110  to thus distribute the active connection information among the nodes  110 . 
     Connection Publishing Triggers 
     In at least some embodiments, a connection publishing may be triggered on a load balancer module  132  by one or more different events. As previously noted, in some embodiments, a connection publishing packet may be generated and sent to a randomly selected load balancer node  110  according to a connection publishing interval, for example at 100 ms or one second intervals, to renew the leases for the TCP connections on the load balancer nodes  110 . In some embodiments, a change in membership of the load balancer nodes  110  may trigger an immediate connection publishing event. In at least some embodiments, the load balancer module  132  may learn about the change from the list of healthy load balancer nodes  110  sent from one of the load balancer nodes  110  that health checks the respective server node  130 . Upon detecting the change according to the list (either a deletion or an addition), the load balancer module  132  may generate a connection publishing packet and send to a load balancer node  110  so that TCP connections affected by the change may be more quickly recovered by the load balancer nodes  110 . 
     Preventing Packet Loops 
     Connection publishing packet loops may occur if the load balancer layer membership changes while processing a connection publishing packet. A first node  110  may receive a connection publishing packet from a load balancer module  132  and send a smaller packet to a second node  110 . However, if the membership has changed, the second node  110  may determine that the packet should go to the first node  110 , and may thus forward the packet to the first node  110 . In at least some embodiments, to prevent this loop from happening, different port numbers may be used for connection publishing packets received from load balancer modules  132  and those received from load balancer nodes  110 , and the load balancer nodes  110  do not redistribute connection publishing packets received from other load balancer nodes  110 . 
     Connection Publishing Packet Distribution Alternatives 
     In the connection publishing methods described above, the load balancer module  132  randomly selects a load balancer node  110  to which a connection publishing packet is sent. However, in some embodiments, other methods may be used to select a load balancer node  110 . For example, in some embodiments, the load balancer node  132  may construct one or more connection publishing packets that are each targeted to a particular ingress node  110  that handles one or more of the active TCP flows, and sent the packet(s) to the target ingress node(s)  110 . The ingress node(s)  110  would then redistribute the active connection information to the primary and secondary flow trackers for the connections. As another example, in some embodiments, instead of sending the connection publishing packet to a single, randomly selected node  110 , each connection publishing packet may be sent by the load balancer module  132  to two or more of the healthy nodes  110 , or to all of the healthy nodes  110 . 
     Load Balancer Node Architecture 
       FIG. 23  illustrates example software stack architecture for a load balancer node  110  according to at least some embodiments, and is not intended to be limiting. In this example software stack architecture, the load balancer node  110  runs within a single Java™ technology process  1102  that uses Java Native Interface (JNI™)  1104  technology to manage a layer of native code that may include load balancer server native code  1106  and core packet processing code  1108 , for example Intel™ Dataplane Development Kit (DPDK) technology code. The native code may interface to two network interface controllers (NICs  1114 A and  1114 B). A first NIC (NIC  1114 A) may face “north”; that is, towards the edge router  104 . A second NIC (NIC  1114 B) may face “south”; that is, towards the server nodes  130 . In at least some embodiment, NICs  1114 A and  1114 B may not maintain TCP stacks. Thus, at least some embodiments may include a third NIC  1114 C that does support TCP connections so that the load balancer node  110  can communicate with processes via a control plane, and vice versa. Alternatively, in some embodiments, only the first, north-facing NIC  1114 A and the second, south-facing NIC  111 B may be implemented in the load balancer node  110 , and the second, south-facing NIC  1114 B may implement a TCP stack via which the load balancer node  110  may communicate with processes via the control plane. Load balancer node  110  also includes operating system (OS) technology software  1112 , e.g. a Linux™ kernel, and a Java Virtual Machine (JVM™) technology software  1110  layer on top of OS technology software  1112  and JNI  1104  technology. 
     In at least some embodiments, the load balancer nodes  110  in the distributed load balancing system may each need to concurrently process many data flows at high packet rates. In at least some embodiments, to achieve the required level of throughput, the load balancer nodes  110  may leverage Intel™ Dataplane Development Kit (DPDK) technology for high performance packet processing. DPDK technology permits a userspace program to read/write packets directly to and from a network interface controller (NIC) and bypasses the many layers of the Linux kernel networking stack (except for the Linus ixgbe base NIC driver). The DPDK approach to packet processing rejects interrupt handler-based input in favor of dedicated CPU cores that directly poll the NIC hardware in a busy loop. This approach may allow for much higher packet rates, at the expense of increasing thermal output by continuously running dedicated CPU cores in a busy loop. DPDK technology may also provide tools for packet processing including CPU core management, lock-free queues, memory pools, and synchronization primitives. As shown in  FIG. 24 , in DPDK technology, a dedicated CPU core  600  may be used for each particular task, and work is passed from one CPU core  600 A to another CPU core  600 B using non-blocking queues  602 . 
     The DPDK queues  602  may be implemented using fast power-of-two ring buffers, and may support single and multiple producer/consumer variants. The multiple producer/consumer variants are not truly lock-free, since they do contain a compare-and-swap (CAS) loop to synchronize access. All packet buffer memory may be pre-allocated in memory pools, so that only pointers to the buffers are read and written to the queues  602 . The memory pools may be implemented as queues, may be optimized to distribute memory across memory channel and rank, and may support non-uniform memory access (NUMA) optimized allocation. In at least some embodiments, the packet buffers may use a method such as an Mbuf paradigm that over-allocates enough headroom and tailroom in each packet buffer to support encapsulate/decapsulate operations that may add/remove outer network layer headers without requiring buffer copies. 
     In at least some embodiments of the load balancer nodes  110 , a core packet processing architecture may be implemented that leverages DPDK technology. Each load balancer node  110  may include at least one multicore packet processor implemented according to the core packet processing architecture. The core packet processing architecture may use a single producer/single consumer paradigm for packet flow through the queues and cores of the multicore packet processor. In this paradigm, each queue inputs to one and only one core, and each core outputs to one and only one core for each other core that it feeds packets to. In addition, memory used by the cores in the multicore packet processor is not shared; each core has its own, separate memory region. Thus, there is no memory or queue sharing between cores, no memory or queue contention, and no need for memory or queue sharing mechanisms such as request for ownership (RFO) or compare-and-swap (CAS).  FIGS. 25 and 26  illustrate example multicore packet processors implemented according to the core packet processing architecture. 
       FIG. 25  illustrates an example multicore packet processor implemented according to the core packet processing architecture that leverages DPDK technology for processing data flows, according to at least some embodiments. The core packet processing architecture may be implemented as a multicore packet processor according to a single producer/single consumer paradigm. In at least some embodiments, as illustrated in  FIG. 23 , the load balancer nodes  110  each have two network interface controllers (NICs)—a north-facing NIC  1114 A that faces the border network/edge router  104  and a south-facing NIC  1114 B that faces the production network/server nodes  130 . In at least some embodiments, the NICs  1114  may be 10 Gpbs NICs. The majority of packets flowing through a load balancer node  110  are received on one of these two NICs (either NIC  1114 A or  1114 B), processed (e.g., encapsulated or decapsulated), and transmitted out the other NIC (either NIC  1114 B or  1114 A). 
     Referring to  FIG. 25 , in at least some embodiments a load balancer node  110  spins up two CPU cores, a receive (RX) core  610  and a transmit (TX) core  630 , for each NIC  1114 . The load balancer node  110  also spins up a number of worker cores  620  that process packets for both NICs  1114  in both directions; in this example four worker cores  620 A through  620 D are used. The receive cores  610  read batches of incoming packets from their input queues as they arrive on the NIC  1114  and distribute the packets to the worker cores  620  that perform the bulk of the work for each packet, with each receive core  610  feeding packets into a respective worker input queue  612  for each worker core  620 . In at least some embodiment, a receive core  610  may perform a layer 4 “flow-hash” technique on each incoming packet (similar to the per-flow hashed multipath routing technique that may be used by the edge router  104  as previously described) to distribute the packets to the worker cores  620  while ensuring that any particular client connection (distinguished by its IP address and port) will be processed by the same worker core  620 . This means that each worker core  620  may always see the same subset of the packets, and eliminates contention on state data managed by the worker core  620  so that no locks are required. The pointers to the received packets may be distributed across the worker queues  622  that the worker cores  620  continuously monitor for new input. The worker cores  620  are responsible for managing the state (e.g. the assigned server node  130 ) for each connection, and may perform UDP encapsulation or decapsulation on the packet before forwarding the packet to one of their outbound queues  632 . The transmit cores  630  cycle through the worker core  620  outbound queues  632  and write the output packets to their corresponding NIC  1114  as they appear on the queues  632 . 
       FIG. 26  illustrates another example multicore packet processor implemented according to the core packet processing architecture that leverages DPDK technology for processing data flows, according to at least some embodiments. The core packet processing architecture may be implemented as a multicore packet processor according to a single producer/single consumer paradigm. In at least some embodiments, in addition to processing the high-throughput client TCP flows, the DPDK core architecture on a load balancer node  110  may also be used to send and receive packets on the north- and south-facing NICs  1114  for other protocols such as ARP, DHCP, and BGP. In the embodiment shown in  FIG. 26 , a worker core  620 A is dedicated to handling the packets for these other protocols. This worker core  620 A may be referred to as a “slow” worker core, since the processing of these packets generally happens at a slower rate than the client TCP flows, while the other worker cores  620 B- 620 D that process only the client TCP flows may be referred to as fast worker cores. The receive cores  610 A and  610 B handling incoming packets on the north-facing and south-facing NICs  1114 , respectively, may identify packets that are to be handled by the slow worker core  620 A and direct the packets to input queues  622  for the slow worker core  620 A. The slow worker core  620 A may also monitor an input queue  622  for packets generated by Java/JNI, and an output queue  634  for output packets to Java/JNI. The slow worker core  620 A also outputs to an input queue  622  for each of the fast worker cores  620 B through  620 D so that the slow worker core  620 A can send packets to each of the fast worker cores  620 B through  620 D, for example connection publishing packets. The slow worker core  620 A also has an outbound queue  632  feeding into each of transmit cores  630 A and  630 B. 
     In at least some embodiments, the third input queue  622  of each fast worker core  620 B through  620 D is an output queue from the slow worker core  620 A. In at least some embodiments, this third input queue  622  may, for example, be used for receiving and processing connection publishing packets, each containing connection state information, by the fast worker queues  620 B through  620 D. For at least some of these connection publishing packets, there may be no output to the transmit cores  630 . Instead, the connection state information in the packets may be consumed by the fast worker core  620 , for example by updating the stored state for one or more packet flows that the respective fast worker core  620  maintains. Thus, the output queues from the slow worker core  620 A that input to the fast worker cores  620 B through  620 D may provide a path other than an input queue  622  directly from a receive core  610  for updating the fast worker cores&#39; stored states. 
     In at least some embodiments, the multicore packet processors of  FIGS. 25 and 26  may filter incoming packets and only process and output packets that are valid. For example, in at least some embodiments, the receive cores  610  may filter out packets that are of a protocol not supported by any of the worker cores  620  and thus not send the packets to the worker cores  620 . In at least some embodiments, the worker cores  620 , when processing packets, may each first analyze the packets read from their respective worker input queues  622  to determine if the packets are to be accepted for further processing and output to the transmit cores  630 , and may only complete the processing and output of packets to the transmit cores  630  that are accepted; the non-accepted packets may be discarded. For example, the worker cores  620  may look at the address information for each packet and only accept packets that are targeted at valid addresses that are being load-balanced, discarding any other packets. 
     Handling Border Gateway Protocol (BGP) Data 
     In at least some embodiments, packet flows associated with a BGP client in and out of the core architecture may be handled as follows. Since the NICs  1114 A and  1114 B are not bound to the Linux kernel, the TCP connection to the edge router  104  is intercepted by core architecture as illustrated in  FIG. 26  and processed by the slow worker core  622 A, which passes the BGP packets up into Java space via output queue  634 . These TCP packets are further processed by one or more modules on the load balancer node  110  before being delivered to the BGP client, including processing by the Linux kernel to manage the TCP connection and effectively translate the packets into a TCP stream. This design allows the BGP client to be written using standard Java TCP socket libraries. 
       FIG. 27  illustrates processing of incoming BGP TCP packets by a load balancer (LB) node process  650 , according to at least some embodiments. A packet from the edge router  104  arrives at the north-facing NIC  640  and goes into input queue  640  for the receive core  652 . The receive core  652  reads the packet from the queue  640 , identified the packet as a BGP packet, and places the packet on an input queue  654  for the slow worker core  656 . The slow worker core  656  validates the packet and places it on the JNI output queue  658 . JNI packet receiver  660  reads the packet from the queue  658  via JNI, mangles the source/destination addresses, and writes the packet to a raw socket  644 . The Linux kernel  646  receives the raw packet, handles it according to the TCP protocol, and appends the payload data to the TCP socket InputStream. The data from the packet is then delivered to the Java TCP socket in the BGP client  662 . 
       FIG. 28  illustrates processing of outgoing BGP TCP packets by a load balancer (LB) node process  650 , according to at least some embodiments. The BGP client  662  writes data to a Java TCP socket of Linux kernel  646 . The Linux kernel  646  handles the data according to the TCP protocol and converts the data into TCP packet(s). In at least some embodiments, the TCP packet(s) match a 127.x.x.x iptables rule. The TCP packet(s) are placed on an output queue  648 , for example a Netfilter LOCAL_OUT queue. A Java thread of JNI packet receiver  670  monitoring the queue  648  via JNI receives the TCP packet(s) and marks each NF_STOLEN to make the kernel  646  forget about them. The Java thread mangles the source/destination addresses and adds the packets(s) to a JNI input queue  672  for slow worker core  656  via JNI. The slow worker core  656  receives the TCP packet(s) from its JNI input queue  672  and places the packets on the outbound queue  664  for the north-facing NIC  640  transmit core  666 . Transmit core  666  reads the TCP packets(s) from its input queue  664  and writes them to the north-facing NIC  640 . The TCP packets are sent by NIC  640  to the edge router  104 . 
     Distributed Load Balancer Simulation and Testing 
     The load balancer described herein is a distributed system that requires the interaction of many independent components (e.g., routers, load balancer nodes, load balancer modules, etc.). To perform testing of the distributed components, logic, and protocols, as well as to simulate scenarios such as node failures, message drops, and delays, embodiments of a test system are described that enable the distributed load balancer to be run in a single process where the interactions can be tested without requiring the code to be deployed to multiple hosts in a complex network topology (e.g., a production network). To accomplish this, a software mechanism referred to as a message bus is described that enables multiple load balancer components to be configured and executed in or as a single process; the single process may be executed on a single host system. The message bus mechanism allows the distributed load balancer system to be tested as a single process, for example on a single host system, while to the load balancer components (e.g., the load balancer nodes and load balancer modules) it appears that they are running on an actual production network. 
     The message bus provides a framework that allows the distributed load balancer to run as a single process. Each of one or more message bus layers in the process simulates a network (e.g., Ethernet) segment between components of the distributed load balancer. The software components of the distributed load balancer system do not have to be written in a special fashion to allow the components to operate within the message bus environment. Instead, the message bus framework provides a component (which may be referred to as a message bus NIC or packet adapter) that intercepts the packets the components of the distributed load balancer system produce, directs the packets into the simulated network provided by a message bus layer instead of into a real physical network, and delivers the packets to the target components. The message bus layers do not implement TCP/IP stack(s) for communications between the components. Instead, the message bus layers interface with the host system&#39;s operating system (OS) and use the host system&#39;s TCP/IP stack. The message bus layers leverage the TCP/IP stack provided by the OS to convert the TCP streams that the clients and servers expect to and from the individual packets that the message bus intercepts and delivers. 
     In at least some embodiments, to interface with the message bus, load balancer components may be provided with at least one message bus network interface controller (NIC), each with a valid media access control (MAC) address, which sends packets to and receives packets from the message bus simulated network environment instead of to and from a physical network. A message bus NIC is a virtual network interface controller that attaches to the message bus instead of to a physical network. Each load balancer component that needs to communicate through the message bus requires a least one message bus NIC. A message bus NIC serves as a pipeline exit to the message bus and as a pipeline entrance to the component. Components can instantiate multiple message bus network interfaces to each message bus NIC. 
     A message bus network interface is a mechanism for components to attach to a message bus via a message bus NIC. A message bus network interface may be synonymous to an interface configuration (ifconfig) interface in Linux technology, with a difference being that the message bus network interface attaches to the message bus instead of to a physical network. A message bus network interface has an IP address, and sits on top of a message bus NIC. The message bus network interface exposes a packet source interface, which can be used by the component to receive packets from the message bus, and a packet sink interface that can be used by the component to send packets into the message bus. 
     Each load balancer node processes individual network packets that are delivered and sent through an implementation of the packet source and packet sink interfaces. When running in the message bus environment, these interfaces are implemented by the message bus network interface that adds or removes the layer 2 Ethernet headers (for the load balancer nodes that expect this to be performed by the kernel network stack). In a production environment as shown in  FIG. 29 , the implementation of the packet source and packet sink interfaces receive and transmit packets on an actual network interface. In a message bus environment as shown in  FIG. 30 , the implementation of the packet source and packet sink interfaces receive packets from and transmit packets onto a message bus layer or layers. 
     For the sake of simplicity, a message bus NIC and message bus interface may collectively be referred to as a message bus packet adapter, or simply packet adapter. See, e.g.,  FIGS. 31 and 32 . 
       FIG. 29  illustrates a load balancing system that includes a distributed load balancer  700  in a production environment, according to at least some embodiments. The load balancer  700  has been simplified for this description. The load balancer  700  may connect to clients  742  on an external network  740  via a border router  702  of a network installation such as a data center that implements the load balancer  700 . The load balancer  700  includes several types of components—at least one edge router  704 , two or more load balancer (LB) nodes  710 , two or more load balancer (LB) modules  732  each implemented on a separate server node (not shown), one or more networking components that form fabric  720  such as routers or switches, and in at least some embodiments a configuration service  722 . In at least some embodiments, each component of the load balancer  700  may be implemented as or on a separate computing device, such as a commodity rack-mounted computing device. 
       FIG. 30  illustrates a distributed load balancer test system  800  that incorporates a message bus mechanism that enables multiple distributed load balancing system components to be configured and executed in or as a single process, according to at least some embodiments. In the load balancer  700  shown in  FIG. 29 , each load balancer software component is installed and executed on a separate computing device (e.g., the load balancer software on the load balancer nodes  710 , and the load balancer modules  732  on the server nodes). To enable these load balancer software components to execute in a single process, each load balancer software component (shown as load balancer (LB) nodes  810  and load balancer (LB) modules  832  in  FIG. 30 ) may include code that abstracts the network connectivity of the components so that the packets in and out of the load balancer software component can also be intercepted and routed through the message bus mechanism instead of being sent and received on a physical network. 
     In at least some embodiments, in the distributed load balancer test system  800 , the message bus mechanism does not implement TCP stack(s) for communications between the components. Instead, the message bus mechanism interfaces with the host system&#39;s operating system 
     (OS) and uses the host system&#39;s TCP stack. In at least some embodiments, the message bus functionality ties in to the kernel (e.g., the Linux kernel) of the host system&#39;s OS below the user layer via IP tables, a functionality of the kernel. The message bus functionality hooks into the IP tables at the kernel level, intercepts packets, and sends the packets up into the message bus process for routing. 
     As shown by simulated edge router  862  and simulated fabric  864  in  FIG. 30 , the functionality of the physical network components (e.g., the edge router  704  and fabric  720  in  FIG. 29 ) may be simulated in software, as can clients  860 , servers  834 , and configuration service  866 . Note, however, that in at least some embodiments actual rather than simulated servers  834  may be used in the distributed load balancer test systems  800 . The message bus layers  850  in  FIG. 30  replace the physical network infrastructure. Thus, the load balancer software components (load balancer nodes  810  and load balancer modules  832 ) may be run in the load balancer test system  800  while unaware that they are not executing in a production network environment as shown in  FIG. 29 . 
     Some components (for example, simulated routers) may be connected to more than one message bus layer  850  in order to pass packets to and receive packets from different message bus layers  850  that simulate network segments. 
     The message bus mechanism implemented in the message bus layers  850  of the distributed load balancing test system  800  simulates the “wire” of a network segment. In at least some embodiments, the message bus mechanism delivers packets to destination components in the distributed load balancing test system  800  based on the components&#39; MAC addresses. Thus, each load balancer software component (load balancer nodes  810  and load balancer modules  832 ) provides a MAC address to the message bus layer(s)  850  to which it is connected so that the load balancer software component can receive packets that are sent to it from other components in the distributed load balancing test system  800 . 
     Message Bus Packet Adapters 
       FIGS. 31 and 32  illustrate message bus packet adapters, according to at least some embodiments. In at least some embodiments, each load balancer (LB) software component processes individual network packets that are delivered and sent through an implementation of the PacketSource and PacketSink interfaces. Referring to  FIG. 31 , when running in the distributed load balancing test system  800 , these interfaces (shown as packet source interface  862  and packet sink interface  864 ) may be implemented by a packet adapter  860  between the message bus layer  850  and the load balancer software component  870  that adds or removes the layer 2 Ethernet headers for the load balancer software components  870  that expect this to be performed by the kernel network stack. In the production environment as illustrated in  FIG. 29 , the implementation of PacketSource and PacketSink for the load balancer software components receives and transmits the packets on actual network interfaces of the physical devices on which the components are implemented. 
     Referring to  FIG. 31 , in at least some embodiments, when a load balancer software component  870  transmits a packet, the thread of execution that calls a send packet method of packet sink interface  864  traverses a chain of functions within the packet adapter  860  and also within the message bus layer  850  to eventually deliver the packet to the destination component by adding the packet to that component&#39;s input queue. In at least some embodiments, when a load balancer software component  870  receives a packet, the load balancer software component  870  calls a receive packet method of the packet source interface  862  and reads packets from its input queue. In at least some embodiments, the message bus mechanism does not require any additional threads of its own to deliver packets. 
     Message Bus Packet Pipelines 
     Referring to  FIG. 32 , in at least some embodiments, the message bus  850  side of the packet source interface  862  and packet sink interface  864  provides a packet pipeline feature. When a load balancer software component  870  sends a packet via packet sink interface  864 , the packet data may traverse a series of stages (packet pipeline  880 ) before reaching the message bus layer  850 . These stages may modify the packet, drop the packet, duplicate the packet, delay the packet, etc. Once a packet traverses packet pipeline  880  and the message bus layer  850  selects a destination component  870 , a second series of pipeline stages (packet pipeline  882 ) associated with the destination component  870  may also be traversed before the packet is added to the destination component  870 &#39;s input queue. 
     Example Provider Network Environments 
     This section describes example provider network environments in which embodiments of the distributed load balancing methods and apparatus may be implemented. However, these example provider network environments are not intended to be limiting. 
       FIG. 33A  illustrates an example provider network environment, according to at least some embodiments. A provider network  1900  may provide resource virtualization to clients via one or more virtualization services  1910  that allow clients to access, purchase, rent, or otherwise obtain instances  1912  of virtualized resources, including but not limited to computation and storage resources, implemented on devices within the provider network or networks in one or more data centers. Private IP addresses  1916  may be associated with the resource instances  1912 ; the private IP addresses are the internal network addresses of the resource instances  1912  on the provider network  1900 . In some embodiments, the provider network  1900  may also provide public IP addresses  1914  and/or public IP address ranges (e.g., Internet Protocol version 4 (IPv4) or Internet Protocol version 6 (IPv6) addresses) that clients may obtain from the provider  1900 . 
     Conventionally, the provider network  1900 , via the virtualization services  1910 , may allow a client of the service provider (e.g., a client that operates client network  1950 A) to dynamically associate at least some public IP addresses  1914  assigned or allocated to the client with particular resource instances  1912  assigned to the client. The provider network  1900  may also allow the client to remap a public IP address  1914 , previously mapped to one virtualized computing resource instance  1912  allocated to the client, to another virtualized computing resource instance  1912  that is also allocated to the client. Using the virtualized computing resource instances  1912  and public IP addresses  1914  provided by the service provider, a client of the service provider such as the operator of client network  1950 A may, for example, implement client-specific applications and present the client&#39;s applications on an intermediate network  1940 , such as the Internet. Other network entities  1920  on the intermediate network  1940  may then generate traffic to a destination public IP address  1914  published by the client network  1950 A; the traffic is routed to the service provider data center, and at the data center is routed, via a network substrate, to the private IP address  1916  of the virtualized computing resource instance  1912  currently mapped to the destination public IP address  1914 . Similarly, response traffic from the virtualized computing resource instance  1912  may be routed via the network substrate back onto the intermediate network  1940  to the source entity  1920 . 
     Private IP addresses, as used herein, refer to the internal network addresses of resource instances in a provider network. Private IP addresses are only routable within the provider network. Network traffic originating outside the provider network is not directly routed to private IP addresses; instead, the traffic uses public IP addresses that are mapped to the resource instances. The provider network may include network devices or appliances that provide network address translation (NAT) or similar functionality to perform the mapping from public IP addresses to private IP addresses and vice versa. 
     Public IP addresses, as used herein, are Internet routable network addresses that are assigned to resource instances, either by the service provider or by the client. Traffic routed to a public IP address is translated, for example via 1:1 network address translation (NAT), and forwarded to the respective private IP address of a resource instance. 
     Some public IP addresses may be assigned by the provider network infrastructure to particular resource instances; these public IP addresses may be referred to as standard public IP addresses, or simply standard IP addresses. In at least some embodiments, the mapping of a standard IP address to a private IP address of a resource instance is the default launch configuration for all a resource instance types. 
     At least some public IP addresses may be allocated to or obtained by clients of the provider network  1900 ; a client may then assign their allocated public IP addresses to particular resource instances allocated to the client. These public IP addresses may be referred to as client public IP addresses, or simply client IP addresses. Instead of being assigned by the provider network  1900  to resource instances as in the case of standard IP addresses, client IP addresses may be assigned to resource instances by the clients, for example via an API provided by the service provider. Unlike standard IP addresses, client IP addresses are allocated to client accounts and can be remapped to other resource instances by the respective clients as necessary or desired. A client IP address is associated with a client&#39;s account, not a particular resource instance, and the client controls that IP address until the client chooses to release it. Unlike conventional static IP addresses, client IP addresses allow the client to mask resource instance or availability zone failures by remapping the client&#39;s public IP addresses to any resource instance associated with the client&#39;s account. The client IP addresses, for example, enable a client to engineer around problems with the client&#39;s resource instances or software by remapping client IP addresses to replacement resource instances. 
       FIG. 33B  illustrates a distributed load balancer implementation in an example provider network environment as shown in  FIG. 33A , according to at least some embodiments. A provider network  1900  may provide a service  1910  to clients  1960 , for example a virtualized storage service. The clients  1960  may access the service  1910 , for example via one or more APIs to the service  1910 , to obtain usage of resources (e.g., storage resources or computation resources) implemented on multiple server nodes  1990  in a production network portion of the provider network  1900 . Server nodes  1990  may each implement a server (not shown), for example a web server or application server, as well as a local load balancer (LB) module  1992 . One or more distributed load balancers  1980  may be implemented in a load balancer layer between the border network and the production network. Border router(s)  1970  may receive packets (e.g., TCP packets) in packet flows from clients  1960  via an intermediate network  1940  such as the Internet, and forward the packets to the edge router(s) of the distributed load balancer(s)  1980  via the border network. The packets may be targeted at the public IP address(es) published by the edge router(s) of the distributed load balancer(s)  1980 . The edge router of each distributed load balancer  1980  may distribute the packet flows among load balancer nodes of the respective distributed load balancer  1980 . In at least some embodiments, each load balancer node that serves as an ingress node advertises the same public IP address to the edge router, and the edge router distributes the packet flows from the clients  1960  among the ingress servers according to a per-flow hashed multipath routing technique, for example an equal-cost multipath (ECMP) hashing technique. The load balancer nodes may use the connection protocol described herein to determine target server nodes  1990  for the packet flows and to facilitate connections between the servers and the clients  1960 . Once a connection is established, the ingress nodes encapsulate and send packets received for the flows to the target server nodes  1990  on the production network, while the flow tracker nodes maintain state for the connections. The load balancer modules  1992  on the server nodes  1990  may make the decisions as to whether the respective servers on the server nodes  1960  accept connections. The load balancer modules receive and decapsulate the packets from the ingress nodes, and send the decapsulated packets (e.g., TCP packets) to the respective servers on the server nodes  1990 . The load balancer modules  1992  may also select load balancer nodes as egress nodes for the packet flows, and encapsulate and send outgoing packets for the flows to the selected egress nodes via the production network. The egress nodes in turn decapsulate the packets and send the decapsulated packets onto the border network for delivery to the respective clients  1960 . 
       FIG. 34A  illustrates an example physical rack implementation of the distributed load balancer and server nodes according to at least some embodiments, and is not intended to be limiting. In at least some embodiments, various components of the distributed load balancer may be implemented on or as commodity rack-mounted computing devices. Rack  190  may include multiple computing devices each serving as a load balancer node (LB nodes  110 A- 110 F), and multiple computing devices each serving as a server node (server nodes  130 A- 130 L). Rack  190  may also include at least one edge router  104 , one or more rack-mounted networking devices (routers, switches, etc.) that form fabric  120 , and one or more other components  180  (other networking devices, patch panels, power supplies, cooling systems, busses, etc.). A network  100  installation such as a data center or centers that implement provider network  1900  of  FIGS. 33A and 33B  may include one or more racks  190 . 
       FIG. 34B  illustrates another example physical rack implementation of the distributed load balancer and server nodes according to at least some embodiments, and is not intended to be limiting.  FIG. 34B  shows the LB nodes  110  and server nodes  130  implemented as slot-mounted computing devices, for example blade servers, in rack  190 . 
       FIG. 35  illustrates an example networking environment in which one, two or more distributed load balancers may be implemented in a network, with the server nodes separately implemented, according to at least some embodiments. In this example, two distributed load balancers  1980 A and  1980 B are shown. The distributed load balancers  1980  each may receive packet flows from clients  1960  via the border network and perform the load balancing methods described herein to distribute the packet flows across multiple server nodes  1990 . In some implementations, each distributed load balancer  1980  may be a rack implementation similar to the racks  190  shown in  FIGS. 34A and 34B , but without the server nodes installed in the load balancer racks. The server nodes  1990  may be rack-mounted computing devices such as Blade servers installed in one or more separate racks within the data center. In some implementations, the server nodes  1990  may implement two or more different services provided by the provider network, with each service fronted by a different one or more of the load balancers  1980 . 
     Illustrative System 
     In at least some embodiments, a server that implements a portion or all of the distributed load balancing methods and apparatus as described herein may include a general-purpose computer system that includes or is configured to access one or more computer-accessible media, such as computer system  2000  illustrated in  FIG. 36 . In the illustrated embodiment, computer system  2000  includes one or more processors  2010  coupled to a system memory  2020  via an input/output (I/O) interface  2030 . Computer system  2000  further includes a network interface  2040  coupled to I/O interface  2030 . 
     In various embodiments, computer system  2000  may be a uniprocessor system including one processor  2010 , or a multiprocessor system including several processors  2010  (e.g., two, four, eight, or another suitable number). Processors  2010  may be any suitable processors capable of executing instructions. For example, in various embodiments, processors  2010  may be general-purpose or embedded processors implementing any of a variety of instruction set architectures (ISAs), such as the x86, PowerPC, SPARC, or MIPS ISAs, or any other suitable ISA. In multiprocessor systems, each of processors  2010  may commonly, but not necessarily, implement the same ISA. 
     System memory  2020  may be configured to store instructions and data accessible by processor(s)  2010 . In various embodiments, system memory  2020  may be implemented using any suitable memory technology, such as static random access memory (SRAM), synchronous dynamic RAM (SDRAM), nonvolatile/Flash-type memory, or any other type of memory. In the illustrated embodiment, program instructions and data implementing one or more desired functions, such as those methods, techniques, and data described above for the distributed load balancing methods and apparatus, are shown stored within system memory  2020  as code  2024  and data  2026 . 
     In one embodiment, I/O interface  2030  may be configured to coordinate I/O traffic between processor  2010 , system memory  2020 , and any peripheral devices in the device, including network interface  2040  or other peripheral interfaces. In some embodiments, I/O interface  2030  may perform any necessary protocol, timing or other data transformations to convert data signals from one component (e.g., system memory  2020 ) into a format suitable for use by another component (e.g., processor  2010 ). In some embodiments, I/O interface  2030  may include support for devices attached through various types of peripheral buses, such as a variant of the Peripheral Component Interconnect (PCI) bus standard or the Universal Serial Bus (USB) standard, for example. In some embodiments, the function of I/O interface  2030  may be split into two or more separate components, such as a north bridge and a south bridge, for example. Also, in some embodiments some or all of the functionality of I/O interface  2030 , such as an interface to system memory  2020 , may be incorporated directly into processor  2010 . 
     Network interface  2040  may be configured to allow data to be exchanged between computer system  2000  and other devices  2060  attached to a network or networks  2050 , such as other computer systems or devices as illustrated in  FIGS. 1 through 35 , for example. In various embodiments, network interface  2040  may support communication via any suitable wired or wireless general data networks, such as types of Ethernet network, for example. Additionally, network interface  2040  may support communication via telecommunications/telephony networks such as analog voice networks or digital fiber communications networks, via storage area networks such as Fibre Channel SANs, or via any other suitable type of network and/or protocol. 
     In some embodiments, system memory  2020  may be one embodiment of a computer-accessible medium configured to store program instructions and data as described above for  FIGS. 1 through 35  for implementing embodiments of a distributed load balancing system. However, in other embodiments, program instructions and/or data may be received, sent or stored upon different types of computer-accessible media. Generally speaking, a computer-accessible medium may include non-transitory storage media or memory media such as magnetic or optical media, e.g., disk or DVD/CD coupled to computer system  2000  via I/O interface  2030 . A non-transitory computer-accessible storage medium may also include any volatile or non-volatile media such as RAM (e.g. SDRAM, DDR SDRAM, RDRAM, SRAM, etc.), ROM, etc, that may be included in some embodiments of computer system  2000  as system memory  2020  or another type of memory. Further, a computer-accessible medium may include transmission media or signals such as electrical, electromagnetic, or digital signals, conveyed via a communication medium such as a network and/or a wireless link, such as may be implemented via network interface  2040 . 
     CONCLUSION 
     Various embodiments may further include receiving, sending or storing instructions and/or data implemented in accordance with the foregoing description upon a computer-accessible medium. Generally speaking, a computer-accessible medium may include storage media or memory media such as magnetic or optical media, e.g., disk or DVD/CD-ROM, volatile or non-volatile media such as RAM (e.g. SDRAM, DDR, RDRAM, SRAM, etc.), ROM, etc, as well as transmission media or signals such as electrical, electromagnetic, or digital signals, conveyed via a communication medium such as network and/or a wireless link. 
     The various methods as illustrated in the Figures and described herein represent exemplary embodiments of methods. The methods may be implemented in software, hardware, or a combination thereof. The order of method may be changed, and various elements may be added, reordered, combined, omitted, modified, etc. 
     Various modifications and changes may be made as would be obvious to a person skilled in the art having the benefit of this disclosure. It is intended to embrace all such modifications and changes and, accordingly, the above description to be regarded in an illustrative rather than a restrictive sense.