Patent Publication Number: US-11659061-B2

Title: Method of adjusting service function chains to improve network performance

Description:
RELATED APPLICATIONS 
     Benefit is claimed under 35 U.S.C. 119(a)-(d) to Foreign application Serial No. 202041002334 filed in India entitled “A METHOD OF ADJUSTING SERVICE FUNCTION CHAINS TO IMPROVE NETWORK PERFORMANCE”, on Jan. 20, 2020, by VMware, Inc., which is herein incorporated in its entirety by reference for all purposes. 
     The present application Ser. No. 16/843,919 is related in subject matter to U.S. patent application Ser. No. 16/843,913, now issued as U.S. Pat. No. 11,153,406, which is incorporated herein by reference. 
     BACKGROUND 
     Datacenters today use service chains that designate a series of service operations to perform on data messages of a network. These service chains are implemented by service paths which each define a specific set of service nodes for a set of data to pass through. Each service node implements one of the service operations designated in the service chain. However, the tools presently used to monitor networks do not include interfaces to monitor the implementation of service chains. Therefore, there is a need in the art for a user interface to troubleshoot the service paths and service nodes implementing a service chain. The present art also does not use service proxies to collect data transmission characteristics of individual service nodes. The lack of service proxies collecting the data leads to inadequate monitoring of service nodes in a service chain. Therefore, there is a need in the art for a method that uses service proxies that monitor the individual service nodes to collect operational statistics and modifies the service paths in response to the collected operational statistics. 
     BRIEF SUMMARY 
     Some embodiments of the invention provide a method for monitoring a particular service chain that includes several service operations to perform on data messages passing through a network. The method provides a user interface to receive a selection of the particular service chain from a plurality of service chains. The particular service chain is implemented by several service paths, each of which includes several service nodes. Each service node performs one of the services of the service chain. The method generates a graphical display of the service paths through the service nodes, and as part of this graphical display, provides operational data relating to operations of the service nodes of the service paths. In some embodiments, the method also provides, as part of the graphical display, operational data relating to operations of the nodes of an individual service path. In some embodiments, the method further includes defining the plurality of service paths for the particular service chain. 
     “Operational data” refers herein to data gathered at least partly while service nodes are in operation. The operational data may include operational statistical data derived from one or more measurements (e.g., performance measurements) of the operations of a service node. In some embodiments, operational data may also include data that is not derived from a measurements of the service nodes, such as an identifier of the service node, a packet type or packet types handled by the service node, etc. 
     The graphical display of some embodiments includes a node representation of each service node and a link connecting the node representations of each successive pair of service nodes in at least one service path. In some embodiments, the graphical display includes a concurrent representation of the plurality of service paths. A single service node may provide a particular service for multiple service paths of the service chain in some embodiments. The method of some embodiments updates the graphical display to add a node representation of any newly implemented service node of the displayed service chain or to remove node representations of any service node(s) that is/are no longer implemented. 
     The method of some embodiments responds to various received user commands. In some embodiments operational data for a service node is provided after receiving a selection of the service node (e.g., via a cursor click or cursor hover operation). The graphical display of some embodiments provides operational data relating to operations of a set of service nodes on an individual service path after receiving a selection of the individual service path. Similarly, the graphical display of some embodiments receives a selection of a particular service. Once a service is selected, the graphical display then provides operational data relating to the operations of a set of service nodes that each implement instantiations of the particular service. 
     The method of some embodiments allows a user to set threshold values for some particular performance parameters. For example, the method may allow a user to set a threshold for throughput of the service chain, aggregate latency of all service nodes of a particular service type, etc. Similarly, the method of some embodiments provides an interface option for setting a threshold for a performance parameter for a particular service type. In some embodiments, the method provides a visual cue to indicate whether a performance parameter has gone beyond a threshold. For example, the method may change the color of one or more graphical elements to indicate the performance parameter has gone beyond the threshold. 
     In addition to monitoring the service chains of a network, the method of some embodiments allows a user to alter the operations of those service chains. For example, the method of some embodiments provides interface options for instantiating an additional service node for a particular service and/or shutting down or restarting an already operating service node. 
     In the method of some embodiments, data may be received from service proxies that monitor service nodes. A service proxy may be implemented on the same device as a service node it monitors, or on a different device from the service node. In some embodiments, each service node has its own service proxy. In other embodiments, a single service proxy may monitor multiple nodes. 
     In some embodiments, a service node (sometimes called a “service instance runtime”) is implemented on a device of the network and the operational data includes operational statistics received from a service proxy implemented on the same device. The device may be a service appliance or a host computer of the network. A service appliance is a device designed to provide a particular service or a small set of services rather than operating as a general purpose computing device. Service devices of some embodiments implement their own service proxies in addition to (or as part of) the service they are designed for. A host computer is a physical computer that implements (hosts) virtual machines (VMs) or containers that implement service nodes. VMs and containers are sometimes collectively referred to as “service machines.” Each host computer of some embodiments implements service proxies for the service nodes implemented on that host computer. 
     In other embodiments, a service proxy is implemented on a separate device from a service node it monitors. For example, service nodes of a service appliance that does not implement a service proxy may be implemented on host computers or other devices of the network. Similarly, the methods of some embodiments may implement a service node on one host machine and a service proxy to monitor that node on another host machine. 
     Implementing service proxies on or off the same device that implements the service nodes they are monitoring are not mutually exclusive options. For example, some embodiments may implement (1) service proxies on a host computer to monitor the service nodes on that host computer and (2) additional service proxies on the host computer to monitor service nodes that are implemented on one or more appliances that lack service proxies of their own. 
     In some embodiments, at least part of the operational data is generated from operational statistics. The operational statistics are based on data received from a service proxy that collects data transmission characteristics relating to a particular service node. These operational statistics may be calculated at one or more service proxies, calculated at a server of the network, or calculated partially at a server and partially at one or more service proxies. In some cases, these calculations may include aggregation of data transmission characteristics (e.g., averages, sums, weighted averages, etc.). The data transmission characteristics of some embodiments include a latency and/or a throughput of a service node. The latency is the time for a query to be sent to the service node and a reply received (e.g., received at the service proxy). A throughput value represents the amount of data in the data messages that passes through a service node in a particular amount of time. The operational data of some embodiments includes data received from the service proxies relating to packets received by a particular service node (e.g., packet type, error rate, drop rate, etc.) and/or data relating to payload size. 
     The method of some embodiments is implemented on a telecommunications network and the particular service chain includes a set of service operations applied to a particular type of data message flow. For example, the method may be implemented on a 5G network, 4G network, or LTE network. 
     Some embodiments of the invention provide a method for monitoring and adjusting a service chain that includes multiple services to perform on data messages passing through a network. The method operates on a service chain implemented by multiple service paths each of which includes multiple service nodes that implement multiple services of the service chain. The method receives operational data relating to a set of service nodes among the multiple service nodes. The method analyzes the operational data. In response to the analysis of the operational data, the method alters the service paths implementing the service chain. 
     The methods of some embodiments alter the service paths in multiple ways. In some embodiments, the method alters the service paths by instantiating a new service node to implement a particular service of the service chain. Instantiating the new service node in some embodiments is in response to operational data indicating that a throughput of one or more service nodes implementing the particular service is beyond a threshold (e.g., lower than a set minimum threshold or above a set maximum threshold) throughput. Instantiating the new service node in some embodiments is in response to operational data indicating that a latency of one or more service nodes implementing the particular service is beyond a threshold latency. 
     The method of some embodiments may alter service paths by deactivating an existing service node that is implementing a particular service. Deactivating the existing service node in some embodiments may be in response to operational data indicating that a throughput of one or more service nodes implementing the particular service is beyond a threshold throughput. In some embodiments, deactivating the existing service node may be in response to operational data indicating that a latency of one or more service nodes implementing the particular service is beyond a threshold latency. 
     Some embodiments alter service paths by restarting an existing service node that is implementing a particular service. For example, the method of such embodiments may restart a service node in response to operational data indicating errors in the existing service node or in response to operational data indicating that the existing service node has fallen below a performance threshold. 
     In some embodiments a particular service node implements parts of multiple service paths of the service chain. In some such embodiments, the method alters the service paths by reducing a number of service paths implemented in part by a particular service node. For example, the method may reduce the number of service paths using a particular node when the node is receiving more than a threshold amount of data in the data messages (e.g., being overloaded). The method of some embodiments, alters the service paths by increasing a number of service paths implemented in part by a particular service node. For example, the method may increase the number of service paths using a particular node in response to the particular service node receiving less than a threshold amount of data in the data messages (e.g., being underutilized). 
     The methods of various embodiments receive and analyze data from various sources and in various ways. In some embodiments, a particular service node is implemented on a device and the operational data for the particular service node is based at least partly on data received from a service proxy implemented on the device. The device in some embodiments may be a service appliance or a host computer of the network. In some embodiments, a particular service node is implemented on a first device and the operational data for the particular service node is based at least partly on data received from a service proxy implemented on a second device. 
     The method of some embodiments bases the operational data on operational statistics received from service proxies. Analyzing the operational data in some embodiments includes aggregating operational statistics data with previously collected operational statistics data. The aggregation may include calculating a weighted sum of the operational statistics data. The operational statistics of some embodiments includes data transmission characteristics of the service nodes received from the service proxies. The data transmission characteristics of a particular service node may include a throughput of the particular service node and/or a latency time for the particular service node. 
     In some embodiments, the service proxy collects other data. The service proxy collects data relating to packets received by a particular service node in some embodiments. The service proxy collects data relating to the sizes of payloads received by a particular service node in some embodiments. 
     Various embodiments include various other elements or limitations. For example, the network of some embodiments is a telecom network and a particular service chain is a set of service functions each applied to a particular type of data message flow. In some embodiments, the method further includes defining the plurality of service paths for a particular service chain. The service nodes of some embodiments include at least one of: a service implemented on a virtual machine (VM), a service implemented on a container, or a service implemented by an appliance. 
     The preceding Summary is intended to serve as a brief introduction to some embodiments of the invention. It is not meant to be an introduction or overview of all inventive subject matter disclosed in this document. The Detailed Description that follows and the Drawings that are referred to in the Detailed Description will further describe the embodiments described in the Summary as well as other embodiments. Accordingly, to understand all the embodiments described by this document, a full review of the Summary, Detailed Description, the Drawings and the Claims is needed. Moreover, the claimed subject matters are not to be limited by the illustrative details in the Summary, Detailed Description and the Drawing. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The novel features of the invention are set forth in the appended claims. However, for purposes of explanation, several embodiments of the invention are set forth in the following figures. 
         FIG.  1    illustrates an example of segregated guest and service planes that are implemented in some embodiments by two logical forwarding elements. 
         FIG.  2    illustrates a data message between two guest virtual machines (GVMs) being redirected along a service path to be processed by service virtual machines (SVMs) of some embodiments. 
         FIG.  3    conceptually illustrates a relationship between a service chain and a set of one or more service paths that implement the service chain in some embodiments. 
         FIG.  4    illustrates an example of a service chain and its associated service paths. 
         FIG.  5    illustrates examples of reverse service paths for the forward service paths illustrated in  FIG.  4   . 
         FIG.  6    illustrates an example of input/output (IO) chain components that implement a service plane in some embodiments. 
         FIG.  7    illustrates a process performed by a service index pre-processor and a service transport layer caller of some embodiments. 
         FIG.  8    illustrates a data flow example corresponding to the process described in  FIG.  7   . 
         FIG.  9    illustrates an operation of a port proxy of some embodiments for formatting a data message for forwarding by a first service node. 
         FIG.  10    conceptually illustrates a process of some embodiments for passing a data message in a service path to a next hop. 
         FIG.  11    illustrates a process that the service proxy of  FIG.  6    performs in some embodiments each time it receives a data message traversing along an ingress path of a service node. 
         FIG.  12    illustrates an example of a data message in some embodiments being forwarded from a first hop service node to a second hop service node. 
         FIG.  13    conceptually illustrates a process that a service proxy performs in some embodiments each time it receives a data message traversing along an egress path of its service node. 
         FIG.  14    conceptually illustrates a process started by an encap processor on a next hop computer that receives an encapsulated data message that needs to be processed by an SVM executing on its computer. 
         FIG.  15    illustrates an example of a data message in some embodiments being forwarded from a second hop service node to a third hop service node. 
         FIG.  16    illustrates an example of a data message in some embodiments being forwarded from a third hop service node back to a first hop service node. 
         FIG.  17    conceptually illustrates a process that a service index post-processor performs in some embodiments. 
         FIG.  18    conceptually illustrates several operations that network managers and controllers perform in some embodiments to define rules for service insertion, next service hop forwarding, and service processing. 
         FIG.  19    illustrates how service paths are dynamically modified in some embodiments. 
         FIG.  20    illustrates an example of a graphical user interface (GUI) of some embodiments that displays elements implementing a service chain. 
         FIG.  21    illustrates a GUI displaying operational data for a service node in response to a user selection of the node. 
         FIG.  22 A  illustrates a GUI displaying operational data for the nodes along a service path in response to a user selection of the service path. 
         FIG.  22 B  illustrates a GUI displaying a visual indicator of a poorly functioning node. 
         FIG.  23    illustrates the example GUI displaying operational data for the nodes that provide a particular service in response to a user selection of one of the nodes implementing the service. 
         FIG.  24    illustrates a GUI adding an additional service node for the selected service in response to a user command. 
         FIG.  25    illustrates a process of some embodiments for selecting a service chain then receiving and displaying operational data for components that implement that service chain. 
         FIG.  26    illustrates an example of a service proxy monitoring a service node. 
         FIG.  27    conceptually illustrates a process of some embodiments for calculating and reporting statistics of a service node at a service proxy. 
         FIG.  28    conceptually illustrates a process of some embodiments for receiving, storing, and analyzing, at a central controller, statistics from a service proxy. 
         FIG.  29    illustrates a process for automatically restarting/remediating an operational service node when the particular service node fails to meet a threshold. 
         FIG.  30    illustrates a process for automatically adding a new service node when the performance of the service nodes implementing a particular service fail to meet a threshold. 
         FIG.  31    illustrates a process for automatically shifting data traffic to underused nodes of a service or recovering resources from an underused node of a service. 
         FIG.  32    conceptually illustrates an electronic system with which some embodiments of the invention are implemented. 
     
    
    
     DETAILED DESCRIPTION 
     In the following detailed description of the invention, numerous details, examples, and embodiments of the invention are set forth and described. However, it will be clear and apparent to one skilled in the art that the invention is not limited to the embodiments set forth and that the invention may be practiced without some of the specific details and examples discussed. 
     Some embodiments of the invention provide a method for monitoring a particular service chain that includes several service operations to perform on data messages passing through a network. The method provides a user interface to receive a selection of the particular service chain from a plurality of service chains. The particular service chain is implemented by several service paths, each of which includes several service nodes. Each service node performs one of the services of the service chain. The method generates a graphical display of the service paths through the service nodes, and as part of this graphical display, provides operational data relating to operations of the service nodes of the service paths. In some embodiments, the method also provides, as part of the graphical display, operational data relating to operations of the nodes of an individual service path. In some embodiments, the method further includes defining the plurality of service paths for the particular service chain. 
     In some embodiments, a service node performs a middlebox service operation, such as a firewall, an intrusion detection system, an intrusion prevention system, a load balancer, an encryptor, a message monitor, a message collector, or any number of other middlebox services. In some embodiments, multiple instances of a category of services may be implemented as separate operations. For example, a service chain may include two separate firewalls from two different vendors or multiple different virus/malware scanners. 
     “Operational data” refers herein to data gathered at least partly while service nodes are in operation. The operational data may include operational statistical data derived from one or more measurements (e.g., performance measurements) of the operations of a service node. In some embodiments, operational data may also include data that is not derived from a measurements of the service nodes, such as an identifier of the service node, a packet type or packet types handled by the service node, etc. 
     Although the discussion below describes receiving the operational data from service proxies in some embodiments, one of ordinary skill in the art will understand that other embodiments receive operational data relating to each node from other system elements. The operational data in some embodiments may include raw data and/or calculated data. Measured values and values calculated from those measured values may be referred to herein as operational statistics. 
     In some embodiments, the user interface and automatic monitoring of the present invention operates in a network that uses service proxies to monitor service nodes that sequentially implement a set of service paths of a service chain. The service chains, service paths, service nodes, network, and host machines, monitored by the service proxies, and which the user interface and automatic monitoring of some embodiments are applied to, are described in the following figures,  FIGS.  1 - 19   . 
       FIG.  1    illustrates an example of segregated guest and service planes that are implemented in some embodiments by two logical forwarding elements (LFEs)  130  and  132 . As shown, two guest machines  102  and  104  and three service machines  106 ,  108 , and  110  execute on three host computers  112 ,  114 , and  116  along with three software forwarding elements  120 ,  122 , and  124 . In this example, the guest machines and service machines are guest virtual machines (GVMs) and service virtual machines (SVMs), but in other embodiments these machines can be other types of machines, such as containers. 
     Also, in this example, each logical forwarding element is a distributed forwarding element that is implemented by configuring multiple software forwarding elements (SFEs) on multiple host computers. To do this, each SFE or a module associated with the SFE in some embodiments is configured to encapsulate the data messages of the LFE with an overlay network header that contains a virtual network identifier (VNI) associated with the overlay network. As such, the LFEs are said to be overlay network constructs that span multiple host computers in the discussion below. 
     The LFEs also span in some embodiments configured hardware forwarding elements (e.g., top of rack switches). In some embodiments, each LFE is a logical switch that is implemented by configuring multiple software switches (called virtual switches or vswitches) or related modules on multiple host computers. In other embodiments, the LFEs can be other types of forwarding elements (e.g., logical routers), or any combination of forwarding elements (e.g., logical switches and/or logical routers) that form logical networks or portions thereof. Many examples of LFEs, logical switches, logical routers, and logical networks exist today, including those provided by VMware&#39;s NSX network and service virtualization platform. 
     As shown, the LFE  130  defines the guest forwarding plane that connects the GVMs  102  and  104  in order to forward data messages between these GVMs. In some embodiments, this LFE is a logical switch that connects to a logical router, which connects the GVMs directly or through a logical gateway to networks outside of the logical switch&#39;s logical network. The LFE  130  is implemented in some embodiments by configuring software switches  120  and  122  and/or their related modules (e.g., related port/VNIC filter modules) on the host computers  112  and  114  to implement a first distributed logical switch. 
       FIG.  1    and other figures discussed below show the source and destination GVMs being on the same logical network and being connected to the same LFE. One of ordinary skill will realize that the service operations of some embodiments do not require the source and destination machines to be connected to the same LFE, or to even be in the same network or the same datacenter. These service operations are performed on data messages that exit the source machine&#39;s network or enter a source machine&#39;s network. The figures depict the source and destination machines as connected to the same LFE to emphasize that the service plane  132  is implemented by a separate logical network than the logical network that forwards the data messages associated with the guest machines. 
     The LFE  132  defines the service forwarding plane that connects the SVMs  106 ,  108 , and  110  in order to forward data messages associated with the GVMs through service paths that include the SVMs. In some embodiments, the LFE  132  is also a logical switch that is implemented by configuring software switches  120 ,  122 , and  124  and/or their related modules on the host computers  112 ,  114 , and  116  to implement a second distributed logical switch. Instead of configuring the same set of SFEs to implement both the guest and service forwarding planes (i.e., the guest and service LFEs), other embodiments configure one set of SFEs on a set of host computers to implement the guest forwarding plane and another set of SFEs on the set of host computers to implement the service forwarding plane. For instance, in some embodiments, each host computer executes a guest software switch and a service software switch, and these two switches and/or their related modules can be configured to implement a guest logical switch and a service logical switch. 
     In some embodiments, the software switches  120 ,  122 , and  124  and/or their related modules can be configured to implement multiple guest forwarding planes (e.g., guest LFEs) and multiple service forwarding planes (e.g., service LFEs) for multiple groups of machines. For instance, for a multi-tenant datacenter, some such embodiments define a guest LFE and a service LFE for each tenant for which at least one chain of services needs to be implemented. For each group of related machines (e.g., for each tenant&#39;s machines), some embodiments define two virtual network identifiers (VNIs) to configure a shared set of software forwarding elements (e.g., software switches) to implement the two different forwarding planes, i.e., the guest forwarding plane and the service forwarding plane. These two VNIs are referred to below as the guest VNI (GVNI) and the service VNI (SVNI). In  FIG.  1   , the guest LFE ports  150  and  152  are associated with the GVNI, while the service LFE ports  154 ,  156 , and  158  are associated with the SVNI, as shown. 
     In some embodiments, the service plane  132  is also implemented by inserting modules in input/output (IO) chains of a GVM&#39;s egress and ingress datapaths to and from an SFE  120  or  122 . In this implementation, the service plane  132  can identify a data message sent from the GVM or received for the GVM, forward the data message to a set of SVMs to perform a chain of services on the data message, and then to return the data message back to the GVM&#39;s datapath so that the data message can be proceed along its datapath to the software switch or to the GVM (i.e., so that the data message can be processed based on the destination network addresses specified by the source GVM). Such a GVM is referred to below as the source GVM as the data message being processed by the service nodes is a data message identified on the GVM&#39;s egress or ingress path. In some embodiments, a GVM&#39;s egress/ingress IO chain is implemented as a set of hooks (function calls) in the GVM&#39;s VNIC (virtual network interface card)  180  or the SFE port associated with the GVM&#39;s VNIC (e.g., the SFE port communicating with the GVM&#39;s VNIC). 
     Before providing an example of the IO chain components of some embodiments that implement the service plane,  FIG.  2    illustrates an example of a data message  202  from the GVM  102  to GVM  104  being redirected along the service plane  132  so that the data message can be processed by SVMs  108  and  110  that perform a chain of two service operations. As shown, the service LFE  132  first forwards the data message to SVM  108 , and then forwards the data message to SVM  110 , before returning the data message back to the egress path of GVM  102  so that the data message can be processed based on the destination network addresses specified by the source GVM  102 . 
     The service LFE in some embodiments forwards the data message between hosts  112 ,  114 , and  116  by using an overlay encapsulation header that stores the SVNI for the service LFE. Also, when the service LFE is a service logical switch, the service forwarding plane in some embodiments uses the MAC addresses associated with the SVMs (e.g., MAC addresses of SVM VNICs) to forward the data message between ports of the service logical switch. In some embodiments, the MAC forwarding also uses service plane MAC address associated with the source GVM, even though this GVM does not directly connect to the service plane but instead connects to the service plane through a port proxy, as further described below. 
     Once the data message  202  returns to the egress path of the GVM  102 , the guest LFE  130  forwards the data message to its destination (e.g., as specified by the destination network address in the data message&#39;s header), which is GVM  104 . The guest LFE  130  in some embodiments forwards the data message between hosts  112  and  114  by using an overlay encapsulation header that stores the GVNI for the guest LFE. Also, when the guest LFE is a logical switch, the guest forwarding plane in some embodiments uses the guest plane MAC addresses associated with the GVMs  102  and  104  to forward the data message (e.g., by using the guest plane MAC address of GVM  104  to forward the data message to the guest forwarding port  152  associated with this GVM). While the service plane of  FIG.  2    captures a data message passing through a GVM&#39;s egress path, the service plane in some embodiments can also capture a data message as it is passing through a GVM&#39;s ingress path before it reaches the GVM&#39;s VNIC. 
     In some embodiments, a chain of service operations is referred to as a service chain. A service chain in some embodiments can be implemented with one or more sets of service nodes (e.g., service machines or appliances), with each set of service nodes defining a service path. Hence, in some embodiments, a service chain can be implemented by each of one or more service paths. Each service path in some embodiments includes one or more service nodes for performing the set of one or more services of the service chain and a particular order through these nodes. 
       FIG.  3    presents an object diagram that illustrates the relationship between a service chain  302  and a set of one or more service paths  304  that implement the service chain. Each service chain has a service chain (SC) identifier  306 , while each service path has a service path identifier (SPI)  308 . Each service path is associated with a set of m service nodes, which, as shown, are identified in terms of service instance endpoints  310 . Service instance endpoints in some embodiments are logical locations in the network where traffic can go or come from a service node connected to the service plane. In some embodiments, a service instance endpoint is one LFE port (e.g., an SFE port) associated with a service node (e.g., a VNIC of an SVM). In these or other embodiments, a service instance endpoint can be associated with two LFE ports used for a service node as further described below for embodiments that use GRE encapsulation. Also, the service endpoints in some embodiments are addressable through MAC addresses associated with the LFE ports or with the SVM VNICs associated with (e.g., communicating with these LFE ports). 
     In some embodiments, each service chain  302  is defined by references to one or more service profiles  312 , with each service profile associated with a service operation in the chain. As described below, a service node in some embodiments (1) receives, from a service manager, a mapping of a service chain identifier to a service profile that it has to implement, and (2) receives, with a data message, a service chain identifier that it maps to the service profile to determine the service operation that it has to perform. In some embodiments, the received mapping is not only based on the service chain identifier (SCI) but is also based on a service index value (that specifies the location of the service node in a service path) and a direction through a service chain (that specifies an order for performing the sequence of services specified by the service chain). The service profile in some embodiments describes the service operation that the service node has to perform. In some embodiments, a service profile can identify a set of rules for a service node to examine. 
     Also, in some embodiments, service insertion rules  314  are defined by reference to service chain identifier  306  for service insertion modules associated with GVMs. Such service insertion modules use these service insertion rules  314  to identify service chains to use to process data messages associated with a source GVM. As mentioned above, the data messages are referred to below as being from a source GVM as the data messages that are processed by the service chains are identified on the egress paths from or ingress paths to the GVMs. 
     As further described below, the service insertion (SI) rules associate flow identifiers with service chain identifiers. In other words, some embodiments try to match a data message&#39;s flow attributes to the flow identifiers (referred to below as rule identifiers of the SI rules) of the service insertion rules, in order to identify a matching service insertion rule (i.e., a rule with a set of flow identifiers that matches the data message&#39;s flow attributes) and to assign this matching rule&#39;s specified service chain as the service chain of the data message. A specific flow identifier (e.g., one defined by reference to a five-tuple identifier) could identify one specific data message flow, while a more general flow identifier (e.g., one defined by reference to less than the five tuples) can identify a set of several different data message flows that match the more general flow identifier. As such, a matching data message flow is any set of data messages that have a common set of attributes that matches a rule identifier of a service insertion rule. 
     As further described below, other embodiments use contextual attributes associated with a data message flow to associate the data message with a service insertion rule. Numerous techniques for capturing and using contextual attributes for performing forwarding and service operations are described in U.S. patent application Ser. No. 15/650,251, now issued as U.S. Pat. No. 10,802,857, which are incorporated herein. Any of these techniques can be used in conjunction with the embodiments described herein. 
     Next hop forwarding rules  316  in some embodiments are defined by reference to the SPI values  308  and service instance endpoints  310 . Specifically, in some embodiments, a service path is selected for a service chain that has been identified for a data message. At each hop, these embodiments use the forwarding rules  316  to identify the next service instance endpoint based on the SPI value for this service path along with a current service index (SI) value, which identifies the location of the hop in the service path. In other words, each forwarding rule in some embodiments has a set of matching criteria defined in terms of the SPI/SI values, and specifies a network address of the next hop service instance endpoint that is associated with these SPI/SI values. To optimize the next hop lookup for the first hop, some embodiments provide to the source GVM&#39;s service insertion module the next hop network address with the SPI, as part of a service path selection process. 
       FIG.  4    illustrates an example of a service chain and its associated service path. As shown, each service chain  405  in some embodiments is defined as a sequential list of service profiles  410 , with each profile in this example related to a different middlebox service (such as firewall, load balancer, intrusion detector, data message monitor, etc.). Also, in this example, each of the M profiles can be implemented by one SVM in a cluster m of VMs. As shown, different clusters for different profiles can have different numbers of SVMs. Also, in some embodiments, one service profile is implemented by one service node (i.e., a cluster of several service nodes is not required to implement a service profile). 
     Since multiple SVMs in a cluster can provide a particular service, some embodiments define for a given service chain, multiple service paths through multiple different combinations of SVMs, with one SVM of each cluster being used in each combination. In the example of  FIG.  4   , there are N service paths associated with the service chain  405 , traversed by data messages originating at a GVM  402  on their way to a GVM  404 . Each service path is identified by a different set of dashed lines in this figure. 
     Specifically, the first service path passes through first SVM  1 , 1  of the first service profile&#39;s cluster to implement the first service of the forward service chain  405 , the first SVM  2 , 1  of the second service profile&#39;s cluster to implement the second service of the forward service chain  405 , and third SVM M, 3  of the Mth service profile&#39;s cluster to implement the Mth service of the forward service chain  405 . The second service path passes through second SVM  1 , 2  of the first service profile&#39;s cluster to implement the first service of the forward service chain  405 , the first SVM  2 , 1  of the second service profile&#39;s cluster to implement the second service of the forward service chain  405 , and first SVM M, 1  of the Mth service profile&#39;s cluster to implement the Mth service of the forward service chain  405 . 
     The third service path passes through third SVM  1 , 3  of the first service profile&#39;s cluster to implement the first service of the forward service chain  405 , the second SVM  2 , 2  of the second service profile&#39;s cluster to implement the second service of the forward service chain  405 , and second SVM M, 2  of the Mth service profile&#39;s cluster to implement the Mth service of the forward service chain  405 . The Nth service path passes through third SVM  1 , 3  of the first service profile&#39;s cluster to implement the first service of the forward service chain  405 , the second SVM  2 , 2  of the second service profile&#39;s cluster to implement the second service of the forward service chain  405 , and fourth SVM M, 4  of the Mth service profile&#39;s cluster to implement the Mth service of the forward service chain  405 . As the example illustrates, different service paths may use the same SVM for a given service operation. However, regardless of the service path that a given data message traverses, the same set of service operations is performed in the same sequence, for paths that are associated with the same service chain and the same service direction. 
     In some embodiments, a service chain has to be performed in a forward direction for data messages from a first GVM to a second GVM, and then in the reverse direction for data messages from the second GVM to the first GVM. In some such embodiments, the service plane selects both the service path for the forward direction and the service path for the reverse direction when it processes the first data message in the flow from the first GVM to the second GVM. Also, in some of these embodiments, the forward and reverse service paths are implemented by the same sets of service nodes but in the reverse order. 
       FIG.  5    illustrates examples of reverse service paths for the forward service paths illustrated in  FIG.  4   . While the forward service paths are for performing M services on data messages from GVM  402  to GVM  404 , the reverse service paths are for performing M services on data messages from GVM  404  to GVM  402 . Also, the order of these services is reversed with the service paths in  FIG.  5    performing service profiles M to  1 , while the service paths in  FIG.  4    perform service profile  1  to M. 
     Also, in the examples of  FIGS.  4  and  5   , each reverse service path has one corresponding forward service path that is implemented by the same exact set of SVMs but in the reverse order, as indicated by the service path legends and the similar dashed lines in these figures. For example, the forward, second service path passes through SVM  1 , 2  for the first service associated with the first profile, SVM  2 , 1  for the second service associated with the second profile, and SVM M, 1  for the Mth service associated with the Mth service profile, while the associated reverse, second service path passes through SVM M, 1  for the first service associated with the Mth service profile, SVM  2 , 1  for the second service associated with the second profile, and SVM  1 , 2  for the second service associated with the first profile. 
     In some embodiments, the same service nodes are used for the forward and reverse paths because at least one of the service nodes (e.g., a firewall SVM) that implements one of the service profiles needs to see the data traffic in both directions between two data endpoints (e.g., two GVMS). In other embodiments, the same service nodes do not need to be used for both directions of data message flows between two data endpoints so long as the same set of service operations are performed in opposite orders. 
       FIG.  6    illustrates an example of the IO chain components that implement the service plane in some embodiments. As shown, the service plane  132  is implemented by software switches  120 ,  122 , and  124  executing on the host computers and two sets of modules  610 ,  612 ,  614 ,  620 ,  624 ,  626 , and  628  on these computers. The implemented service plane in this example as well some of the other examples illustrated in some of the subsequent figures is an overlay logical L2 service plane. One of ordinary skill will realize that other embodiments are implemented by other types of service planes, such as overlay L3 service planes, or overlay networks with multiple L2 logical switches and one or more logical L3 routers. 
     In  FIG.  6   , the software switches  120 ,  122 , and  124  and modules  610 ,  612 ,  614 ,  620 ,  624 ,  626 , and  628  implement two different layers of the service plane, which are the service insertion layer  602  and the service transport layer  604 . The service insertion layer  602  (1) identifies the service chain for a data message, (2) selects the service path to use to perform the service operations of the service chain, (3) identifies the next-hop service nodes at each hop in the selected service path (including the identification of the source host computer to which the data message should be returned upon the completion of the service chain), and (4) for the service path, specifies the service metadata (SMD) header attributes for the data message. The SMD attributes in some embodiments include the network service header (NSH) attributes per RFC (Request for Comments) 8300 of IETF (Internet Engineering Task Force). 
     The service transport layer  604 , on the other hand, formulates the service overlay encapsulation header and encapsulates the data message with this header so that it can pass between service hops. In some embodiments, the service transport layer  604  modifies the SMD header to produce the service overlay encapsulation header. For instance, in some of these embodiments, the overlay encapsulation header is a Geneve header with the SMD attributes stored in a TLV (type, length, value) section of the Geneve header. In other embodiments, the service transport layer  604  adds the service overlay encapsulation header to an SMD header that is first used to encapsulate the data message. Also, when traversing between two hops (e.g., between two service nodes) executing on the same host computer, the service transport layer in several embodiments described below does not encapsulate the data message with an overlay encapsulation header in some embodiments. In other embodiments, even when traversing between two hops on the same host computer, the service transport layer encapsulates the data message with an overlay encapsulation header. 
     In some embodiments, the service insertion (SI) layer  602  includes an SI pre-processor  610  and an SI post-processor  612 , in each the two IO chains  650  and  652  (i.e., the egress IO chain  650  and the ingress IO chain  652 ) of a GVM for which one or more service chains are defined. The SI layer  602  also includes a service proxy  614  for each service node connected to the service plane (e.g., for each SVM with a VNIC paired with a service plane LFE port). The service transport (ST) layer  604  includes one STL port proxy  620  on each host computer that has one or more possible source GVMs for which one or more service chains are defined. The ST layer  604  also has (1) an STL caller  624  in each IO chain of each source GVM, (2) an STL module  626  in the IO chain of each SVM, and (3) one or more encap processors  628 . 
     For a data message that passes through a GVM&#39;s ingress or egress datapath, the SI pre-processor  610  on this datapath performs several operations. It identifies the service chain for the data message and selects the service path for the identified service chain. The pre-processor also identifies the network address for a first hop service node in the selected service path and specifies the SMD attributes for the data message. The SMD attributes include in some embodiments the service chain identifier (SCI), the SPI and SI values, and the direction (e.g., forward or reverse) for processing the service operations of the service chain. In some embodiments, the SPI value identifies the service path while the SI value specifies the number of service nodes. 
     After the SI pre-processor completes its operation, the STL caller  624  in the same datapath calls the STL port proxy  620  to relay the SMD attributes and first hop&#39;s network address that the pre-processor identified, so that the port proxy can forward the SMD attributes through the service plane to the first hop. The port proxy formats the data message for forwarding to the first service node. In some embodiments, this formatting comprises replacing the original source and destination MAC addresses in the data message with a service plane MAC address that is associated with the source GVM  102  and the MAC address of the first hop service node. This formatting also stores a set of attributes for the data message that should be processed by other service transport layer modules (e.g., the other STL modules, etc.) on the same host computer. These data message attributes include the SMD attributes as well as the original source and destination MAC addresses. 
     The STL port proxy  620  passes the formatted data message along with its stored attributes to the software switch  120 . Based on the destination MAC address (i.e., the first hop MAC address) of the formatted data message, the software switch delivers the data message to the switch port associated with the first hop SVM. When the first hop is on the same host computer as the port proxy  620 , the data message is provided to the STL module  626  in the ingress IO chain of the first hop&#39;s service node on the same host computer. When the first hop is not on the same host computer, the data message is encapsulated with an encapsulating header and forwarded to the next hop, as further described below. 
     Each hop&#39;s STL module  626  re-formats the data message by replacing the service plane source MAC address and service plane destination MAC address (i.e., its service node&#39;s MAC address) with the original source and destination MAC addresses of the data message. It then passes this re-formatted data message with its accompanying SMD attributes to its hop&#39;s service proxy  614 . This service proxy is in the IO chain of the ingress datapath of the GVM. For purposes of preventing the illustration in  FIG.  6    from being overcomplicated with unnecessary detail, the ingress and egress paths of each SVM in this example are combined in this figure, unlike the ingress and egress paths  650  and  652  of the GVM  102 . 
     The service proxy  614  encapsulates the received data message with an encapsulating NSH header that stores the data message&#39;s SMD attributes and provides this encapsulated data message to its service node when the service node can support NSH headers. When the service node is an SVM, the service proxy in some embodiments supplies the data messages and its NSH header to the SVM&#39;s VNIC through a VNIC injection process, as further described below. When the service node cannot process NSH headers, the service proxy  614  stores the SMD attributes into a legacy QinQ encapsulating header or a GRE encapsulating header, and then passes the encapsulated data message to the VNIC of the SVM. These headers will be further described below. 
     In some embodiments, the service proxy  614  of each service hop segregates the service node for that hop from the service transport layer. This segregation improves the security of both the SVM and the service transport layer. It also allows the service proxy to ensure that the data messages that are provided to its SVM are formatted properly, which is especially important for legacy SVMs that do not support the newer NSH format. 
     The service proxy  614  in some embodiments also performs liveness detection signaling with its service node to ensure that the service node is operational. In some embodiments, the service proxy sends a data message with a liveness value to its service node at least once in each recurring time period. To do this, the service proxy sets and resets a timer to ensure that it has sent a liveness signal for each time period to its service node. Each liveness value is accompanied with a liveness sequence number to allow the service proxy to keep track of liveness responses provided by the SVM. Each time the service node replies to a liveness signal, it provides to the service proxy the same liveness value in a responsive data message in some embodiments or its corresponding value in the responsive data message in other embodiments. Also, with each liveness responsive data message, the service node provides the same sequence number in some embodiments, or an incremented version of the sequence number provided by the service proxy in other embodiments. 
     As further described below, the service proxy of some embodiments piggybacks some of its liveness detection signaling on each data message that it passes to its service node from the service forwarding plane. Each time that the service proxy sends a liveness signal to its service node, it resets its liveness timer. Each time the service node processes the data message, it provides the processed data message back to the service node with the responsive liveness value and associated sequence number (incremented in some embodiments, or non-incremented in other embodiments, as mentioned above). 
     In some embodiments, the service proxy registers a liveness detection failure when the service node does not respond to its liveness signal within a particular time (e.g., within 0.3 seconds). After registering two successive liveness detection failures, the service proxy in some embodiments notifies a local control plane (LCP) module executing on its host the SVM has failed so that the LCP can notify a central control plane (CCP) server. In response to such a notification, the CCP removes the SVM and the service paths on which SVM resides from the forwarding and path selection rules in the data plane, and if needed, generates additional service paths for the failed SVM&#39;s associated service chain. Also, in some embodiments, the service proxy sends an in-band data message back to the source GVM to program its classifier to not select the service path on which the failed service node resides. 
     In some embodiments, the service proxy also performs flow programming at the behest of its service node. This flow programming in some embodiments involves modifying how the source GVM&#39;s IO chain selects service chains, service paths, and/or forwards data message flows along service paths. In other embodiments, this flow programming involves other modifications to how a data message flow is processed by the service plane. Flow programming will be further described below. 
     Upon receiving a data message and its SMD attributes (in an encapsulating NSH header or some other encapsulating header), the SVM performs its service operation. In some embodiments, the SVM uses mapping records that it receives from its service manager to map the SCI, SI and direction values in the SMD attributes to a service profile, and then maps this service profile to one of its rule sets, which it then examines to identify one or more service rules to process. In some embodiments, each service rule has a rule identifier that is defined in terms of data message attributes (e.g., five tuple attributes, which are the source and destination IP address, source and destination port addresses and the protocol). The SVM in some embodiments compares the rule&#39;s identifier with the attributes of the data message to identify a matching rule. Upon identifying one or more matching rules, the SVM in some embodiments performs an action specified by the highest priority matching rule. For instance, a firewall SVM might specify that the data message should be allowed to pass, should be dropped and/or should be redirected. 
     Once the SVM has completed its service operation, the SVM forwards the data message along its egress datapath. The service proxy in the egress datapath&#39;s IO chain then captures this data message and for this data message, identifies the network address of the next hop in the service path. To do this, the service proxy in some embodiments decrements the SI value, and then uses this decremented value along with the SPI value in the data message&#39;s stored attribute set to identify an exact match forwarding rule that identifies a next hop network address. In some embodiments, the SVM can decrement the SI value. For such cases, the service proxy in some embodiments can be configured not to decrement the SI value when its corresponding SVM decremented it. 
     In either configuration, the service proxy identifies the next hop network address by using the appropriate SPI/SI values to identify the next-hop forwarding rule applicable to the data message. When the proxy&#39;s service node is on multiple service paths, the proxy&#39;s forwarding rule storage stores multiple exact match forwarding rules that can specify different next hop network addresses for different SPI/SI values associated with different service paths. Assuming that the decremented SI value is not zero, the next hop in the service path is another service node. Hence, the proxy in some embodiments provides the next hop&#39;s MAC address to the proxy&#39;s associated STL module  626  in the SVM&#39;s egress datapath. This module then re-formats the data message, by specifying the SVM&#39;s MAC address and the next hop&#39;s MAC address as the source and destination MAC addresses and storing the original source and destination MAC addresses of the data message in the stored set of attributes stored for the data message. The STL module  626  then forwards the data message along the egress path, where it reaches the software switch, which then has to forward the data message and its stored attributes to the next hop service node. 
     When the next hop is on the same host computer, the software switch passes the data message and its attributes to the port that connects to the STL module of the next hop&#39;s service node, as described above. On the other hand, when the next hop service node is on another host computer, the software switch provides the data message to the uplink port that connects to the VTEP (VXLAN Tunnel Endpoint) that communicates through an overlay network tunnel with a VTEP on the other host computer. An encap processor  628  then captures this data message along the egress path of this port, defines an encapsulating overlay header for this data message and encapsulates the data message with this overlay header. In some embodiments, the overlay header is a single header that stores both SMD and STL attributes. For instance, in some embodiments, the overlay header is a Geneve header that stores the SMD and STL attributes in one or more TLVs. 
     As mentioned above, the SMD attributes in some embodiments include the SCI value, the SPI value, the SI value, and the service direction. Also, in some embodiments, the STL attributes includes the original L2 source MAC address, the original L2 destination MAC address, the data message direction, and the service-plane source MAC address of the source GVM. In some embodiments, the service direction and the service-plane source MAC address are already part of the SMD attributes. The service transport layer in some embodiments needs these attributes with each processed data message, in order to recreate the original data message and later at the end of the service-path, to return the data message to the original host to resume along its datapath. 
     When the encapsulated data message is received at the next hop&#39;s host computer, the data message is captured by the encap processor  628  of the software switch&#39;s downlink port that connects to the VTEP that received the data message from the prior hop&#39;s VTEP. This encap processor removes the encapsulation header from the data message and stores the STL and SMD attributes as the set of attributes of the data message. It then passes the decapsulated message to the downlink port, which then passes it to the software switch to forward to the next hop&#39;s switch port. From there the data message is processed by the STL module and service proxy before reaching the service node, as described above. 
     When the service proxy determines that the decremented SI value is zero, the service proxy matches the decremented SI value and the embedded SPI value with a rule that directs the service proxy to identify the next hop as the service plane MAC address of the source GVM. In some embodiments, this determination is not specified by a forwarding entry of a forwarding table, but rather is hard coded into the logic of the service proxy. Hence, when the SI value is zero, the proxy provides the source GVM&#39;s service plane MAC address to its associated STL module  626  to use to forward the data message back to the GVM&#39;s host computer. The STL module then defines the message&#39;s destination MAC (DMAC) address as the source GVM&#39;s service plane MAC address while defining the message&#39;s source MAC (SMAC) address as the service plane MAC address associated with its service node (e.g., the service plane MAC of the software switch&#39;s port associated with the service node). It also stores the original SMAC and DMAC of the data message in the attribute set of the data message. 
     The STL module then passes the formatted data message and its attributes along the egress path, where it reaches it associated software switch port. The software switch then passes this message to its uplink port. The encap processor  628  of this port then captures this data message, defines an encapsulating overlay header for this data message and encapsulates the data message with this overlay header. As mentioned above, this overlay header is a Geneve header that stores the SMD and STL attributes in one or more TLVs. This encapsulated data message then traverses the overlay network to reach the source GVM&#39;s host computer, where this data message is decapsulated by the downlink port&#39;s encap processor, and is then provided to the software switch, which then forwards it to the port proxy. 
     Once the port proxy  620  receives the decapsulated data message, it identifies the GVM associated with this data message from the original source MAC address that is now part of the decapsulated data message&#39;s stored attributes. In some embodiments, the port proxy has a record that maps the original source MAC address and service direction in the SMD attributes of a received data to a GVM on its host (e.g., to a software switch port associated with a guest forwarding plane and a GVM on its host). The port proxy then formats the data message to include its original SMAC and DMAC and provides the data message back to the source GVM&#39;s IO chain. The SI post-processor  612  in this IO chain then processes this data message, before returning this data message to the egress datapath of the GVM. The operations of this post-processor will be further described below. 
     One of ordinary skill will realize that the service insertion layer and service transport layer in other embodiments are implemented differently than the exemplary implementations described above. For instance, instead of using an L2 overlay (L2 transport layer) that relies on MAC addresses to traverse the different service hops, other embodiments use an L3 overlay (L3 transport layer) that uses L3 and/or L4 network addresses to identify successive service hops. Also, the above-described service insertion and/or transport modules can be configured to operate differently. 
     A more detailed example of the operations of the service insertion and service transport layers will now be described by reference to  FIGS.  7 - 16   .  FIG.  7    illustrates a process  700  performed by the SI pre-processor  610  and STL caller  624  of some embodiments. This process is described below by reference to the data flow example illustrated in  FIG.  8   . The process  700  starts when the SI pre-processor  610  is called to analyze a data message that is sent along the ingress or egress datapath of a GVM. 
     As shown, the process  700  initially determines (at  705 ) whether the pre-processor  610  has previously selected a service chain and a service path for the data message&#39;s flow and stored the SMD attributes for the selected service chain and path. In some embodiments, the process  700  makes this determination by using the data message&#39;s attributes (e.g., its five tuple attributes) to try to identify a record for the message&#39;s flow in a connection tracker that stores records of message flows for which service chains and paths were previously selected, and SMD attributes were previously stored for these chains and paths in the connection tracker records. 
       FIG.  8    illustrates the pre-processor  610  receiving a data message  802  along the egress datapath of the GVM  102 . It also shows the pre-processor initially checking a connection tracking storage  804  to try to find a connection record that has a flow identifier (e.g., a five-tuple identifier) that matches a set of attributes (e.g., five tuple attributes) of the received data message. In this example, the pre-processor  610  cannot find such a connection record as the received data message is the first data message for its flow. 
     When the process  700  determines (at  705 ) that the connection storage  804  has a connection record that matches the received data message, the process retrieves (at  710 ) the SMD attributes from this record, or from another record referenced by the matching connection record. The SMD attributes in some embodiments include the SCI, SPI, SI and direction values. From  710 , the process transitions to  740 , which will be described below. 
     On the other hand, when the process  700  determines (at  705 ) that the connection storage  804  does not have a connection record that matches the received data message, the process performs (at  715 ) a classification operation that tries to match the data message to a service insertion rule in a SI rule storage, which is illustrated in  FIG.  8    as storage  806 . In some embodiments, the SI rule storage  806  stores service insertion rules  822  that have rule identifiers defined in terms of one or more data message flow attributes (e.g., one or more of the five tuple attributes or portions thereof). Each service rule also specifies a SCI that identifies a service chain that is applicable to data message flows that match the rule identifier of the service rule. 
     At  720 , the process determines whether the classification operation matches the data message&#39;s attributes to the rule identifier of a service insertion rule that requires a service chain to be performed on the data message. When the classification operation does not identify a service insertion rule that requires a service chain to be performed on the data message, the process  700  ends. In some embodiments, the SI rule storage  806  has a default low priority rule that matches any data message when the data message&#39;s attributes do not match any higher priority SI rule, and this default low priority rule specifies that no service chain has been defined for the data message&#39;s flow. No service chain is defined for a data message flow in some embodiments when no service operations needs to be performed on the data message flow. 
     On the other hand, when the classification operation matches the data message&#39;s attributes to the rule identifier of a service insertion rule that requires a service chain to be performed on the data message, the process  700  performs ( 725 ) a path selection operation to select a service path for the service chain specified by the service insertion rule identified at  715 . As shown in  FIG.  8   , the pre-processor  610  performs a path-selection operation by examining a path storage table  808  that identifies one or more service paths for each service chain identifier. 
     Each service path is specified in terms of its SPI value. When multiple service paths are specified for a service chain, the path storage  808  stores for each service chain a set of selection metrics  820  for selecting one SPI from the available SPIs. Different embodiments use different selection metrics. For instance, some embodiments use a selection metric that costs a service path based on the number of hosts on which the service nodes of the service path execute. In other embodiments, these selection metrics are weight values that allow the pre-processor to select SPIs for a service chain in a load balanced manner that is dictated by these weight values. For instance, in some embodiments, these weight values are generated by a central control plane based on the load on each of the service nodes in the service path and/or based on other costs (such as number of hosts traversed by the service path, etc.). 
     In some of these embodiments, the pre-processor maintains a record of previous selections that it has made for a particular service chain, and selects subsequent service paths based on these previous selections. For example, for four service paths, the weight values might be 1, 2, 2, 1, which specify that on six successive SPI selections for a service chain, the first SPI should be selected once, the second and third SPIs should then be selected twice each, and the fourth SPI should be selected one. The next SPI selection for this service chain will then select the first SPI, as the selection mechanism is round robin. 
     In other embodiments, the weight values are associated with a numerical range (e.g., a range of hash values) and a number is randomly or deterministically generated for each data message flow to map the data message flow to a numerical range and thereby to its associated SPI. In still other embodiments, the hosts LCP selects one service path for each service chain identifier from the pool of available service paths, and hence stores just one SPI for each SCI in the path table  808 . The LCP in these embodiments selects the service path for each service chain based on costs (such as the number of hosts traversed by each service path and/or the load on the service nodes of the service paths). 
     After identifying a service path for the identified service chain, the process  700  next identifies (at  730 ) the network address for the first hop of the selected service path. In some embodiments, the MAC address for this hop is stored in the same record as the selected path&#39;s SPI. Hence, in these embodiments, this MAC address is retrieved from the path selection storage  808  with the selected SPI. In other embodiments, the pre-processor retrieves the first hop&#39;s MAC address from an exact match forwarding table  810  that stores next hop network addresses for associated pairs of SPI/SI values, as shown in  FIG.  8   . In some embodiments, the initial SI values for the service chains are stored in the SI rules of the SI rule storage  806 , while in other embodiments, these initial SI values are stored with the SPI values in that path table  808 . 
     At  735 , the process  700  specifies the SMD attributes for the data message, and associates these attributes with the data message. As mentioned above, the SMD attributes include in some embodiments the SCI, the SPI, SI and direction values. The service directions for service paths are stored with the SPI values in the path table  808  as the directions through the service chains are dependent on the service paths. Also, as mentioned below, a service chain in some embodiments has to be performed in a forward direction for data messages from a first GVM to a second GVM, and then in the reverse direction for data messages from the second GVM to the first GVM. For such service chains, the pre-processor  610  selects both the service path for the forward direction and the service path for the reverse direction when it processes the first data message in the flow from the first GVM to the second GVM. 
     After the SI pre-processor completes its operation, the STL caller  624  in the same datapath calls (at  740 ) the STL port proxy  620  to relay the SMD attributes and first hop&#39;s network address that the pre-processor identified, so that the port proxy can forward the SMD attributes through the service plane to the first hop. The operation of the port proxy  620  as well as other modules in the service insertion layers and service transport layers will be described by reference to  FIGS.  9 - 16   . These figures describe an example of processing the data message from GVM  102  through a service path that includes the SVM  106 , then SVM  108  and then SVM  110 . 
     In these figures, each GVM is a compute machine of a tenant in a multi-tenant datacenter, and connects to the software switch through a switch port that is associated with a guest VNI (GVNI) of the tenant. Also, in these figures, each SVM is a service machine for processing the GVM message traffic, and connects to the software switch through a switch port that is associated with a service VNI (SVNI) of the tenant. As mentioned above and further described below, some embodiments use the GVNI for performing the guest logical forwarding operations (i.e., for establishing a guest logical forwarding element, e.g., a logical switch or router, or a guest logical network) for the tenant, while using the SVNI for performing the service logical forwarding operations for the tenant (i.e., for establishing a service logical forwarding element, e.g., a logical switch or router, or a service logical network). 
     Both of these logical network identifiers (i.e., the GVNI and SVNI) are generated for the tenant by the management or control plane in some embodiments. The management or control plane of some embodiments generates different GVNIs and SVNIs for different tenants such that no two tenants have the same GVNI or SVNI. In some embodiments, each SVM is dedicated to one tenant, while in other embodiments, an SVM can be used by multiple tenants. In the multi-tenant situation, each SVM can connect to different ports of different service planes (e.g., different logical switches) for different tenants. 
     As shown in  FIG.  9   , the port proxy  620  formats the data message for forwarding to the first service node, by replacing the original source and destination MAC addresses in the data message with a service plane MAC address that is associated with the source GVM  102  and the MAC address of the first hop service node. This operation is depicted as operation  1005  in the process  1000  of  FIG.  10   . This process  1000  is a process that the port proxy  620  or STL module  626  starts whenever an SI module (such as an SI pre-processor  610  or a SI proxy  614 ) is done processing a data message. 
     In this process  1000 , the port proxy also adds (at  1010 ) the original source and destination MAC addresses of the data message to the set of attributes for the data message that should be processed by other service transport layer modules (e.g., the vswitch, other STL modules, the encap processor, etc.) on the same host computer. The reformatted data message  902  and the augmented attributed set  904  are depicted in  FIG.  9   . 
     After reformatting the data message and augmenting its attribute set, the port proxy  620  passes (at  1015 ) the formatted data message along with its stored attribute set along its egress path where it reaches the software switch  120 . Based on the destination MAC address (e.g., the first hop MAC address) of the formatted data message, the software switch determines (at  1020 ) whether the next hop&#39;s port is local. This is the case for the example illustrated in  FIG.  9   . Hence, the software switch delivers (at  1025 ) the data message to the switch port associated with the first hop SVM  106 . This port then sends the data message along the SVM&#39;s ingress path, where the data message  902  and its augmented attribute set  904  is identified by the STL module  626  through a function call of the ingress IO chain of the first hop&#39;s SVM, as shown in  FIG.  9   . 
     This STL module  626  then re-formats (at  1030 ) the data message by replacing the GVM&#39;s service plane MAC address and the first hop MAC address (i.e., the MAC address of SVM  106 ) with the original source and destination MAC addresses of the data message, which it retrieves from the augmented attribute set  904 . In retrieving the original SMAC and DMAC addresses, the STL module  626  modifies the data message&#39;s attribute set. The reformatted data message  906  and the modified attributed set  908  are depicted in  FIG.  9   . The STL module then passes this re-formatted data message with its accompanying SMD attributes along the SVM&#39;s ingress path, where it is next processed by this hop&#39;s ingress service proxy  614 . 
       FIG.  11    illustrates a process  1100  that the service proxy  614  performs in some embodiments each time it receives a data message traversing along the ingress path of a service node. As shown, the service proxy initially makes (at  1105 ) a copy of the data message if necessary. For instance, in some embodiments, the service node only needs to receive a copy of the data message to perform its operations. One example of such a service node would a monitoring SVM that needs to obtain a data message copy for its message monitoring or mirroring operation. 
     In these embodiments, the service proxy copies the data messages and performs the remaining operations  1110 - 1125  with respect to this copy, while passing the original data message to the next service hop or back to the source GVM. To forward the original data message to the next service hop or back to the GVM, the service proxy has to perform a next-hop lookup based on the SPI/SI values and then provide the next-hop address (e.g., the next service hop&#39;s address or the service plane MAC of the source GVM) to the STL module to forward. These look up and forwarding operations are similar to those described below by reference to  FIGS.  12 - 14   . 
     Next, at  1110 , the service proxy sets a liveness attribute in the stored SMD attribute set of the data message (which, in some embodiments, might be the data message copy at this point). This liveness attribute is a value that directs the service node to provide a responsive liveness value (the same value or related value) with the data message once it has processed the data message. With this liveness attribute, the service proxy also provides a sequence number, which the service node has to return, or increment and then return, with the responsive liveness value, as described above. 
     At  1115 , the service proxy formats the data message, if necessary, to put it in a form that can be processed by the service node. For instance, when the service node does not know the current next hop MAC that is set as the destination MAC of the data message, the service proxy changes the destination MAC of the message to a destination MAC associated with the service node. 
     After formatting the data message to sanitize it for forwarding to the service node, the service proxy  614  encapsulates (at  1120 ) the data message with one of three encapsulation headers that it can be configured to use, and passes (at  1125 ) the encapsulated message along the service node&#39;s ingress path so that it can be forwarded to the service node.  FIG.  9    illustrates the encapsulated data message  920  passing from the service proxy to the SVM  106  with a native NSH encapsulation header. As shown, the encapsulating header  922  includes the service chain identifier, the service index, service chain direction and liveness signal. Numerous techniques for applying and using encapsulating headers are described in U.S. patent application Ser. No. 16/444,826, now issued as U.S. Pat. No. 11,042,397, which is incorporated herein by reference. Any of these techniques can be used in conjunction with the embodiments described herein. 
     The SVMs of some embodiments perform service operations on data messages, then encapsulate the message with an encapsulating header and provide the encapsulated message to the service proxy. Various service operations performed by SVMs of some embodiments are described in U.S. patent application Ser. No. 16/444,826, which is incorporated herein by reference. Any of these operations can be performed by SVMs in conjunction with the embodiments described herein. 
     In some embodiments, the SVM can also set flow programming attribute(s) in the encapsulating header to direct the service proxy to modify the service processing of the data message&#39;s flow. This flow programming will be further described below. After encapsulating the data message, the SVM forwards the data message along its egress path.  FIG.  12    illustrates an example of SVM  106  returning the encapsulated data message  1202  with the SMD and liveness attributes in its encapsulating header  1204 . 
       FIG.  13    illustrates a process  1300  that the service proxy  614  performs in some embodiments each time it receives a data message traversing along the egress path of its service node. As shown, the service proxy in some embodiments initially (at  1305 ) removes the encapsulation header from the data message, removes the SMD attributes from this header, and stores these attributes in an attribute set that it creates for the data message. In some embodiments, the service proxy retrieves (at  1305 ) some or all of the SMD attributes (e.g., the SPI value, the service plane MAC address of the source GVM) for the data message from a previous record that the service proxy created before giving the data message to the service node along the ingress path.  FIG.  12    illustrates an example of the attribute set  1206  that the service proxy  614  creates for the decapsulated data message  1207 . 
     Next,  1310 , the process resets the liveness timer (e.g., a timer that expires every 0.25 seconds) that it maintains to account for the liveness value that it has received from the service node, which signifies that this node is still operational. With this liveness value, the service proxy receives from the service node a sequence number, which the process validates to ensure that it is the next liveness value that needs to be received. 
     At  1315 , the process determines whether the SVM specified any flow programming attribute(s), which require the service proxy to direct the SI post processor  612  for the source GVM to perform flow programming by sending to the post processor  612  in-band data messages. In some embodiments, the service proxy sends an in-band flow programming control signal with another data message that it generates to send back to the source GVM, where it will be intercepted by its post processor  612 . 
     When the source GVM receives the data message with the flow programming control signal, its post processor can uniquely identify the data message flow to which it applies by using a flow identifier that is unique to this flow. As further described below, this flow identifier is derived partially based on a unique identifier of the source GVM. The unique flow identifier also allows other service plane modules, such as the service nodes, service proxies and STL modules, to uniquely identify each data message flow. This unique flow identifier in some embodiments is part of the SMD attributes that are passed between the service hops of a service path and passed back to the source GVM. 
     In some embodiments, however, the service proxy sends the in-band flow programming control signal with the current data message that it is processing. In some of these embodiments, the service proxy does this only when its associated service node is the last hop service node of the service path, while in other embodiments it does this even when its service node is not the last hop service node. When its service node is not the last hop service node of the service path, the service proxy embeds the flow programming in the SMD attributes of the data message, which in some embodiments eventually get forwarded to the source GVM&#39;s SI post processor as part of the data message encapsulation header when the last hop service is performed. Even in this situation, the service proxy of the last hop in other embodiments sends the flow programming signal as a separate message. 
     The flow programming signals will be further described below by reference to  FIG.  17   . Also, as further described below, the service proxy also sends flow programming signals back to the source GVM when it detects that its service node has failed so that the classifier at the source GVM can select another service path for the current data message flow, as well as other data message flows. In such a situation, the service proxy also notifies the LCP on its host computer, so that the LCP can notify the CCP and the CCP, in turn, can modify the service paths specified for service chains that use the failed service node. 
     At  1320 , the process  1300  determines whether its service node specified that the data message should be dropped. If so, the process drops the data message and then ends. Otherwise, assuming the data message should not be dropped and should continue along its service path, the service proxy in some embodiments decrements (at  1325 ) the SI value in case the service node has not decremented the SI value, and then uses (at  1330 ) this decremented value along with the SPI value in the data message&#39;s stored attribute set to identify an exact match forwarding rule that identifies a next hop network address. When the proxy&#39;s service node is on multiple service paths, the proxy&#39;s forwarding rule storage stores multiple exact match forwarding rules that can specify different next hop network addresses for different SPI/SI values. 
     When the decremented SI value is zero, the service proxy in some embodiments that matches the decremented SI value and the embedded SPI value with a rule that directs the service proxy to identify the next hop as the service plane MAC address of the source GVM. This rule in some embodiments does not provide a MAC address, but rather refers to the service plane MAC address that is part of the SMD attribute set stored for the data message. In some embodiments, this instructions for returning the data message to the service plane MAC address of the source GVM when the SI value is zero is not specified by a forwarding entry of a forwarding table, but rather is hard coded into the logic of the service proxy. 
     At  1330 , the service proxy stores the next hop network address (e.g., MAC address) in the attribute set that is stored for the data message.  FIG.  12    illustrates an example of the service proxy  614  storing the next hop MAC address associated with the next service node in the attribute set  1206  of the decapsulated data message  1207 . After identifying the next hop network address, the service proxy returns (at  1335 ) the data message to the egress path of its service node, and the process  1300  ends. 
     Once the service proxy returns the data message to the service node&#39;s egress path, the STL module  626  receives this data message and commences the process  1000  of  FIG.  10   . The STL module  626  performs the first three operations  1005 - 1015  of this process each time it receives a data message from a service insertion layer. Specifically, the STL module formats (at  1005 ) the data message for forwarding to the next hop service node, by replacing the original source and destination MAC addresses in the data message with a service plane MAC addresses of the current service hop and the next service hop (i.e., the hop1mac and hop2mac addresses in the example illustrated in  FIG.  12   ). 
     At  1010 , the STL module also adds the original source and destination MAC addresses of the data message to the set of attributes for the data message that should be processed by other service transport layer modules (e.g., the vswitch, the encap processor, etc.) on the same host computer. The reformatted data message  1208  and the augmented attributed set  1210  are depicted in  FIG.  12   . After reformatting the data message and augmenting its attribute set, the STL module  626  passes (at  1015 ) the formatted data message along the egress path, where it next reaches the software switch  120 . 
     Based on the destination MAC address (i.e., the next hop MAC address) of the formatted data message, the software switch determines (at  1020 ) that the next hop&#39;s port is not local. Hence, the software switch provides (at  1035 ) the data message to the uplink port  1250  that connects to a VTEP1 that communicates through an overlay network tunnel with a VTEP2 on host  114 , as illustrated in the example of  FIG.  12   . As shown, an STL encap processor  628  along the egress path of this uplink port (at  1040 ) receives this data message (e.g., is called as one of the hooks specified for the uplink port), defines an encapsulating overlay header  1240  for this data message and encapsulates the data message with this overlay header. 
     In some embodiments, the overlay header is a Geneve header that stores the SMD and STL attributes in one or more of its TLVs. As mentioned above, the SMD attributes in some embodiments include the SCI value, the SPI value, the SI value, and the service direction. Also, in some embodiments, the STL attributes includes the original L2 source MAC address and the original L2 destination MAC address.  FIG.  12    illustrates an example of this encapsulating header. 
     When the encapsulated data message is received at the next hop&#39;s host computer  114 , the data message is captured by the STL encap processor  628  of (e.g., defined as a hook for) a downlink port  1252  that connects to the VTEP connecting through the overlay network tunnel to the prior hop&#39;s VTEP.  FIG.  14    illustrates a process  1400  started by an encap processor  628  on a next hop computer that receives an encapsulated data message that needs to be processed by an SVM executing on its computer. 
     As shown, this encap processor removes (at  1405 ) the encapsulation header from the data message, and stores (at  1405 ) the STL and SMD attributes as the associated set of attributes of the data message. It then passes (at  1410 ) the decapsulated message to the downlink port, which then passes it to the software switch to forward (at  1415 ) to its port that is connected to the next hop SVM (i.e., that is associated with the destination MAC address). This port then passes the data message  1208  and the attribute set  1210  to the ingress path of the next hop SVM, as shown in the example of  FIG.  12    for the SVM  108 . 
     The STL module  626  on this ingress path then re-formats (at  1420 ) the data message by replacing the previous and current hop service plane MAC address (i.e., the hop1mac and hop2mac) with the original source and destination MAC addresses of the data message, which it retrieves from the data message attribute set. In retrieving the original SMAC and DMAC addresses, the STL module  626  modifies the data message&#39;s attribute set. The reformatted data message  1230  and the modified attributed set  1232  are depicted in  FIG.  12   . The STL module then passes this re-formatted data message with its accompanying SMD attributes along the SVM&#39;s ingress path, where it is next processed by this hop&#39;s ingress service proxy  614 . 
     The operation of this service proxy is as described above by reference to  FIGS.  9  and  11   .  FIG.  12    shows the service proxy of SVM  108  on host  114  passing an encapsulated data message to the SVM. The encapsulating header of this data message is supported by the SVM  108  and stores the SCI, SI, service direction and liveness values. In some embodiments, the SVMs that are part of the same service path support different encapsulating headers. In some of these embodiments, the service proxies along a service path can encapsulate the data message with different encapsulating headers before passing the data message to their associated SVMs. For instance, in one case, the first hop service proxy passes to the SVM  106  the data message with an NSH encapsulating header, while the second hop service proxy passes to the SVM  108  the data message with a QinQ encapsulating header. 
     Once the SVM  108  performs its service operation on the data message, the SVM sends the processed data message along its egress data path, as shown in  FIG.  15   . As shown, the service proxy then identifies the MAC address of the next service hop and adds this MAC address to the stored attribute set for the data message. At this point, the next hop is the third service hop, which corresponds to the SVM  110 . This proxy identifies this MAC by decrementing the SI value (when the SVM  108  did not decrement the SI value) and then using the embedded SPI value and decremented SI value to lookup a forwarding rule that provides the next hop&#39;s MAC address. The STL module in this egress path then replaces the original SMAC and DMAC in the data message with the current hop and next hop MAC addresses (i.e., the hop2mac and the hop3mac in the example of  FIG.  15   ), stores the original SMAC and DMAC in the stored attribute set of the data message, and then passes the data message along the egress path where it is received by the software switch  122 . 
     The software switch then determines that the next hop is associated with its uplink port  1252 , and hence passes the data message to this port. As shown in  FIG.  15   , the encap processor  628  on the egress path of this port (e.g., specified as a hook on this egress path) then encapsulates the data message with a Geneve header that stores the SMD and STL attributes in one or more of TLVs and specifies that the data message is traversing from this port&#39;s associated VTEP2 to VTEP3 that is associated with port  1254  of host  116 . 
     The STL encap processor  628  in the ingress path of port  1254  then removes the encapsulation header from the data message and stores the STL and SMD attributes as the associated set of attributes of the data message. It then passes the decapsulated message to the port  1254 , which then passes it to the software switch  124  to forward to its port connected to the next hop SVM  110  (i.e., to its port associated with the service plane DMAC). This port then passes the data message and attribute set to the ingress path of this SVM, as shown in  FIG.  15   . 
     The STL module  626  in this ingress path replaces the previous and current hop service plane MAC address (i.e., the hop2mac and hop3mac) with the original source and destination MAC addresses of the data message, which it retrieves from the data message attribute set. The STL module  626  also modifies the data message&#39;s attribute set by removing the original SMAC and DMAC addresses, and then passes the re-formatted data message with its accompanying SMD attributes along the SVM&#39;s ingress path for this hop&#39;s ingress service proxy  614  to process. This service proxy passes to the SVM  110  an encapsulated data message with an encapsulating header supported by the SVM  110  and storing the SCI, SI, service direction and liveness values. 
     Once the SVM  110  performs its service operation on this data message, the SVM sends the processed data message along its egress data path, as shown in  FIG.  16   . The service proxy decrements the SI value when assuming that the SVM  110  has not done so already. In this example, the decremented SI value is now zero. In some embodiments, the service proxy then matches this SI value and the SPI value to a rule identifier of a forwarding rule that specifies that it should select the service plane MAC (spmac) of the source GVM as the next hop MAC address. In other embodiments, the hardcoded logic of the service proxy directs it to identify the service plane MAC of the source GVM as the next hop MAC. In either case, the service proxy adds the source GVM&#39;s service plane MAC to the attribute set of the data message. 
     The STL module next replaces the original SMAC and DMAC in the data message with the third hop MAC address and the source GVM&#39;s service plane MAC, stores the original SMAC and DMAC in the stored attribute set of the data message, and then passes the data message to its software switch  124 . The software switch then determines that the next hop is associated with its port  1254 , and hence passes the data message to this port. As shown in  FIG.  16   , the encap processor  628  on the egress path of this port then encapsulates the data message with a Geneve header that stores the SMD and STL attributes in one or more TLVs and specifies that the data message is traversing from this port&#39;s associated VTEP3 to VTEP1 that is associated with port  1250  of host  112 . 
     The STL encap processor  628  in the ingress path of port  1250  then removes the encapsulation header from the data message and stores the STL and SMD attributes as the associated set of attributes of the data message. It then passes the decapsulated message to the port  1250 , which then passes it to the software switch  120  to forward to its port connected to the port proxy  620 . This port then passes the data message and attribute set to the port proxy  620 , as shown in  FIG.  16   . 
     The port proxy  620  then replaces the previous and current hop service plane MAC address (i.e., the hop3mac and spmac) with the original source and destination MAC addresses of the data message, which it retrieves from the data message attribute set. The port proxy  620  also modifies the data message&#39;s attribute set to remove the original SMAC and DMAC, and then passes this re-formatted data message with its accompanying SMD attributes back to the STL caller  624  that called it in the first place. In some embodiments, the port proxy uses a connection record that it created when the STL caller originally called it, to identify the STL caller to call back. In other embodiments, the port proxy uses a mapping table that maps each service plane MAC with a GVM&#39;s STL caller. The mapping table in some embodiments has records that associate service plane MACs and service directions with guest forwarding plane port identifiers associated with the GVMs. 
     Once called, the STL caller passes the data message along the egress path of GVM  102 , where it will next be forwarded to the SI post-processor  612 .  FIG.  17    illustrates a process  1700  that the SI post-processor  612  performs in some embodiments. The post-processor performs this process  1700  each time it receives a data message that is passed to it along a GVM&#39;s IO chain. As shown, the post processor  612  in some embodiments initially determines (at  1705 ) whether it needs to examine the received data message for SI post processing. This is because as a module along a GVM&#39;s IO chain, the post processor will get called for all data message flows that pass along this IO chain and some of these data message might not match an SI rule that requires service insertion operations to be performed on them. In some embodiments, the process  1700  determines (at  1705 ) whether it needs to process the data message by determining whether the data message has associated service metadata. If not, the process transitions to  1720 , which will be described below. 
     When the SI post processor  612  determines that it needs to process the data message, the process determines (at  1710 ) whether the SMD metadata associated with the data message specifies a flow programming tag that requires the post processor to perform a flow programming operation. In some embodiments, such a flow programming tag would be specified in the data message&#39;s SMD attributes by a service node to change the service path processing at the source GVM, or by a service proxy for the same reason when it detects failure of its service node. When the flow programming tag does not specify any flow programming, the process transitions to  1720 , which will be described below. 
     Otherwise, when the flow programming tag specifies a flow programming operation, the process  1700  performs this operation, and then transitions to  1720 . The flow programming operation entails in some embodiments modifying the connection record in the connection tracking storage  804  to specify the desired operation and/or SMD attributes (e.g., allow, drop, etc.) for the data message&#39;s flow. The post processor&#39;s writing to the connection tracker  804  is depicted in  FIG.  16   . As mentioned above and further described below, the SMD metadata for the processed data message includes a flow identifier that uniquely identifies the data message&#39;s flow by being at least partially derived from the unique service plane identifier of the source GVM. The post processor  612  uses this flow identifier to match the data message&#39;s flow in the connection tracker in some embodiments. 
     In some embodiments, the flow programming tag can specify the following operations (1) NONE when no action is required (which causes no flow programming operation to be performed), (2) DROP when no further data messages of this flow should be forwarded along the service chain and instead should be dropped at the source GVM, (3) ACCEPT when no further data messages of this flow should be forwarded along the service chain and instead the flow should be accepted at the source GVM. In some embodiments, the flow programming tag can also specify DROP_MESSAGE. The DROP_MESSAGE is used when the service node needs to communicate with the proxy (e.g. to respond to a ping request) and wants the user data message (if any) to be dropped, even though no flow programming at the source is desired. 
     In some embodiments, an additional action is available for the service proxies to internally communicate failure of their SVMs. This action would direct the SI post processor in some embodiments to select another service path (e.g., another SPI) for the data message&#39;s flow. This action in some embodiments is carried in-band with a user data message by setting an appropriate metadata field in some embodiments. For instance, as further described below, the service proxies communicate with the post processor of the source GVM through OAM (Operation, Administration, and Maintenance) metadata of the NSH attributes through in-band data message traffic over the data plane. Given that by design flow programming actions are affected by signaling delays and are subject to loss, an SVM or service proxy might still see data messages belonging to a flow that was expected to be dropped, accepted or re-directed at the source for some time after communicating the flow programming action to the proxy. In this case, the service plane should continue set action to drop, allow or redirect at the source. 
     The process  1700  transitions to  1720  after completing the flow programming operation. It also transitions to  1720  when it determines (at  1705 ) that no SI post processing needs to be performed on the data message or determines that no flow programming needs to be performed for this data message. At  1720 , the process  1700  lets the data message through the egress path of GVM  102 , and then ends. 
     The examples described above by reference to  FIGS.  8 ,  9 ,  12 ,  15 , and  16    show service plane operations that are performed on a data message that is identified along the egress path of a source GVM. These service plane operations (described by reference to  FIGS.  7 ,  10 ,  11 ,  13 ,  14  and  17   ) are equally applicable to data messages that are identified as they traverse along the ingress path of a source GVM. To perform these ingress side operations, the SI pre and post processors  610  and  612  on the ingress path are flipped as compared to the locations of these two processors on the egress path. Specifically, as shown in  FIG.  6   , the preprocessor  610  receives a data message that enters the GVM&#39;s ingress path from the software switch port that is associated with this GVM&#39;s VNIC, while the post processor  612  passes the processed data message along the ingress IO chain to the GVM&#39;s VNIC. 
     However, the service insertion and service transport operations for the ingress side processing are similar to the egress side processing of data messages to and from a particular GVM. In some cases, this GVM exchanges data messages with another GVM. As described above by reference to  FIGS.  4  and  5   , the service plane can be directed to perform the same service chain on the data messages in each direction, but in the opposite order. In such cases, the service nodes for the service path on the ingress side perform a series of service operations for a first direction of the service chain for data messages that the other GVM sends to the particular GVM, while the service nodes for the service path on the egress side perform the same series of service operations but in a second, opposite direction through the service chain. Also, as mentioned above, the two sets of service nodes for the forward and reverse directions include the same service nodes in some embodiments. 
       FIG.  18    conceptually illustrates several operations that the network managers and controllers perform in some embodiments to define rules for service insertion, next service hop forwarding, and service processing. As shown, these operations are performed by a service registrator  1804 , a service chain creator  1806 , a service rule creator  1808 , a service path generator  1810 , a service plane rule generator  1812 , and a rule distributor  1814 . In some embodiments, each of these operators can be implemented by one or more modules of a network manager or controller and/or can be implemented by one or more standalone servers. 
     Through a service partner interface  1802  (e.g., a set of APIs or a partner user interface (UI) portal), the service registrator  1804  receives vendor templates  1805  that specify services that different service partners perform. These templates define the partner services in terms of one or more service descriptors, including service profiles. The registrator  1804  stores the service profiles in a profile storage  1807  for the service chain creator  1806  to use to define service chains. 
     Specifically, through a user interface  1818  (e.g., a set of APIs or a UI portal), the service chain creator  1806  receives from a network administrator (e.g., a datacenter administrator, a tenant administrator, etc.) one or more service chain definitions. In some embodiments, each service chain definition associates a service chain identifier, which identified the service chain, with an ordered sequence of one or more service profiles. Each service profile in a defined service chain is associated with a service operation that needs to be performed by a service node. The service chain creator  1806  stores the definition of each service chain in the service chain storage  1820 . 
     Through the user interface  1818  (e.g., a set of APIs or a UI portal), the service rule creator  1808  receives from a network administrator (e.g., a datacenter administrator, a tenant administrator, etc.) one or more service insertion rules. In some embodiments, each service insertion rule associates a set of data message flow attributes with a service chain identifier. The flow attributes in some embodiments are flow header attributes, like L2 attributes or L3/L4 attributes (e.g., five tuple attributes). In these or other embodiments, the flow attributes are contextual attributes (e.g., AppID, process ID, active directory ID, etc.). Numerous techniques for capturing and using contextual attributes for performing forwarding and service operations are described in U.S. patent application Ser. No. 15/650,251, which are incorporated herein. Any of these techniques can be used in conjunction with the embodiments described herein. 
     The service rule creator  1808  generates one or more service insertion rules and stores these rules in the SI rule storage  1822 . In some embodiments, each service insertion rule has a rule identifier and a service chain identifier. The rule identifier in some embodiments can be defined in terms of flow identifiers (e.g., header attributes, contextual attributes, etc.) that identify data message flow(s) to which the SI rule is applicable. The service chain identifier of each SI rule, on the other hand, identifies the service chain that has to be performed by the service plane for any data message flow that matches the rule identifier of the SI rule. 
     For each service chain that is part of a service rule, the service path generator  1812  generates one or more service paths, with each path identifying one or more service instance endpoints for one or more service nodes to perform the service operations specified by the chain&#39;s sequence of service profiles. In some embodiments, the process that generates the service paths for a service chain accounts for one or more criteria, such as (1) the data message processing load on the service nodes (e.g., SVMs) that are candidate service nodes for the service paths, (2) the number of host computers crossed by the data messages of a flow as they traverse each candidate service path, etc. 
     The generation of these service paths is further described in U.S. patent application Ser. No. 16/282,802, now issued as U.S. Pat. No. 11,012,351, which is incorporated herein by reference. As described in this patent application, some embodiments identify the service paths to use for a particular GVM on a particular host based on one or more metrics, such as host crossing count (indicating how many times a data message traversing the service path crosses hosts), a locality count (indicating how many of the SIRs along this path are located on the local host), etc. Other embodiments identify service paths (i.e., select service nodes for service paths) based on other metrics, such as financial and licensing metrics. 
     The service path generator  1812  stores the identity of the generated service paths in the service path storage  1824 . This storage in some embodiments associates each service chain identifier to one or more service path identifiers, and for each service path (i.e., each SPI) it provides a list of service instance endpoints that define the service path. Some embodiments store the service path definitions in one data storage, while storing the association between the service chain and its service paths in another data storage. 
     The service rule generator  1810  then generates rules for service insertion, next service hop forwarding, and service processing from the rules stored in storages  1820 ,  1822  and  1824 , and stores these rules in rule storages  1826 ,  1828  and  1830 , from where the rule distributor  1814  can retrieve these rules and distribute them to the SI pre-processors, service proxies and service nodes. The distributor  1814  also distributes in some embodiments the path definitions from the service path storage  1824 . The path definitions in some embodiments includes the first hop network address (e.g., MAC address) of the first hop along each path. In some embodiments, the service rule generator  1810  and/or the rule distributor  1814  specify and distribute different sets of service paths for the same service chain to different host computers, as different sets of service paths are optimal or preferred for different host computers. 
     In some embodiments, the SI classification rules that are stored in the rule storage  1826  associate flow identifiers with service chain identifiers. Hence, in some embodiments, the rule generator  1810  retrieves these rules form the storage  1822  and stores them in the classification rule storage  1826 . In some embodiments, the rule distributor  1814  directly retrieves the classification rules from the SI rule storage  1822 . For these embodiments, the depiction of the SI classification rule storage  1826  is more of a conceptual illustration to highlight the three type of the distributed rules, along with the next-hop forwarding rules and the service node rules. 
     In some embodiments, the service rule generator  1810  generates the next hop forwarding rules for each hop service proxy of each service path for each service chain. As mentioned above, each service proxy&#39;s forwarding table in some embodiments has a forwarding rule that identifies the next hop network address for each service path on which the proxy&#39;s associated service node resides. Each such forwarding rule maps the current SPI/SI values to the next hop network address. The service rule generator  1810  generates these rules. For the embodiments in which the SI pre-processor has to look-up the first hop network address, the service rule generator also generates the first hop look-up rule for the SI pre-processor. 
     Also, in some embodiments, the service rule generator  1810  generates for the service nodes service rules that map service chain identifier, service index values and service directions to service profiles of the service nodes. To do this, the service rule generator uses the service chain and service path definitions from the storages  1820  and  1824 , as well as the service profile definitions from the service profile storage  1807 . In some embodiments, the rule distributor forwards the service node rules to a service node through a service manager of the service node when such a service manager exists. The service profile definitions are also distributed by the distributor  1814  to the host computers (e.g., to their LCPs) in some embodiments, so that these host computers (e.g., the LCPs) can use these service profiles to configure their service proxies, e.g., to configure the service proxies to forward received data messages to their service nodes, or to copy the received data messages and forward the copies to their service nodes, while forwarding the original received data messages to their next service node hops or back to their source GVMs when they are the last hops. 
     In some embodiments, the management and control plane dynamically modify the service paths for a service chain, based on the status of the service nodes of the service paths and the data message processing loads on these service nodes.  FIG.  19    illustrates how service paths are dynamically modified in some embodiments. In these embodiments, a central control plane  1900  works with a local control plane  1910  on the host computers  1920  to define service paths for a service chain, and to modify these service paths. The CCP  1900  in some embodiments is a cluster of servers (e.g., three servers) that provide control plane operations for defining configurations based on service rules specified by network administrators through a cluster of management servers that provide management operations. 
     As shown, the CCP has a status updater  1902  that receives service node status data from status publishers  1903  on the host computers  1920 . As mentioned above, each time that a service proxy determines that its associated service node has failed (e.g., each time a service node fails to respond to the service proxy&#39;s liveness signal twice in a row), the service proxy notifies the LCP  1910  of its host. The LCP then has its status publisher  1903  notify the CCP&#39;s status updater  1902  of the service node&#39;s failure. 
     The status updater  1902  relays any service node failures to the service path generator  1812 , which in some embodiments is part of the CCP along with the SP rule generator  1810  and a statistic collector  1904 . Each time a service node fails, the service path generator removes from the service path storage  1824  its previously defined service paths that use this service node. For each removed service path, the service path generator  1812  deletes or deactivates the removed path&#39;s SPI value for the service chain identifier of the corresponding service chain. 
     In some embodiments, each removed service path is removed (e.g., deleted or deactivated) from the records of all hosts that previously received forwarding rules or path definitions that were for this service path. In some embodiments, the CCP (e.g., the service path generator  1810  or the rule distributor  1814 ) directs these hosts to remove the service path from the forwarding and path definition rules of their forwarding rule storages  1928  and path definition storage  808 . The LCP of the failed service node in some embodiments removes the service path from its forwarding and path definition rules, while in other embodiments even this LCP waits for instructions to do so from the CCP. 
     Each host  1920  also has a statistics publisher  1905  that publishes data message load statistics that the service proxies generate for their service nodes in some embodiments. Each time a service proxy receives a data message that has been processed by its service node, the service proxy in some embodiments increments statistics (e.g., data message count, byte count, etc.) that it maintains in a statistic storage  1907  for its service node. In some embodiments, the statistics publisher  1905  periodically or on-demand retrieves the collected statistics from the storage  1907  and forwards these statistics to a statistic collector  1904  of the CCP. In some embodiments, the statistics collector  1904  receives (through the management plane) statistics that the service managers of the service nodes receive from the service nodes. 
     The statistics collector  1904  relays the collected statistics to the service path generator  1812 . As mentioned above, the service path generator in some embodiments defines the service paths through the service nodes based in part on the data message load on the service nodes. For instance, when the data message load on a service node exceeds a threshold value, the service path generator performs one or more actions in some embodiments to reduce the load on this service node. For instance, in some embodiments, it stops adding the service node to any new service paths that it might define. In these or other embodiments, it also directs the distributor  1814  to remove the service paths that use this service node from some or all of the hosts. 
     Conjunctively or alternatively, the service path generator directs a CCP module (e.g., the distributor  1814 ) to direct the LCPs of one or more host computers to adjust the selection criteria  820  used for selecting service paths that the LCPs generate in order to control how the SI pre-processor performs its path selections. In other embodiments, the service path generator or another CCP module aggregates the load statistics for each service node and distributes the aggregated load to host LCPs along with their associated SPI values so that the LCPs can analyze these statistics and adjust the path selection criteria that they generate. In some embodiments, each LCP uses or has a path evaluator  1915  to generate the path selection criteria to evaluate and select paths based on service node statistics, and/or based on other criteria, such as number of hosts traversed by each service path. 
     In some embodiments, the servers that implement the management plane, the control plane, the service managers are in the same datacenter as the host computers on which the guest and service machines and modules (e.g., GVMs, SVMs, service proxies, port proxies, STL modules, SFEs, etc.) execute. In these embodiments, the management plane servers, the control plane servers, the service managers and the host computer modules (e.g., the LCPs, SVMs, GVMs, hypervisor modules, etc.) communicate with each other through the shared network infrastructure (e.g., the switches, routers, wired and wireless links, etc.) of the datacenter. 
     In other embodiments, the management plane servers, the control plane servers, the service managers and/or the host computers operate in different datacenters (e.g., enterprise private datacenters and public cloud datacenters). In some such embodiments, management plane servers, the control plane servers, the service managers and/or the host computer modules (e.g., the LCPs, SVMs, GVMs, hypervisor modules, etc.) communicate with each other through network infrastructures outside of their respective datacenters. Also, some such embodiments implement the service transport layer as a distributed logical L3 routers and/or network that spans multiple datacenters (e.g., multiple private datacenters, multiple public datacenters, and/or multiple private/public datacenters). 
     In order to properly control and coordinate the above described service chains, some embodiments provide a graphical user interface (GUI). The GUI of some embodiments allows a user to select a particular service chain.  FIG.  20    illustrates an example of a GUI  2000  of some embodiments that displays the elements implementing a selected service chain. GUI  2000  includes service nodes  2010 A- 2030 B, links  2035 , and path selectors  2040 A- 2040 C. 
     Each of the service nodes  2010 A- 2030 B is a visual representation of a single instantiation of a service. Service nodes  2010 A- 2010 C represent the nodes implementing a first service operation of the service chain, service nodes  2020 A- 2020 B represent the nodes implementing a second service operation, and service nodes  2030 A- 2030 B represent the nodes implementing a third service operation. 
     In the GUI  2000 , between the various service nodes  2010 A- 2030 B are links  2035  connecting each successive pair of service nodes. The links  2035  of GUI  2000  provide a concurrent representation of multiple service paths in the service chain. The links  2035  are shown in  FIGS.  20 - 24    as directly connecting service nodes. However, one of ordinary skill in the art will understand that the links  2035  identify a sequence of service operations of the service chain rather than representing direct communication between service nodes. In some network systems, service nodes of a service path may communicate directly. However, in other network systems the processed data from one service node may be sent to one or more intermediary systems before those systems send the processed data to a service node implementing the next service in the service path. The links in the illustrated embodiments are shown between service nodes. However, in other embodiments, there may be similar links or other indicators after the service nodes on the end nodes of a service chain to indicate that data messages are sent to other entities (e.g., to be sent to other virtual machines on the network, machines external to the network, etc.) after the last service node in the service chain. Examples of links shown after end nodes of a service chain are illustrated in  FIGS.  22 A- 22 B . 
     Each service path of the service chain passes through one service node of each service. The GUI  2000  provides path selectors  2040 A- 2040 C as user selectable objects that each allow a user to select a particular path to view. For reasons of clarity and space, the figures herein include path selectors for only the service nodes of the first service (i.e., service nodes  2010 A- 2010 C). However, one of ordinary skill in the art will understand that other embodiments may include path selectors for other nodes. The effects of user interaction with the path selectors  2040 A- 2040 C are further described with respect to  FIGS.  22 A and  22 B , below. 
     In addition to displaying the service nodes and links of a service chain, the GUI  2000  also displays operational data for various components of the service chain in response to user commands.  FIG.  21    illustrates the GUI  2000  displaying operational data for a service node in response to a user selection of the node.  FIG.  21    shows service node representation  2020 A, data display area  2110 , and cursor  2120 . Service node  2020 A is one of two nodes that implement the second service in the service chain. Data display area  2110  is a pop-up display that shows operational data for service node  2020 A. Cursor  2120  represents a cursor of a control device used to interact with the GUI  2000 . 
     The GUI  2000  displays a data display area with operational data relating to a particular node when a user selects that node. As shown in this figure, the cursor  2120  hovers (e.g., is placed over) over the service node  2020 A to select it. In response to the selection, the GUI  2000  provides the data display area  2110  with operational data relating to service node  2020 A. The operational data in the display area  2110  may include a latency measurement of the node, throughput of the node, error measurements, or other data about the node. 
     In addition to latency and throughput, the display area of some embodiments may provide various types of additional counters, such as (1) initial counters, (2) error counters, (3) latency and byte counters, and/or (4) liveness check counters. Initial counters provide the user traffic and liveness packet counts to/from a serviceVM vNIC. Examples of initial counters in some embodiments include rx_from_svm, rx_from_iochain, to_svm_liveness_pkts, and from_svm_liveness_pkts. Error counters show measurements of different types of errors that can cause packet drop by the serviceVM proxy (e.g., the proxy sitting on the service VM vNIC). Examples of error counters in some embodiment include native_length_patch_fail, bad_eth_hdr, quench, get_nsh_fail, ingress_inject_fail, copy_fail, nsh_ttl_expired, zero_si, unknown_next_hop, set_nsh_fail, push_headroom_fail, insufficient_decap_bytes, insufficient_spf_bytes, and insufficient_map_bytes, partial_copy. Latency and bytes counters can be utilized to show latency cause and throughput per hop in the service chain. In some embodiments, the latency and byte counters can also be utilized for the whole chain (i.e., to evaluate the performance of one chain with the performance of another chain). The previously mentioned latency (shown in ms) and throughput counters are examples of latency and byte counters. Other examples of latency and byte counters in some embodiments include service_latency_seconds, rx_bytes_from_svm, and rx_bytes_from_iochain. The liveness check counters are used for debugging whether liveness checks are happening as expected (when liveness checks are enabled). Examples of liveness check counters in some embodiments include liveness_checks_performed, svm_alive, liveness_bitmap, and sequence_number. 
     In some embodiments, any or all of these counters can be exposed in the GUI and in some embodiments can be extracted from the same CLI. In some embodiments, all of the above counters can help with isolating any specific hop in the service chain; for determining packet drops, latency, throughput, and/or overall liveness status. Further description of how the operational data is generated is described with respect to  FIGS.  26 - 28   , below. 
     Although the GUI of the illustrated embodiment displays operational data in response to a hover operation, one of ordinary skill in the art will understand that other user command operations are used in other embodiments. For example, in some embodiments, a cursor-click with a particular button, a keyboard hotkey, a click-and-drag operation, rather than a hover operation, are used to select a node. 
     In the preceding figure, the GUI  2000  displays operational data for a single service node. However, the GUI of some embodiments allows a user to view operational data for all service nodes on a particular service path.  FIG.  22 A  illustrates the example GUI  2000  displaying operational data for the nodes along a service path in response to a user selection of the service path.  FIG.  22 A  shows path selectors  2040 B, including path selector  2210 , selected path  2215 , data display areas  2220  and  2225 , and aggregate display area  2228 . As previously mentioned with respect to the links of  FIG.  20   , the illustrated paths of  FIGS.  22 A and  22 B  identify the order in which the services of the nodes they connect are performed and do not indicate direct communication between the linked nodes. 
     In the GUI  2000  the number of path selectors associated with a service node of the first service indicate how many service paths include that particular service node. Thus, the selectors indicate four paths that include service node  2010 A, two paths that include service node  2010 B, and three paths that include service node  2010 C. In this figure, the user has selected path selector  2210 , one of the service path selectors associated with service node  2010 B, with a cursor operation (e.g., hover, click, etc.). In response to this selection, the GUI  2000  has highlighted the selected service path  2215 . The path  2215  goes through service nodes  2010 B,  2020 A, and  2030 B then out of node  2030 B. After leaving node  2030 B, data packets are forwarded on toward their destinations. One of ordinary skill in the art will understand that although the illustrated implementation shows path  2215  extending beyond node  2030 B, in other embodiments the display shows the path  2215  stopping at  2030 B. 
     In some embodiments, the paths are bi-directional. That is, an initial set of packets are sent along the path, through service nodes  2010 B,  2020 A, and  2030 B in that order, while reply packets (packets with source and destination reversed) are sent back through the same service path in the reverse order, through service nodes  2030 B,  2020 A, and  2010 B in that order. 
     The GUI  2000  provides operational data for each service node along the selected path  2215 . That is, the GUI  2000  displays operational data for: (1) node  2010 B in display area  2220 ; (2) node  2020 A in display area  2110 ; and (3) node  2030 B in display area  2225 . Thus the user can determine the operational performance of all nodes along the service path by reading the operational statistics for each node. In addition to displaying operational data for each individual node in the path, the GUI  2000  also displays an aggregate display area  2228 . The aggregate display area  2228  provides operational data that is the aggregate of the individual paths. The display area  2228  identifies which service path is currently selected for display (here Service Path  6 ) and the policy rule that causes data to be sent along this path (here, Policy Rule  12 ). The counters of the aggregate display area  2228  are individually computed according to stored formulas. How an aggregate value is determined depends on what value is being measured. For example, the overall latency of a path is the sum of the latencies of the individual nodes. However, the throughput of a path is the minimum throughput of the individual nodes rather than the sum of the throughputs. 
     In order to allow problematic nodes to be identified more easily than by reaching all the operational statistics for each node, the GUI of some embodiments provides a visual (e.g., color coded) indicator of whether the nodes are operating within preset thresholds.  FIG.  22 B  illustrates an example GUI displaying a visual indicator of a poorly functioning node.  FIG.  22 B  shows path selector  2230  (one of the set of path selectors  2040 A), selected path  2235 , data display areas  2240  and  2245 , aggregate display area  2248 , and visual indicators  2250 - 2265 . 
     In this figure, in response to a user selection of path selector  2230 , the GUI  2000  has highlighted the selected service path  2235 . The path  2235  goes through service nodes  2010 A,  2020 A, and  2030 A. The operational data displayed by the GUI  2000  includes color coded visual indicators  2250 ,  2255 , and  2260  in addition to the numerical indicators of service node performance. The specific color of each indicator  2250 ,  2255 , and  2260  identifies whether an operational statistic of its corresponding node has gone beyond a threshold. In the illustrated example, a threshold latency of 30 ms has been set as an upper limit. Accordingly, as nodes  2010 A and  2020 A each have a latency below 30 ms (as shown in display areas  2240  and  2110 ), the indicators  2250  and  2255  are white circles. In contrast, node  2030 A has a latency of 34.5 ms (as shown in display area  2245 ). 34.5 ms is above the threshold, therefore the indicator  2260  is a gray circle to draw the user&#39;s attention to node  2030 A. 
     The thresholds of some embodiments are set by a user. In other embodiments the threshold may be set by a network virtualization manager or other network system (e.g., based on past performance data, pre-programmed threshold levels, etc.). Some embodiments provide multiple thresholds and multiple colors/patterns to indicate them. For example, in some embodiments, a node with latency below a 30 ms threshold will have a green indicator, a node with a latency between 30 and 50 ms will have a yellow indicator, and a node with a latency above 50 ms will have a red indicator. 
     Aggregate display area  2248  also includes a color coded visual indicator  2265 . Here, the indicator is a gray circle. Various embodiments may display a colored visual indicator in the aggregate display area for different reasons. For example, in some embodiments, the color of a visual indicator may indicate that an aggregate statistic exceeds a particular threshold (e.g., total latency for the path is above 60 ms). Alternatively, or in addition, to identifying an out of bounds aggregate statistic the visual indicator may be set to the color of the node with the worst performance (in this figure, node  2030 A). 
     In addition to or instead of path selectors for selecting a service path, the GUI of some embodiments provides controls that allow a user to select an entire set of service nodes that each implement a particular service operation.  FIG.  23    illustrates GUI  2000  displaying operational data for all the nodes that provide a particular service in response to a user selection of one of the nodes implementing that service. The figure shows cursor  2120 , node  2020 A, display areas  2110  and  2310 , and service control object  2320 . 
     In  FIG.  23   , the GUI  2000  receives a cursor click that selects node  2020 A. In response, the GUI  2000  displays operational data for node  2020 A in display area  2110 , and operational data for node  2020 B (that provides the same service) in display area  2310 . By displaying operational data for an entire service, the GUI  2000  allows a user to determine whether that service is inadequate for the data coming through the service chain and whether individual nodes of the service are functioning properly. When the service is performing inadequately, the user may decide to add additional nodes for the service (service  2 ). Accordingly, the GUI  2000  provides a service control object  2320  for that service. Further details about the control object  2320  is illustrated in the following figures. 
       FIG.  24    illustrates GUI  2000  adding an additional service node for the selected service in response to a user command. In this figure, the cursor  2120  has just clicked on service control object  2320 . In response, a new service node and at least one new service path has been added to the service chain. The GUI  2000  provides a new service node representation  2410  for the newly implemented node. Adding new node  2410  also creates new service paths. Previously, with two nodes of service  2  and two nodes of service  3 , each node of service  1  was the start of four (two options for service  2  times two options for service  3 ) potential service paths of the service chain. In  FIG.  24   , with three nodes for service  2 , each node of service  1  is the start of six (three options for service  2  times two options for service  3 ) potential service paths of the service chain. Accordingly, the GUI has also added two new path selectors  2430  to each set of path selectors  2040 A- 2040 C to identify new paths that use the new links  2440  that run through new service node  2410 . 
     The GUIs of some embodiments use a different pattern or color from previously displayed paths, or use other visual indicators for newly added paths. For example, the new links  2440  and the path selectors  2430  are shown in  FIG.  24    with dotted lines as an indicator of a newly added link/node. The GUIs of some other embodiments do not visually differentiate new links/nodes from previously existing links/nodes. 
     In addition to or instead of providing controls to add a node, some embodiments allow a user to command that an operational node restart or shut down. GUIs of different embodiments receive restart or shutdown commands in different ways, such as (1) through a particular cursor operation on the node to be restarted/shut down, (2) selection of a shutdown option from a menu of options, or (3) some combination of cursor options and menu items. In some such embodiments, a central controller will flag (as inactive) paths through a node that is about to be restarted or shut down, in order to stop data traffic from using that node while it is restarting/shutting down. 
     The above figures illustrate a GUI displaying a particular service chain. The GUIs of some embodiments allow a user to select one out of multiple service chains of a network.  FIG.  25    illustrates a process  2500  of some embodiments for selecting a service chain, then receiving and displaying operational data for components that implement that service chain. The process  2500  starts when a server of the network (e.g., a server implementing a network controller or manager) receives (at  2505 ) a selection of a service chain. The service chain may be one of multiple service chains operating on a network or network segment. In some embodiments, the selection can be received via a menu of service chains, a set of graphical representations of service chains or some other selection method. 
     After a service chain is selected, the process  2500  then identifies (at  2510 ) data collected for components of that service chain. In some embodiments, the process  2500  identifies this data in a database such as the one described with respect to  FIG.  28   , below. This data may include throughput values, latency values, other statistical data collected by proxy servers or calculated from measurements by the proxy servers, averages or weighted averages of such statistical data, and/or other operational data about service nodes (e.g., what type of packets each service node processes, what service is provided by each service node, etc.). In some embodiments, the process  2500  retrieves the identified data in this operation. In other embodiments, the process identifies which data in the database should be accessed for each component, but does not retrieve the operational data for a component until the component is selected by a user. 
     After identifying the relevant data for the selected service chain, the process  2500  then generates (at  2515 ) a graphical display of the service chain and its components. In some embodiments, as described with respect to  FIGS.  20 - 24   , above, the graphical display includes representations of the service nodes implementing a service chain and links between the nodes. 
     The process  2500  receives (at  2520 ) a selection of a service chain component (e.g., as shown in  FIG.  21   , above). In some embodiments, multiple service chain components can be selected at the same time (e.g., as shown in  FIGS.  22 A- 23   , above). The process then displays (at  2525 ) operational data collected for the selected service chain component(s). In some embodiments, displaying the operational data in operation  2525  includes periodically updating the displayed data as new data about the selected component(s) becomes available. 
     After displaying the operational data for the selected component(s), the process  2500  then determines (at  2530 ) whether there are any more selections of service chain components. If there are more selections, the process  2500  returns to operation  2525  and displays operational data for the selected service chain component. If there are no further selections the process ends. In some embodiments, when the process  2500  ends, the GUI will continue to display the operational data for the previously selected components and may also update the displayed operational data. 
     Although the above described process  2500  is in a particular order for purposed of description, one of ordinary skill in the art will understand that other embodiments may perform such a process in a different order. The  FIGS.  20 - 25    describe GUI displays in terms of components of a single service chain. However, one of ordinary skill in the art will understand that a particular service node may implement part of multiple service chains, rather than being exclusive to one service chain. 
     As previously mentioned, the operational data displayed in the above described GUIs may be collected by service proxies, each of which monitors a corresponding service node. The following three figures describe a service proxy and processes for collating and analyzing the data from these service proxies. 
     Each service proxy in some embodiments monitors its respective service node to determine whether the service node is functioning and to generate multiple statistics regarding the operation of its service node.  FIG.  26    illustrates an example of a service proxy  2610  monitoring its service node  2605 . As shown, the service proxy  2610  passes data packets  2620  to and from the service node  2605 . While doing this, the service node in some embodiments generates statistics regarding the number of data packets  2620 , the size (e.g., bits, bytes, etc.) of the payloads of those packets, and the throughput of the service node. In some embodiments, the service node computes a throughput value by dividing the number of bits/bytes in a packet&#39;s payload by the time that it takes the service node to process the packet (i.e., by the duration of time from when the packet leaves the service proxy to when it returns to the service proxy from the service node). Some embodiments compute a throughput value of a service node by measuring the total amount of data (in bits or bytes) in all payloads of all data messages are processed by a service node over some regular period (e.g., 1 second, 5 seconds, 10 seconds, etc.) then dividing by the duration of that period. Other embodiments may count the bits/bytes in the payloads of data messages until some threshold number of bits/bytes are processed, while measuring the time the service node takes to process those data messages, then divide the number of bits/bytes processed by the measured time. Other embodiments may measure the number of bits/bytes in a set number of packets (e.g., 1 packet, 1024 packets, etc.) processed by the service node while measuring the time it takes the service node to process that number of packets, then divide the bits/bytes in those packets by the time it took the service node to process them. 
     In some embodiments, the service proxy  2610  stores the generated statistics (e.g., number of data packets, number of bytes, and throughput) in a data storage  2615  of the service proxy  2610 . As mentioned above, the service proxy in some embodiments periodically exchanges liveness messages  2625  to perform periodic liveness checks to ensure that the service node is operational. From these liveness messaging exchange, the service proxy generates latency metric values that gauge the responsiveness of the service node, and stores an average latency metric value in its data storage  2615 . Periodically the service proxy  2610  provides the statistics (e.g., the latency values, the throughput, etc.) that it maintains in its storage  2615  to the controller cluster, which then assesses this data to determine whether a service chain&#39;s implementation should be modified. 
       FIG.  27    conceptually illustrates a process  2700  that the service proxy performs in some embodiments to calculate and report statistics for its associated service node. In some embodiments, the service proxy continuously performs the process  2700  while the proxy and its corresponding service node are operational. The process  2700  starts when the process generates (at  2705 ) a statistical measurement regarding the operation of its service node. In some embodiments, the measurement generated at  2705  can be either a latency value or a throughput value of the service node. The process  2700  then stores (at  2710 ) the generated measurement in the data storage  2615  in  FIG.  26    when it is the first measurement of its kind (i.e., it is the first latency value or first throughput value) for the reporting period. 
     When this measurement is not the first measurement of its kind for the reporting period, the process averages (at  2710 ) the generated measurement (i.e., averages the generated latency) with the previous measurement or average measurement (i.e., with the previous latency or average latency) stored in the data storage  2615 . In some embodiments, the service proxy calculates weighted averages of each measurement to be more heavily weighted to recent throughput or latency measurements (i.e., uses a larger weight value for the measurement value computed at  2705  than the measurement value or average measurement value retrieved from the storage  2615 ). After generating an average measurement value at  2710 , the process (at  2710 ) stores back in the data storage  2615  the average measurement value. 
     The process  2700  next determines (at  2715 ) whether it is time to report the statistics. If it is not time to report the statistics, the process  2700  returns to operations  2705  where it waits until it generates another measurement value (e.g., another latency or throughput value) for its service proxy. On the other hand, when the process determines (at  2715 ) that it is time to report the generated statistics, the process  2700  reports (at  2720 ) the generated measurements (e.g., the generated latency and throughput measurements) to the controller cluster. It then returns to operation  2705 , where it waits until it generates another measurement value (e.g., another latency or throughput value) for its service proxy. 
     After the service proxies of some embodiments collect operational statistics, these statistics are sent to a central controller for further aggregation, storage in a database, and analysis. The database of some embodiments may be used to provide operational data to a user, as previously described. In addition to or instead of using the database for providing operational data to a user, the aggregated statistics in the database may be analyzed in order to automatically adjust a service chain for better results. 
       FIG.  28    conceptually illustrates a process  2800  of some embodiments for receiving, storing, and analyzing, at a central controller, statistics from a service proxy. The process  2800  starts when the central controller receives (at  2805 ) operational statistics from a service proxy. In some embodiments, these operational statistics are forwarded periodically by the service proxy to the controller cluster. In other embodiments, the operational statistics may be received after the controller commands the service proxy to send the statistics. 
     After receiving the statistics from the service proxy, the process  2800  stores (at  2810 ) them in a database. If any previous statistics from that proxy are stored in the database, the process generates blended averages of the statistics and stores the blended averages in the database. By storing blended averages, the central controller further smooths the data received from the service proxy. After storing the statistics, the process  2800  determines (at  2815 ) whether it is time to analyze them. If it is not time to analyze the stored statistics, then the process  2800  returns to operation  2805  to receive more operational statistics. If it is time to analyze the stored statistics, then the process  2800  calls (at  2820 ) one or more analysis engines to analyze collected statistics. The central controller will modify service paths or nodes if the analysis indicates that changes to the service paths are needed. One of ordinary skill in the art will understand that although one iteration through operations  2805 - 2815  receives and blends statistics from a single node, that the analysis engines called in operation  2820  may analyze statistics from multiple nodes (e.g., all nodes in a service chain). The network of some embodiments uses a single server (e.g., a server implementing the central controller) to both collect and analyze the data relating to a particular service chain. In other embodiments, the analysis engines run on a separate server or servers from the central controller. Examples of analysis of statistics of nodes and modification of service paths and service nodes are described in the following three figures. 
     One problem encountered with service chains is a failure of a single service node. When this failure results in a service node that is inoperative, too slow, generating too many errors, or otherwise falling below a performance threshold, the system of some embodiments alters the service paths of the service chain by restarting or remediating the failing service node. 
       FIG.  29    illustrates a process for automatically restarting/remediating an operational service node when the particular service node is fails to meet a threshold. Not operating properly could include, for example, producing excessive errors, operating too slowly, pausing unexpectedly, etc. 
     The process  2900  analyzes (at  2905 ) data transmission characteristics of operational service nodes of a service. The data transmission characteristics received from service proxies for the nodes. Although the process  2900  analyzes nodes of a particular service, in some embodiments a similar process analyzes all nodes in a service chain. As mentioned with respect to  FIG.  28   , above, the analysis may be by an analysis engine called by a central controller. 
     The process  2900  selects (at  2910 ) a particular node. The processes  2900  of some embodiments may have some pre-filtering to select the node to be examined. For example, in some embodiments, an analysis engine may select a node if the service proxy of that node has flagged it as failing. In other embodiments, the process  2900  selects each node of a service until all the nodes have been selected. 
     If the performance of the node meets (at  2915 ) a threshold, then the process  2900  proceeds to operation  2930 . If the performance of the service node fails to meet (at  2915 ) the threshold, then the process  2900  proceeds to operation  2920 . The process  2900  then flags (at  2920 ) the service paths using the failing node as inactive. The service paths are flagged as inactive in order to stop data traffic through the node before restarting or otherwise remediating whatever problem is causing the node to fail. The networks of some embodiments use a centralized service to generate a service path index. The networks of other embodiments generate service paths dynamically in a decentralized manner. In embodiments that use a single service path index, the process  2900  flags service paths as inactive by identifying which specific paths in the service path index pass through the failing node and designating those paths as inactive. In embodiments where the service paths are generated dynamically in a decentralized manner, the process  2900  flags service paths as inactive by notifying the service path generators not to use the failing node as part of a service path. 
     The process then restarts the node or performs other actions that remediate the problem with the node (at  2925 ). If (at  2930 ) the process has not analyzed all the nodes, then the process returns to operation  2910  and selects another node. If (at  2930 ) the process has analyzed all the nodes, then the process  2900  ends. 
     Even when all nodes of a service operation are functioning properly, the load on that service may be more than the existing nodes of the service can handle. Accordingly, in some embodiments, analysis is directed toward a service operation as a whole, rather than at individual nodes of that service (e.g., after the individual nodes are found to be working properly). In such cases, the method of some embodiments adds new service nodes and new service paths to handle the load. Instantiating the new service node in some embodiments is in response to operational data indicating that a throughput or latency of one or more service nodes implementing the particular service is beyond a threshold (e.g., lower than a set minimum threshold or above a set maximum threshold). 
       FIG.  30    illustrates a process  3000  for automatically adding a new service node when the collective performance of service nodes implementing a service path fail to meet a threshold. The process  3000  is used in some embodiments to identify and remediate cases where the nodes that perform a particular service are collectively overwhelmed by the amount of data to be processed by that service. 
     The process  3000  analyzes (at  3005 ) data transmission characteristics of operational service nodes of a service. The data transmission characteristics are received from service proxies for the nodes. If the performance of the service nodes performing that service meets (at  3010 ) a threshold, then the process  3000  ends. If the performance of the service nodes of a service fails to meet (at  3010 ) the threshold, then the process  3000  proceeds to operation  3015 . 
     The process  3000  then instantiates (at  3015 ) a new node for the service. The process  3000  also defines (at  3020 ) new service paths through the new operational node. In some embodiments, defining new service paths for a new operational node includes eliminating one or more existing service paths through already operational service nodes. In other embodiments, existing service paths are not eliminated, but the amount of data messages through the paths is reduced. The central controllers of some embodiments may reduce the amount of data messages through existing paths and increase the amount of data messages through new paths by adjusting a set of weighting values in selection metrics that are then distributed to host machines of the network that use those selection metrics to determine when to add a new data flow to a service path. 
     Another issue for a network implementing service chains is that the flow of data traffic for a particular service chain will not be constant. When more data traffic is sent through a service chain, the network may add additional service nodes to handle the increased traffic. However, when the data traffic drops for a particular chain, there may be more service nodes for a particular service node than are needed to process the data traffic at that time. Accordingly, the method of some embodiments may deactivate an existing service node that is implementing a particular service and eliminate the paths through that node. Deactivating the existing service node in some embodiments may be in response to operational data indicating that a throughput or latency of one or more service nodes implementing the particular service is beyond a threshold. 
     In some cases, a particular node may be underused, not because there is too little data traffic for the service, but because other nodes of the service are being overused. Therefore, in some embodiments when a node is found to be underused, the analysis engines determine whether other nodes of that service are overused (indicating rebalancing of data allocation is needed). If none of the nodes are overused, then the underused node is shut down. 
       FIG.  31    illustrates a process for automatically shifting data traffic to underused nodes of a service or recovering resources from an underused node of a service. Some embodiments shift data traffic to an underused node by (1) eliminating or reducing the use of paths through other, overused, nodes of the service and (2) instantiating new paths that pass through the underused node. 
     The process  3100  analyzes (at  3105 ) data transmission characteristics of operational service nodes of a service. The data transmission characteristics are received from service proxies for the nodes. The process  3100  determines (at  3110 ) whether any node is underused. An example of an indicator of an underused node would be a low throughput resulting from a low amount of data messages received rather than from a delay in processing the data messages that are received. 
     If no node is underused, the process  3100  ends. If a node is underused, the process  3100  determines (at  3115 ) whether any other node(s) is/are overused. In some embodiments, a node may be overused because too many service paths are using that node. In some embodiments, in addition to or instead of too many service paths using a node, the service paths using that node may be handling too much data. 
     If another node is overused, the process eliminates (at  3120 ) or reduces the amount of data message traffic sent on, one or more paths through the overused node. Some or all of the data message traffic through those path is then routed through existing or new path(s) through the underused node. That is, the process increases the load of the underused node while decreasing the lode of the overused node. If no other node is overused, the process shuts down (at  3125 ) the underused node. In some embodiments, shutting down the underused node is preceded by an operation to flag the paths through the node as inactive, as described with respect to operation  2920  of  FIG.  29   , above. 
     In some embodiments, the data sent along a particular path is sent on a per flow basis. That is, when a new flow (e.g., a flow defined by a five-tuple with source and destination IP addresses, source and destination port numbers, and packet protocol, or a tuple with fewer/more attributes) is received by the service chain, the flow is assigned to a particular path by a service insertion pre-processor of a host. The service insertion pre-processor of some embodiments uses selection metrics stored in a path table on the host to determine which path to assign a particular flow to. In some embodiments, the selection metrics are set by a network controller on a server, such as the central controller described with respect to  FIG.  28   . The central controller of some embodiments, after having the collected statistics analyzed, changes the weights of the selection criteria in the selection metric in favor of paths through underutilized nodes and against paths through overused nodes. The network controller then distributes the updated selection metrics (e.g., through a rule distributor  1814  of  FIG.  18   ) path tables of the hosts. 
     Many of the above-described features and applications are implemented as software processes that are specified as a set of instructions recorded on a computer readable storage medium (also referred to as computer readable medium). When these instructions are executed by one or more processing unit(s) (e.g., one or more processors, cores of processors, or other processing units), they cause the processing unit(s) to perform the actions indicated in the instructions. Examples of computer readable media include, but are not limited to, CD-ROMs, flash drives, RAM chips, hard drives, EPROMs, etc. The computer readable media does not include carrier waves and electronic signals passing wirelessly or over wired connections. 
     In this specification, the term “software” is meant to include firmware residing in read-only memory or applications stored in magnetic storage, which can be read into memory for processing by a processor. Also, in some embodiments, multiple software inventions can be implemented as sub-parts of a larger program while remaining distinct software inventions. In some embodiments, multiple software inventions can also be implemented as separate programs. Finally, any combination of separate programs that together implement a software invention described here is within the scope of the invention. In some embodiments, the software programs, when installed to operate on one or more electronic systems, define one or more specific machine implementations that execute and perform the operations of the software programs. 
       FIG.  32    conceptually illustrates a computer system  3200  with which some embodiments of the invention are implemented. The computer system  3200  can be used to implement any of the above-described hosts, controllers, and managers. As such, it can be used to execute any of the above described processes. This computer system includes various types of non-transitory machine readable media and interfaces for various other types of machine readable media. Computer system  3200  includes a bus  3205 , processing unit(s)  3210 , a system memory  3225 , a read-only memory  3230 , a permanent storage device  3235 , input devices  3240 , and output devices  3245 . 
     The bus  3205  collectively represents all system, peripheral, and chipset buses that communicatively connect the numerous internal devices of the computer system  3200 . For instance, the bus  3205  communicatively connects the processing unit(s)  3210  with the read-only memory  3230 , the system memory  3225 , and the permanent storage device  3235 . 
     From these various memory units, the processing unit(s)  3210  retrieve instructions to execute and data to process in order to execute the processes of the invention. The processing unit(s) may be a single processor or a multi-core processor in different embodiments. The read-only-memory (ROM)  3230  stores static data and instructions that are needed by the processing unit(s)  3210  and other modules of the computer system. The permanent storage device  3235 , on the other hand, is a read-and-write memory device. This device is a non-volatile memory unit that stores instructions and data even when the computer system  3200  is off. Some embodiments of the invention use a mass-storage device (such as a magnetic or optical disk and its corresponding disk drive) as the permanent storage device  3235 . 
     Other embodiments use a removable storage device (such as a flash drive, etc.) as the permanent storage device. Like the permanent storage device  3235 , the system memory  3225  is a read-and-write memory device. However, unlike storage device  3235 , the system memory is a volatile read-and-write memory, such a random access memory. The system memory stores some of the instructions and data that the processor needs at runtime. In some embodiments, the invention&#39;s processes are stored in the system memory  3225 , the permanent storage device  3235 , and/or the read-only memory  3230 . From these various memory units, the processing unit(s)  3210  retrieve instructions to execute and data to process in order to execute the processes of some embodiments. 
     The bus  3205  also connects to the input and output devices  3240  and  3245 . The input devices enable the user to communicate information and select commands to the computer system. The input devices  3240  include alphanumeric keyboards and pointing devices (also called “cursor control devices”). The output devices  3245  display images generated by the computer system. The output devices include printers and display devices, such as cathode ray tubes (CRT) or liquid crystal displays (LCD). Some embodiments include devices such as a touchscreen that function as both input and output devices. 
     Finally, as shown in  FIG.  32   , bus  3205  also couples computer system  3200  to a network  3265  through a network adapter (not shown). In this manner, the computer can be a part of a network of computers (such as a local area network (“LAN”), a wide area network (“WAN”), or an Intranet, or a network of networks, such as the Internet. Any or all components of computer system  3200  may be used in conjunction with the invention. 
     Some embodiments include electronic components, such as microprocessors, storage and memory that store computer program instructions in a machine-readable or computer-readable medium (alternatively referred to as computer-readable storage media, machine-readable media, or machine-readable storage media). Some examples of such computer-readable media include RAM, ROM, read-only compact discs (CD-ROM), recordable compact discs (CD-R), rewritable compact discs (CD-RW), read-only digital versatile discs (e.g., DVD-ROM, dual-layer DVD-ROM), a variety of recordable/rewritable DVDs (e.g., DVD-RAM, DVD-RW, DVD+RW, etc.), flash memory (e.g., SD cards, mini-SD cards, micro-SD cards, etc.), magnetic and/or solid state hard drives, read-only and recordable Blu-Ray® discs, ultra-density optical discs, and any other optical or magnetic media. The computer-readable media may store a computer program that is executable by at least one processing unit and includes sets of instructions for performing various operations. Examples of computer programs or computer code include machine code, such as is produced by a compiler, and files including higher-level code that are executed by a computer, an electronic component, or a microprocessor using an interpreter. 
     While the above discussion primarily refers to microprocessor or multi-core processors that execute software, some embodiments are performed by one or more integrated circuits, such as application specific integrated circuits (ASICs) or field programmable gate arrays (FPGAs). In some embodiments, such integrated circuits execute instructions that are stored on the circuit itself. 
     As used in this specification, the terms “computer”, “server”, “processor”, and “memory” all refer to electronic or other technological devices. These terms exclude people or groups of people. For the purposes of the specification, the terms display or displaying means displaying on an electronic device. As used in this specification, the terms “computer readable medium,” “computer readable media,” and “machine readable medium” are entirely restricted to tangible, physical objects that store information in a form that is readable by a computer. These terms exclude any wireless signals, wired download signals, and any other ephemeral or transitory signals. 
     While the invention has been described with reference to numerous specific details, one of ordinary skill in the art will recognize that the invention can be embodied in other specific forms without departing from the spirit of the invention. For instance, several figures conceptually illustrate processes. The specific operations of these processes may not be performed in the exact order shown and described. The specific operations may not be performed in one continuous series of operations, and different specific operations may be performed in different embodiments. Furthermore, the process could be implemented using several sub-processes, or as part of a larger macro process. 
     Even though the service insertion rules in several of the above-described examples provide service chain identifiers, some of the inventions described herein can be implemented by having a service insertion rule provide the service identifiers (e.g., SPIs) of the different services specified by the service insertion rule. Similarly, several of the above-described embodiments perform distributed service routing that relies at each service hop identifying a next service hop by performing an exact match based on the SPI/SI values. However, some of the inventions described herein can be implemented by having the service insertion pre-processor embed all the service hop identifiers (e.g., service hop MAC addresses) as the data message&#39;s service attribute set and/or in the data message&#39;s encapsulating service header. 
     In addition, some embodiments decrement the SI value differently (e.g., at different times) than the approaches described above. Also, instead of performing the next hop lookup just based on the SPI and SI values, some embodiments perform this lookup based on the SPI, SI and service direction values as these embodiments use a common SPI value for both the forward and reverse directions of data messages flowing between two machines. 
     The above-described methodology is used in some embodiments to express path information in single tenant environments. Thus, one of ordinary skill will realize that some embodiments of the invention are equally applicable to single tenant datacenters. Conversely, in some embodiments, the above-described methodology is used to carry path information across different datacenters of different datacenter providers when one entity (e.g., one corporation) is a tenant in multiple different datacenters of different providers. In these embodiments, the tenant identifiers that are embedded in the tunnel headers have to be unique across the datacenters, or have to be translated when they traverse from one datacenter to the next. Thus, one of ordinary skill in the art would understand that the invention is not to be limited by the foregoing illustrative details, but rather is to be defined by the appended claims.