Service function chaining

Service function chaining

Service function chain (SFC) essentially refers to the software-defined networking (SDN) version of policy-based routing (PBR). In many cases, SFC involves security, although it can include a variety of other features.

Fundamentally, SFC routes packets through one or more service functions instead of conventional routing that routes packets using destination IP address. Service functions essentially emulate a series of physical network devices with cables linking them together.

A basic example of SFC involves routing packets from one location to another through a firewall that lacks a “next hop” IP address from a conventional routing perspective. A more complex example involves an ordered series of service functions, each implemented using multiple instances (VMs). Packets must flow through one instance and a hashing algorithm distributes flows across multiple instances at each hop.


All OpenStack Networking services and OpenStack Compute instances connect to a virtual network via ports making it possible to create a traffic steering model for service chaining using only ports. Including these ports in a port chain enables steering of traffic through one or more instances providing service functions.

A port chain, or service function path, consists of the following:

  • A set of ports that define the sequence of service functions.
  • A set of flow classifiers that specify the classified traffic flows entering the chain.

If a service function involves a pair of ports, the first port acts as the ingress port of the service function and the second port acts as the egress port. If both ports use the same value, they function as a single virtual bidirectional port.

A port chain is a unidirectional service chain. The first port acts as the head of the service function chain and the second port acts as the tail of the service function chain. A bidirectional service function chain consists of two unidirectional port chains.

A flow classifier can only belong to one port chain to prevent ambiguity as to which chain should handle packets in the flow. A check prevents such ambiguity. However, you can associate multiple flow classifiers with a port chain because multiple flows can request the same service function path.

Currently, SFC lacks support for multi-project service functions.

The port chain plug-in supports backing service providers including the OVS driver and a variety of SDN controller drivers. The common driver API enables different drivers to provide different implementations for the service chain path rendering.

Port chain architecture Port chain model

See the developer documentation for more information.


Port chain

  • id - Port chain ID
  • tenant_id - Project ID
  • name - Readable name
  • description - Readable description
  • port_pair_groups - List of port pair group IDs
  • flow_classifiers - List of flow classifier IDs
  • chain_parameters - Dictionary of chain parameters

A port chain consists of a sequence of port pair groups. Each port pair group is a hop in the port chain. A group of port pairs represents service functions providing equivalent functionality. For example, a group of firewall service functions.

A flow classifier identifies a flow. A port chain can contain multiple flow classifiers. Omitting the flow classifier effectively prevents steering of traffic through the port chain.

The chain_parameters attribute contains one or more parameters for the port chain. Currently, it only supports a correlation parameter that defaults to mpls for consistency with Open vSwitch (OVS) capabilities. Future values for the correlation parameter may include the network service header (NSH).

Port pair group

  • id - Port pair group ID
  • tenant_id - Project ID
  • name - Readable name
  • description - Readable description
  • port_pairs - List of service function port pairs

A port pair group may contain one or more port pairs. Multiple port pairs enable load balancing/distribution over a set of functionally equivalent service functions.

Port pair

  • id - Port pair ID
  • tenant_id - Project ID
  • name - Readable name
  • description - Readable description
  • ingress - Ingress port
  • egress - Egress port
  • service_function_parameters - Dictionary of service function parameters

A port pair represents a service function instance that includes an ingress and egress port. A service function containing a bidirectional port uses the same ingress and egress port.

The service_function_parameters attribute includes one or more parameters for the service function. Currently, it only supports a correlation parameter that determines association of a packet with a chain. This parameter defaults to none for legacy service functions that lack support for correlation such as the NSH. If set to none, the data plane implementation must provide service function proxy functionality.

Flow classifier

  • id - Flow classifier ID
  • tenant_id - Project ID
  • name - Readable name
  • description - Readable description
  • ethertype - Ethertype (IPv4/IPv6)
  • protocol - IP protocol
  • source_port_range_min - Minimum source protocol port
  • source_port_range_max - Maximum source protocol port
  • destination_port_range_min - Minimum destination protocol port
  • destination_port_range_max - Maximum destination protocol port
  • source_ip_prefix - Source IP address or prefix
  • destination_ip_prefix - Destination IP address or prefix
  • logical_source_port - Source port
  • logical_destination_port - Destination port
  • l7_parameters - Dictionary of L7 parameters

A combination of the source attributes defines the source of the flow. A combination of the destination attributes defines the destination of the flow. The l7_parameters attribute is a place holder that may be used to support flow classification using layer 7 fields, such as a URL. If unspecified, the logical_source_port and logical_destination_port attributes default to none, the ethertype attribute defaults to IPv4, and all other attributes default to a wildcard value.


Create a port chain

The following example uses the neutron command-line interface (CLI) to create a port chain consisting of three service function instances to handle HTTP (TCP) traffic flows from to

  • Instance 1
    • Name: vm1
    • Function: Firewall
    • Port pair: [p1, p2]
  • Instance 2
    • Name: vm2
    • Function: Firewall
    • Port pair: [p3, p4]
  • Instance 3
    • Name: vm3
    • Function: Intrusion detection system (IDS)
    • Port pair: [p5, p6]


The example network net1 must exist before creating ports on it.

  1. Source the credentials of the project that owns the net1 network.

  2. Create ports on network net1 and record the UUID values.

    $ openstack port create p1 --network net1
    $ openstack port create p2 --network net1
    $ openstack port create p3 --network net1
    $ openstack port create p4 --network net1
    $ openstack port create p5 --network net1
    $ openstack port create p6 --network net1
  3. Launch service function instance vm1 using ports p1 and p2, vm2 using ports p3 and p4, and vm3 using ports p5 and p6.

    $ openstack server create --nic port-id=P1_ID --nic port-id=P2_ID vm1
    $ openstack server create --nic port-id=P3_ID --nic port-id=P4_ID vm2
    $ openstack server create --nic port-id=P5_ID --nic port-id=P6_ID vm3

    Replace P1_ID, P2_ID, P3_ID, P4_ID, P5_ID, and P6_ID with the UUIDs of the respective ports.


    This command requires additional options to successfully launch an instance. See the CLI reference for more information.

    Alternatively, you can launch each instance with one network interface and attach additional ports later.

  4. Create flow classifier FC1 that matches the appropriate packet headers.

    $ neutron flow-classifier-create \
      --description "HTTP traffic from to" \
      --ethertype IPv4 \
      --source-ip-prefix \
      --destination-ip-prefix \
      --protocol tcp \
      --source-port 1000:1000 \
      --destination-port 80:80 FC1
  5. Create port pair PP1 with ports p1 and p2, PP2 with ports p3 and p4, and PP3 with ports p5 and p6.

    $ neutron port-pair-create \
      --description "Firewall SF instance 1" \
      --ingress p1 \
      --egress p2 PP1
    $ neutron port-pair-create \
      --description "Firewall SF instance 2" \
      --ingress p3 \
      --egress p4 PP2
    $ neutron port-pair-create \
      --description "IDS SF instance" \
      --ingress p5 \
      --egress p6 PP3
  6. Create port pair group PPG1 with port pair PP1 and PP2 and PPG2 with port pair PP3.

    $ neutron port-pair-group-create \
      --port-pair PP1 --port-pair PP2 PPG1
    $ neutron port-pair-group-create \
      --port-pair PP3 PPG2


    You can repeat the --port-pair option for multiple port pairs of functionally equivalent service functions.

  7. Create port chain PC1 with port pair groups PPG1 and PPG2 and flow classifier FC1.

    $ neutron port-chain-create \
      --port-pair-group PPG1 --port-pair-group PPG2 \
      --flow-classifier FC1 PC1


    You can repeat the --port-pair-group option to specify additional port pair groups in the port chain. A port chain must contain at least one port pair group.

    You can repeat the --flow-classifier option to specify multiple flow classifiers for a port chain. Each flow classifier identifies a flow.

Update a port chain or port pair group

  • Use the neutron port-chain-update command to dynamically add or remove port pair groups or flow classifiers on a port chain.

    • For example, add port pair group PPG3 to port chain PC1:

      $ neutron port-chain-update \
        --port-pair-group PPG1 --port-pair-group PPG2 --port-pair-group PPG3 \
        --flow-classifier FC1 PC1
    • For example, add flow classifier FC2 to port chain PC1:

      $ neutron port-chain-update \
        --port-pair-group PPG1 --port-pair-group PPG2 \
        --flow-classifier FC1 --flow-classifier FC2 PC1

      SFC steers traffic matching the additional flow classifier to the port pair groups in the port chain.

  • Use the neutron port-pair-group-update command to perform dynamic scale-out or scale-in operations by adding or removing port pairs on a port pair group.

    $ neutron port-pair-group-update \
      --port-pair PP1 --port-pair PP2 --port-pair PP4 PPG1

    SFC performs load balancing/distribution over the additional service functions in the port pair group.

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