Control plane

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In network routing, the control plane is the part of the router architecture that is concerned with establishing the network topology, or the information in a routing table that defines what to do with incoming packets. Control plane functions, such as participating in routing protocols, run in the architectural control element. [1] In most cases, the routing table contains a list of destination addresses and the outgoing interface or interfaces associated with each. Control plane logic also can identify certain packets to be discarded, as well as preferential treatment of certain packets for which a high quality of service is defined by such mechanisms as differentiated services.

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Depending on the specific router implementation, there may be a separate forwarding information base that is populated by the control plane, but used by the high-speed forwarding plane to look up packets and decide how to handle them.

In computing, the control plane is the part of the software that configures and shuts down the data plane. [2] By contrast, the data plane is the part of the software that processes the data requests. [3] The data plane is also sometimes referred to as the forwarding plane.

The distinction has proven useful in the networking field where it originated, as it separates the concerns: the data plane is optimized for speed of processing, and for simplicity and regularity. The control plane is optimized for customizability, handling policies, handling exceptional situations, and in general facilitating and simplifying the data plane processing. [4] [5]

The conceptual separation of the data plane from the control plane has been done for years. [6] An early example is Unix, where the basic file operations are open, close for the control plane and read write for the data plane. [7]

Building the unicast routing table

A major function of the control plane is deciding which routes go into the main routing table. "Main" refers to the table that holds the unicast routes that are active. Multicast routing may require an additional routing table for multicast routes. Several routing protocols e.g. IS-IS, OSPF and BGP maintain internal databases of candidate routes which are promoted when a route fails or when a routing policy is changed.

Several different information sources may provide information about a route to a given destination, but the router must select the "best" route to install into the routing table. In some cases, there may be multiple routes of equal "quality", and the router may install all of them and load-share across them.

Sources of routing information

There are three general sources of routing information:

Local interface information

Routers forward traffic that enters on an input interface and leaves on an output interface, subject to filtering and other local rules. While routers usually forward from one physical (e.g., Ethernet, serial) to another physical interface, it is also possible to define multiple logical interfaces on a physical interface. A physical Ethernet interface, for example, can have logical interfaces in several virtual LANs defined by IEEE 802.1Q VLAN headers.

When an interface has an address configured in a subnet, such as 192.0.2.1 in the 192.0.2.0/24 (i.e., subnet mask 255.255.255.0) subnet, and that interface is considered "up" by the router, the router thus has a directly connected route to 192.0.2.0/24. If a routing protocol offered another router's route to that same subnet, the routing table installation software will normally ignore the dynamic route and prefer the directly connected route.

There also may be software-only interfaces on the router, which it treats as if they were locally connected. For example, most implementations have a "null" software-defined interface. Packets having this interface as a next hop will be discarded, which can be a very efficient way to filter traffic. Routers usually can route traffic faster than they can examine it and compare it to filters, so, if the criterion for discarding is the packet's destination address, "blackholing" the traffic will be more efficient than explicit filters.

Other software defined interfaces that are treated as directly connected, as long as they are active, are interfaces associated with tunneling protocols such as Generic Routing Encapsulation (GRE) or Multiprotocol Label Switching (MPLS). Loopback interfaces are virtual interfaces that are considered directly connected interfaces.

Static routes

Router configuration rules may contain static routes. A static route minimally has a destination address, a prefix length or subnet mask, and a definition where to send packets for the route. That definition can refer to a local interface on the router, or a next-hop address that could be on the far end of a subnet to which the router is connected. The next-hop address could also be on a subnet that is directly connected, and, before the router can determine if the static route is usable, it must do a recursive lookup of the next hop address in the local routing table. If the next-hop address is reachable, the static route is usable, but if the next-hop is unreachable, the route is ignored.

Static routes also may have preference factors used to select the best static route to the same destination. One application is called a floating static route, where the static route is less preferred than a route from any routing protocol. The static route, which might use a dialup link or other slow medium, activates only when the dynamic routing protocol(s) cannot provide a route to the destination.

Static routes that are more preferred than any dynamic route also can be very useful, especially when using traffic engineering principles to make certain traffic go over a specific path with an engineered quality of service.

Dynamic routing protocols

See routing protocols. The routing table manager, according to implementation and configuration rules, may select a particular route or routes from those advertised by various routing protocols.

Installing unicast routes

Different implementations have different sets of preferences for routing information, and these are not standardized among IP routers. It is fair to say that subnets on directly connected active interfaces are always preferred. Beyond that, however, there will be differences.

Implementers generally have a numerical preference, which Cisco calls an "administrative distance", for route selection. The lower the preference, the more desirable the route. Cisco's IOS [8] implementation makes exterior BGP the most preferred source of dynamic routing information, while Nortel RS [9] makes intra-area OSPF most preferred.

The general order of selecting routes to install is:

  1. If the route is not in the routing table, install it.
  2. If the route is "more specific" than an existing route, install it in addition to the existing routes. "More specific" means that it has a longer prefix. A /28 route, with a subnet mask of 255.255.255.240, is more specific than a /24 route, with a subnet mask of 255.255.255.0.
  3. If the route is of equal specificity to a route already in the routing table, but comes from a more preferred source of routing information, replace the route in the table.
  4. If the route is of equal specificity to a route in the routing table, yet comes from a source of the same preference,
    1. Discard it if the route has a higher metric than the existing route
    2. Replace the existing route if the new route has a lower metric
    3. If the routes are of equal metric and the router supports load-sharing, add the new route and designate it as part of a load-sharing group. Typically, implementations will support a maximum number of routes that load-share to the same destination. If that maximum is already in the table, the new route is usually dropped.

Routing table vs. forwarding information base

See forwarding plane for more detail, but each implementation has its own means of updating the forwarding information base (FIB) with new routes installed in the routing table. If the FIB is in one-to-one correspondence with the RIB, the new route is installed in the FIB after it is in the RIB. If the FIB is smaller than the RIB, and the FIB uses a hash table or other data structure that does not easily update, the existing FIB might be invalidated and replaced with a new one computed from the updated RIB.

Multicast routing tables

Multicast routing builds on unicast routing. Each multicast group to which the local router can route has a multicast routing table entry with a next hop for the group, rather than for a specific destination as in unicast routing.

There can be multicast static routes as well as learning dynamic multicast routes from a protocol such as Protocol Independent Multicast (PIM).

See also

Related Research Articles

An Internet Protocol address is a numerical label such as 192.0.2.1 that is assigned to a device connected to a computer network that uses the Internet Protocol for communication. IP addresses serve two main functions: network interface identification, and location addressing.

<span class="mw-page-title-main">IPv4</span> Fourth version of the Internet Protocol

Internet Protocol version 4 (IPv4) is the first version of the Internet Protocol (IP) as a standalone specification. It is one of the core protocols of standards-based internetworking methods in the Internet and other packet-switched networks. IPv4 was the first version deployed for production on SATNET in 1982 and on the ARPANET in January 1983. It is still used to route most Internet traffic today, even with the ongoing deployment of Internet Protocol version 6 (IPv6), its successor.

<span class="mw-page-title-main">Multicast</span> Computer networking technique

In computer networking, multicast is a type of group communication where data transmission is addressed to a group of destination computers simultaneously. Multicast can be one-to-many or many-to-many distribution. Multicast differs from physical layer point-to-multipoint communication.

<span class="mw-page-title-main">Router (computing)</span> Device that forwards data packets between computer networks

A router is a computer and networking device that forwards data packets between computer networks, including internetworks such as the global Internet.

Routing is the process of selecting a path for traffic in a network or between or across multiple networks. Broadly, routing is performed in many types of networks, including circuit-switched networks, such as the public switched telephone network (PSTN), and computer networks, such as the Internet.

Open Shortest Path First (OSPF) is a routing protocol for Internet Protocol (IP) networks. It uses a link state routing (LSR) algorithm and falls into the group of interior gateway protocols (IGPs), operating within a single autonomous system (AS).

The Routing Information Protocol (RIP) is one of the oldest distance-vector routing protocols which employs the hop count as a routing metric. RIP prevents routing loops by implementing a limit on the number of hops allowed in a path from source to destination. The largest number of hops allowed for RIP is 15, which limits the size of networks that RIP can support.

<span class="mw-page-title-main">Routing table</span> Data table stored in a router that lists the routes to network destinations

In computer networking, a routing table, or routing information base (RIB), is a data table stored in a router or a network host that lists the routes to particular network destinations, and in some cases, metrics (distances) associated with those routes. The routing table contains information about the topology of the network immediately around it.

Label Distribution Protocol (LDP) is a protocol in which routers capable of Multiprotocol Label Switching (MPLS) exchange label mapping information. Two routers with an established session are called LDP peers and the exchange of information is bi-directional. LDP is used to build and maintain label-switched path (LSP) databases that are used to forward traffic through MPLS networks.

<span class="mw-page-title-main">Protocol-Independent Multicast</span> Multicast routing protocol

Protocol-Independent Multicast (PIM) is a family of multicast routing protocols for Internet Protocol (IP) networks that provide one-to-many and many-to-many distribution of data over a LAN, WAN or the Internet. It is termed protocol-independent because PIM does not include its own topology discovery mechanism, but instead uses routing information supplied by other routing protocols. PIM is not dependent on a specific unicast routing protocol; it can make use of any unicast routing protocol in use on the network. PIM does not build its own routing tables. PIM uses the unicast routing table for reverse-path forwarding.

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<span class="mw-page-title-main">Data plane</span> Router architecture

In routing, the data plane, sometimes called the forwarding plane or user plane, defines the part of the router architecture that decides what to do with packets arriving on an inbound interface. Most commonly, it refers to a table in which the router looks up the destination address of the incoming packet and retrieves the information necessary to determine the path from the receiving element, through the internal forwarding fabric of the router, and to the proper outgoing interface(s).

Static routing describes a process by which routing is configured with fixed values which do not change at runtime unless manually edited. Static routes are used with and without dynamic Routing protocols and usually share the same routing table as those protocols. Routes require at least two attributes; the destination and the gateway, but may contain additional attributes such as a metric. Some implementations treat the network address and subnet mask as separate values, however in practice both of the values have to be considered for any given routing decision to determine the longest prefix match. Static routes together with connected routes and routes from configuration protocols such as DHCP or Router Advertisements provide the routes which are then redistributed using dynamic routing protocols. While static routes are entered into the system and remain there until removed or changed manually, dynamic routing protocols create and delete routes dynamically at runtime without intervention. Thus the term static here refers to the nature of remaining unchanged by the system itself. The most prominent example of a static route is a default route which is often used on devices with a statically configured IP address to provide the device with access to the rest of the network or the internet by default. In contrast to a so called connected route which is automatically generated upon address assignment based on the used subnet mask, a static route must be manually configured. Due to this the configuration may fail if there is no route to the provided gateway at the time of configuration, other than the connected route which will always succeed as it does not require a gateway. The gateway of a static route need not be an address, but can also specify an interface in most implementations.

IEEE 802.1aq is an amendment to the IEEE 802.1Q networking standard which adds support for Shortest Path Bridging (SPB). This technology is intended to simplify the creation and configuration of Ethernet networks while enabling multipath routing.

IP routing is the application of routing methodologies to IP networks. This involves not only protocols and technologies but includes the policies of the worldwide organization and configuration of Internet infrastructure. In each IP network node, IP routing involves the determination of a suitable path for a network packet from a source to its destination in an IP network. The process uses static configuration rules or dynamically obtained from routing protocols to select specific packet forwarding methods to direct traffic to the next available intermediate network node one hop closer to the desired final destination, a total path potentially spanning multiple computer networks.

In computer networking, a unicast flood occurs when a switch receives a unicast frame and the switch does not know that the addressee is on any particular switch port. Since the switch has no information regarding which port, if any, the addressee might be reached through, it forwards the frame through all ports aside from the one through which the frame was received.

References

  1. ↑ Forwarding and Control Element Separation (ForCES) Framework, RFC 3746, Network Working Group, April 2004
  2. ↑ Do, Truong-Xuan; Kim, Younghan (2017-06-01). "Control and data plane separation architecture for supporting multicast listeners over distributed mobility management". ICT Express. Special Issue on Patents, Standardization and Open Problems in ICT Practices. 3 (2): 90–95. doi: 10.1016/j.icte.2017.06.001 . ISSN   2405-9595.
  3. ↑ Conran, Matt (2019-02-25). "Named data networking: Stateful forwarding plane for datagram delivery". Network World. Retrieved 2019-10-14.
  4. ↑ Xia, Wenfeng; Wen, Yoggang; Heng Foh, Chuan; Niyato, Dusit; Xie, Haiyong (2015). "A Survey on Software-Defined Networking". IEEE Communications Surveys & Tutorials. 17 (1): 27–46. doi: 10.1109/COMST.2014.2330903 . S2CID   4269723.
  5. ↑ Ahmad, Ijaz; Namal, Suneth; Ylianttila, Mika; Gurtov, Andrei (2015). "Security in Software-Defined Networks: A Survey" (PDF). IEEE Communications Surveys & Tutorials. 17 (4): 2317–2342. doi:10.1109/COMST.2015.2474118. S2CID   2138863.
  6. ↑ Do, Truong-Xuan; Kim, Younghan (2017-06-01). "Control and data plane separation architecture for supporting multicast listeners over distributed mobility management". ICT Express. Special Issue on Patents, Standardization and Open Problems in ICT Practices. 3 (2): 90–95. doi: 10.1016/j.icte.2017.06.001 . ISSN   2405-9595.
  7. ↑ Bach, Maurice J. (1986). The Design of the Unix Operating System. Prentice-Hall. Bibcode:1986duos.book.....B.
  8. ↑ Configuring IP Routing Protocol-Independent Features, Cisco Systems,July 2006
  9. ↑ Nortel Ethernet Routing Switch 8600 Configuring IP Routing Operations, Nortel Networks, January 2007
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