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In routing, the forwarding plane, sometimes called the data 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). The IP Multimedia Subsystem architecture uses the term transport plane to describe a function roughly equivalent to the routing control plane. In certain cases the table may specify that a packet is to be discarded. In such cases, the router may return an ICMP "destination unreachable" or other appropriate code. Some security policies, however, dictate that the router should drop the packet silently, in order that a potential attacker does not become aware that a target is being protected. The incoming forwarding element will also decrement the time-to-live (TTL) field of the packet, and, if the new value is zero, discard the packet. While the Internet Protocol (IP) specification indicates that an Internet Control Message Protocol (ICMP) Time exceeded message should be sent to the originator of the packet (i.e. the node indicated by the source address), the router may be configured to drop the packet silently (again according to security policies). Depending on the specific router implementation, the table in which the destination address is looked up could be the routing table (also known as the routing information base, RIB), or a separate forwarding information base (FIB) that is populated (i.e., loaded) by the routing control plane, but used by the forwarding plane for look-ups at much higher speeds. Before or after examining the destination, other tables may be consulted to make decisions to drop the packet based on other characteristics, such as the source address, the IP protocol identifier field, or Transmission Control Protocol (TCP) or User Datagram Protocol (UDP) port number. Forwarding plane functions run in the forwarding element.〔(Forwarding and Control Element Separation (ForCES) Framework ), RFC 3746, Network Working Group, April 2004〕 High-performance routers often have multiple distributed forwarding elements, so that the router increases performance with parallel processing. The outgoing interface will encapsulate the packet in the appropriate data link protocol. Depending on the router software and its configuration, functions, usually implemented at the outgoing interface, may set various packet fields, such as the DSCP field used by differentiated services. In general, the passage from the input interface directly to an output interface, through the fabric with minimum modification at the output interface, is called the ''fast path'' of the router. If the packet needs significant processing, such as segmentation or encryption, it may go onto a slower path, which is sometimes called the ''services plane'' of the router. Service planes can make forwarding or processing decisions based on higher-layer information, such as a Web URL contained in the packet payload. ==Issues in router forwarding performance== Vendors design router products for specific markets. Design of routers intended for home use, perhaps supporting several PCs and VoIP telephony, is driven by keeping the cost as low as possible. In such a router, there is no separate forwarding fabric, and there is only one active forwarding path: into the main processor and out of the main processor. Routers for more demanding applications accept greater cost and complexity to get higher throughput in their forwarding planes. Several design factors affect router forwarding performance: * Data link layer processing and extracting the packet * Decoding the packet header * Looking up the destination address in the packet header * Analyzing other fields in the packet * Sending the packet through the "fabric" interconnecting the ingress and egress interfaces * Processing and data link encapsulation at the egress interface Routers may have one or more processors. In a uniprocessor design, these performance parameters are affected not just by the processor speed, but by competition for the processor. Higher-performance routers invariably have multiple processing elements, which may be general-purpose processor chips or specialized application-specific integrated circuits (ASIC). Very high performance products have multiple processing elements on each interface card. In such designs, the main processor does not participate in forwarding, but only in control plane and management processing. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「forwarding plane」の詳細全文を読む スポンサード リンク
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