(1) Field of the Invention
The present invention relates to a packet routing apparatus. More particularly, the invention relates to a packet routing apparatus suitable for relaying a variable length packet between lines of different communication protocols.
(2) Description of the Related Art
In an IP network using the TCP/IP protocol, various applications of electric mails, WWW (World Wide Web), and the like can be used. The IP network has advantages such as lower communication cost as compared with a conventional telephone-based switching network. Consequently, traffic has been being explosively increased in recent years.
A router disposed in each of nodes of an IP network determines the outgoing route of a received packet on the basis of the IP header of the packet. A software router is in the main stream of the conventional routers and the software router determines the outgoing route of a received packet by a CPU connected to an input/output line interface via a bus. To address an explosive increase in traffic, in recent years, a hardware router which increases the speed of packet transfer by analyzing the header of an IP packet by hardware is in the limelight. An example of the technique of analyzing the header by hardware is disclosed in Japanese Unexamined Patent Application No. 2000-151709.
One of transfer method for transmitting IP packets on an IP network adopts encapsulation of an IP packet with the header of a lower layer. For encapsulation of IP packets, connection-oriented protocols such as an ATM (Asynchronous Transfer Mode) communication, a frame relay of an HDLC, and PPP (Point to Point Protocol) are used. According to an MPLS (Multi-Protocol Label Switching) system examined by the IFTF (Internet Engineering Task Force), an IP packet is encapsulated by using a shim header including a label.
When IP packets are encapsulated with the header of a lower layer, multiplex transfer of IP packets can be realized between nodes. By reducing the number of destination indication bits, the transfer speed can be increased. When a connection-oriented packet transfer is performed in a lower layer, a closed network can be maintained. In this case, by using the function of the lower layer, a virtual private network (VPN) can be realized on an IP network.
As communication services such as frame relay and ATM are improved by major carriers, the configuration of an intracorporate communication network employing the frame relay communication or ATM communication for the lower layer and using the TCP/IP protocol in the upper layer is increasing.
In the case of realizing the VPN on the Internet, a node (hereinbelow, called an edge node) disposed at the entrance of a network (hereinbelow, called a core network) of an Internet service provider (ISP) for providing VPN services encapsulates IP packets. Each of nodes (hereinbelow, called core nodes) in the core network transfers a received packet in accordance with an encapsulation header. An edge node disposed at the exit of the core network removes the encapsulation header.